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Searching for Axion Dark Matter using Radio Telescopes | We investigate the use of next generation radio telescopes such as the Square
Kilometre Array (SKA) to detect axion two-photon coupling in the astrophysical
environment. The uncertainty surrounding astrophysical magnetic fields presents
new challenges, but with a frequency range corresponding to axions of mass
$1.7-57\mu$eV and a spectral profile with a number of distinguishing features,
SKA-mid offers a tantalising opportunity to constrain axion dark matter
properties. To determine the sensitivity of SKA-mid to an axion signal, we
consider observations of the Galactic centre and interstellar medium, and find
that this new telescope could allow us to probe axion couplings
$\gtrsim10^{-16}$GeV$^{-1}$.
|
Candidate planetary nebulae in the IPHAS photometric catalogue | Context. We have carried out a semi-automated search for planetary nebulae
(PNe) in the INT Photometric H-Alpha Survey (IPHAS) catalogue. We present the
PN search and the list of selected candidates. We cross correlate the selected
candidates with a number of existing infrared galactic surveys in order to gain
further insight into the nature of the candidates. Spectroscopy of a subset of
objects is used to estimate the number of PNe present in the entire candidate
list.
Aims. The overall aim of the IPHAS PN project is to carry out a deep census
of PNe in the northern Galactic plane, an area where PN detections are clearly
lacking.
Methods. The PN search is carried out on the IPHAS photometric catalogues.
The candidate selection is based on the IPHAS and 2MASS/UKIDSS colours of the
objects and the final candidate selection is made visually.
Results. From the original list of ~600 million IPHAS detections we have
selected a total of 1005 objects. Of these, 224 are known objects, leaving us
with 781 PN candidates. Based on the initial follow-up spectroscopy, we expect
the list to include very young and proto-PNe in addition to genuine, normal PNe
(~16 %) and emission line objects other than PNe. We present additional
criteria to select the most probable PN candidates from our candidate list.
|
Large-scale Map of Millimeter-wavelength Hydrogen Radio Recombination
Lines around a Young Massive Star Cluster | We report the first map of large-scale (10 pc in length) emission of
millimeter-wavelength hydrogen recombination lines (mm-RRLs) toward the giant H
II region around the W43-Main young massive star cluster (YMC). Our mm-RRL data
come from the IRAM 30 m telescope and are analyzed together with radio
continuum and cm-RRL data from the Karl G. Jansky Very Large Array and
HCO$^{+}$ 1-0 line emission data from the IRAM 30 m. The mm-RRLs reveal an
expanding wind-blown ionized gas shell with an electron density ~70-1500
cm$^{-3}$ driven by the WR/OB cluster, which produces a total Ly$\alpha$ photon
flux of 1.5 x 10$^{50}$ s$^{-1}$. This shell is interacting with the dense
neutral molecular gas in the W43-Main dense cloud. Combining the high spectral
and angular resolution mm-RRL and cm-RRL cubes, we derive the two-dimensional
relative distributions of dynamical and pressure broadening of the ionized gas
emission and find that the RRL line shapes are dominated by pressure broadening
(4-55 km s$^{-1}$) near the YMC and by dynamical broadening (8-36 km s$^{-1}$)
near the shell's edge. Ionized gas clumps hosting ultra-compact H II regions
found at the edge of the shell suggest that large-scale ionized gas motion
triggers the formation of new star generation near the periphery of the shell.
|
Calibrating the Star Formation Rate at z=1 from Optical Data | We present a star-formation rate calibration based on optical data that is
consistent with average observed rates in both the red and blue galaxy
populations at z~1. The motivation for this study is to calculate SFRs for
DEEP2 Redshift Survey galaxies in the 0.7<z<1.4 redshift range, but our results
are generally applicable to similar optically-selected galaxy samples without
requiring UV or IR data. Using SFRs fit from UV/optical SEDs in the AEGIS
survey, we explore the behavior of restframe B-band magnitude, observed [OII]
luminosity, and restframe (U-B) color with SED-fit SFR for both red sequence
and blue cloud galaxies. We find that a SFR calibration can be calculated for
all z~1 DEEP2 galaxies using a simultaneous fit in M_B and restframe colors
with residual errors that are within the SFR measurement error. The resulting
SFR calibration produces fit residual errors of 0.3 dex RMS scatter for the
full color-independent sample with minimal correlated residual error in L[OII]
or stellar mass. We then compare the calibrated z~1 SFRs to two diagnostics
that use L[OII] as a tracer in local galaxies and correct for dust extinction
at intermediate redshifts through either galaxy B-band luminosity or stellar
mass. We find that a L[OII] - M_B SFR calibration commonly used in the
literature agrees well with our calculated SFRs after correcting for the
average B-band luminosity evolution in L* galaxies. However, we find better
agreement with a local L[OII]-based SFR calibration that includes stellar mass
to correct for reddening effects, indicating that stellar mass is a better
tracer of dust extinction for all galaxy types and less affected by systematic
evolution than galaxy luminosity from z=1 to the current epoch.
|
uGMRT search for cold gas at z~1-1.4 towards red quasars | We present results from our search for HI 21-cm and OH 18-cm absorption at
z~1-1.4 towards red quasars showing strong MgII absorption using uGMRT. The
quasars J1501+1822 and J1521+5508 show multiple strong associated MgII
absorption at z~1.1 and signature of reddening in their optical spectra. We
report the detection of HI 21-cm absorption towards J1521+5508 at the systemic
redshift of the quasar, with N(HI) =(1.2 +/- 0.2) x 10^{20} cm^{-2} for spin
temperature of 100 K and unit covering factor. The HI 21-cm absorption is
offset from the blueshifted strong MgII absorbers by >~1500 km/s. We do not
detect HI 21-cm absorption at the redshift of the associated MgII absorption
and at the systemic redshift towards J1501+1822. We argue that lack of
one-to-one correspondence between MgII and HI 21-cm absorption could be related
with clumpiness of the neutral gas and the radio and optical sightlines probing
different volume of the gas. We find that the presence of strong associated
MgII absorption and reddening in the optical spectra of the quasars lead to an
increased detection rate of associated HI 21-cm absorption at z>=1. We also
report non-detection of intervening OH absorption ([OH]/[HI] <= (1-4) x
10^{-8}) at z = 1.3 towards two red quasars, J0850+5159 and J0852+3435, which
show strong metal and HI 21-cm absorption and the 2175 A dust extinction bump.
|
Measuring the extragalactic background light from very high energy
gamma-ray observations of blazars | The extragalactic background light (EBL) contains important information about
stellar and galaxy evolution. It leaves imprint on the very high energy
$\gamma$-ray spectra from sources at cosmological distances due to the process
of pair production. In this work we propose to {\em measure} the EBL directly
by extracting the collective attenuation effects in a number of $\gamma$-ray
sources at different redshifts. Using a Markov Chain Monte Carlo fitting
method, the EBL intensities and the intrinsic spectral parameters of
$\gamma$-ray sources are derived simultaneously. No prior shape of EBL is
assumed in the fit. With this method, we can for the first time to derive the
spectral shape of the EBL model-independently. Our result shows the expected
features predicted by the present EBL models and thus support the understanding
of the EBL origin.
|
Foregrounds for observations of the cosmological 21 cm line: I. First
Westerbork measurements of Galactic emission at 150 MHz in a low latitude
field | We present the first results from a series of observations conducted with the
Westerbork telescope in the 140--160 MHz range with a 2 arcmin resolution aimed
at characterizing the properties of the foregrounds for epoch of reionization
experiments. For the first time we have detected fluctuations in the Galactic
diffuse emission on scales greater than 13 arcmin at 150 MHz, in the low
Galactic latitude area known as Fan region. Those fluctuations have an $rms$ of
14 K. The total intensity power spectrum shows a power--law behaviour down to
$\ell \sim 900$ with slope $\beta^I_\ell = -2.2 \pm 0.3$. The detection of
diffuse emission at smaller angular scales is limited by residual point
sources. We measured an $rms$ confusion noise of $\sim$3 mJy beam$^{-1}$.
Diffuse polarized emission was also detected for the first time at this
frequency. The polarized signal shows complex structure both spatially and
along the line of sight. The polarization power spectrum shows a power--law
behaviour down to $\ell \sim 2700$ with slope $\beta^P_\ell = -1.65 \pm 0.15$.
The $rms$ of polarization fluctuations is 7.2 K on 4 arcmin scales. By
extrapolating the measured spectrum of total intensity emission, we find a
contamination on the cosmological signal of $\delta T= \sqrt{\ell (\ell+1)
C^I_\ell / 2\pi} \sim 5.7$ K on 5 arcmin scales and a corresponding $rms$ value
of $\sim$18.3 K at the same angular scale. The level of the polarization power
spectrum is $\delta T \sim 3.3$ K on 5 arcmin scales. Given its exceptionally
bright polarized signal, the Fan region is likely to represent an upper limit
on the sky brightness at moderate and high Galactic latitude.
|
The mass budget for intermediate-mass black holes in dense star clusters | Intermediate-mass black holes (IMBHs) could form via runaway merging of
massive stars in a young massive star cluster (YMC). We combine a suite of
numerical simulations of YMC formation with a semi-analytic model for dynamical
friction and merging of massive stars and evolution of a central quasi-star, to
predict how final quasi-star and relic IMBH masses scale with cluster
properties (and compare with observations). The simulations argue that inner
YMC density profiles at formation are steep (approaching isothermal), producing
some efficient merging even in clusters with relatively low effective
densities, unlike models which assume flat central profiles resembling those of
globular clusters (GCs) {\em after} central relaxation. Our results can be
approximated by simple analytic scalings, with $M_{\rm IMBH} \propto v_{\rm
cl}^{3/2}$ where $v_{\rm cl}^{2} = G\,M_{\rm cl}/r_{\rm h}$ is the circular
velocity in terms of initial cluster mass $M_{\rm cl}$ and half-mass radius
$r_{\rm h}$. While this suggests IMBH formation is {\em possible} even in
typical clusters, we show that predicted IMBH masses for these systems are
small, $\sim 100-1000\,M_{\odot}$ or $\sim 0.0003\,M_{\rm cl}$, below even the
most conservative observational upper limits in all known cases. The IMBH mass
could reach $\gtrsim 10^{4}\,M_{\odot}$ in the centers nuclear star clusters,
ultra-compact dwarfs, or compact ellipticals, but in all these cases the
prediction remains far below the present observed supermassive BH masses in
these systems.
|
Lithium Diffusion in the Post-Recombination Universe and Spatial
Variation of [Li/H] | The observed amount of lithium for low metallicity population II stars (known
as the Spite plateau) is a factor of $\sim 3-5$ lower than the predictions of
the standard cosmology. Since the observations are limited to the local
Universe (halo stars, globular clusters and satellites of the Milky Way) it is
possible that certain physical processes may have led to the spatial separation
of lithium and local reduction of [Li/H]. We study the question of lithium
diffusion after the cosmological recombination in sub-Jeans dark matter haloes,
taking into account that more than 95% of lithium remains in the singly-ionized
state at all times. Large scattering cross sections on the rest of the ionized
gas leads to strong coupling of lithium to protons and its initial direction of
diffusion coincides with that of H$^+$. In the rest frame of the neutral gas
this leads to the diffusion of H$^+$ and Li$^+$ out of overdensities with the
trend of reducing [Li/H] in the minima of gravitational wells relative to the
primordial value. We quantify this process and argue that, with certain
qualifications, it may have played a significant role in creating local lithium
deficiency within the primordial dark matter haloes, comparable to those
observed along the Spite plateau.
|
On $S$-Matrix Exclusion of de Sitter and Naturalness | The cosmological constant puzzle, traditionally viewed as a naturalness
problem, is evidently nullified by the $S$-matrix formulation of quantum
gravity/string theory. We point out an implication of this fact for another
naturalness puzzle, the Hierarchy Problem between the weak and Planck scales.
By eliminating the landscape of de Sitter vacua and eternal inflation, the
$S$-matrix formulation exhibits an obvious tension with the explanations based
on anthropic selection or cosmological relaxation of the Higgs mass. This
sharpens the Hierarchy Problem in a profound way. On one hand, it strengthens
the case for explanations based on new physics not far from the weak scale. At
the same time, it opens up a question, whether instead the hierarchy is imposed
by the $S$-matrix consistency between the Standard Model and gravity.
|
On the cosmological backreaction for large distance modifications of
gravity | Every theory that modifies gravity at cosmological distances and that is not
already ruled out by the Solar system observations must exhibit some nonlinear
mechanism that turns off the modification close to a compact matter source.
Given this nonlinearity, one might expect such a theory to show a large
gravitational backreaction, i.e. an order one influence of the small scale
inhomogeneities on the large scale evolution of the Universe. We argue that
this is not necessarily the case. If the dominant nonlinear terms in the
equations obey a shift symmetry, the averaged effect of the nonlinearities can
be small, although the effect on small scales is large. This happens for DGP
(-like) modifications and so called f(G) (or Gauss-Bonnet) models. For both
type of models the shift symmetry is part of the larger "Galilean" symmetry.
|
A sample of galaxy pairs identified from the LAMOST spectral survey and
the Sloan Digital Sky Survey | A small fraction($<10\%$) of SDSS main sample galaxies(MGs) have not been
targeted with spectroscopy due to the the fiber collision effect. These
galaxies have been compiled into the input catalog of the LAMOST extra-galactic
survey and named as the complementary galaxy sample. In this paper, we
introduce the project and the status of the spectroscopies of the complementary
galaxies in the first two years of the LAMOST spectral survey(till Sep. of
2014). Moreover, we present a sample of 1,102 galaxy pairs identified from the
LAMOST complementary galaxies and SDSS MGs, which are defined as that the two
members have a projected distance smaller than 100 kpc and the recessional
velocity difference smaller than 500 $\rm kms^{-1}$. Compared with the SDSS
only selected galaxy pairs, the LAMOST-SDSS pairs take the advantages of not
being biased toward large separations and therefor play as a useful supplement
to the statistical studies of galaxy interaction and galaxy merging.
|
Angular Correlation Function Estimators Accounting for Contamination
from Probabilistic Distance Measurements | With the advent of surveys containing millions to billions of galaxies, it is
imperative to develop analysis techniques that utilize the available
statistical power. In galaxy clustering, even small sample contamination
arising from distance uncertainties can lead to large artifacts, which the
standard estimator does not account for. We first introduce a formalism, termed
decontamination, that corrects for sample contamination by utilizing the
observed cross-correlations in the contaminated samples; this corrects any
correlation function estimator for contamination. Using this formalism, we
present a new estimator that uses the standard estimator to measure correlation
functions in the contaminated samples but then corrects for contamination. We
also introduce a weighted estimator that assigns each galaxy a weight in each
redshift bin based on its probability of being in that bin. We demonstrate that
these estimators effectively recover the true correlation functions and their
covariance matrices. Our estimators can correct for sample contamination caused
by misclassification between object types as well as photometric redshifts;
they should be particularly helpful for studies of galaxy evolution and
baryonic acoustic oscillations, where forward-modeling the clustering signal
using the contaminated redshift distribution is undesirable.
|
Investigating the sources of low-energy events in a SuperCDMS-HVeV
detector | Recent experiments searching for sub-GeV/$c^2$ dark matter have observed
event excesses close to their respective energy thresholds. Although specific
to the individual technologies, the measured excess event rates have been
consistently reported at or below event energies of a few-hundred eV, or with
charges of a few electron-hole pairs. In the present work, we operated a 1-gram
silicon SuperCDMS-HVeV detector at three voltages across the crystal (0 V, 60 V
and 100 V). The 0 V data show an excess of events in the tens of eV region.
Despite this event excess, we demonstrate the ability to set a competitive
exclusion limit on the spin-independent dark matter--nucleon elastic scattering
cross section for dark matter masses of $\mathcal{O}(100)$ MeV/$c^2$, enabled
by operation of the detector at 0 V potential and achievement of a very low
$\mathcal{O}(10)$ eV threshold for nuclear recoils. Comparing the data acquired
at 0 V, 60 V and 100 V potentials across the crystal, we investigated possible
sources of the unexpected events observed at low energy. The data indicate that
the dominant contribution to the excess is consistent with a hypothesized
luminescence from the printed circuit boards used in the detector holder.
|
Cosmological Particle Production and Pairwise Hotspots on the CMB | Heavy particles with masses much bigger than the inflationary Hubble scale
$H_*$, can get non-adiabatically pair produced during inflation through their
couplings to the inflaton. If such couplings give rise to time-dependent masses
for the heavy particles, then following their production, the heavy particles
modify the curvature perturbation around their locations in a time-dependent
and scale non-invariant manner. This results into a non-trivial spatial profile
of the curvature perturbation that is preserved on superhorizon scales and
eventually generates localized hot or cold spots on the CMB. We explore this
phenomenon by studying the inflationary production of heavy scalars and derive
the final temperature profile of the spots on the CMB by taking into account
the subhorizon evolution, focusing in particular on the parameter space where
pairwise hot spots (PHS) arise. When the heavy scalar has an $\mathcal{O}(1)$
coupling to the inflaton, we show that for an idealized situation where the
dominant background to the PHS signal comes from the standard CMB fluctuations
themselves, a simple position space search based on applying a temperature cut,
can be sensitive to heavy particle masses $M_0/H_*\sim\mathcal{O}(100)$. The
corresponding PHS signal also modifies the CMB power spectra and bispectra,
although the corrections are below (outside) the sensitivity of current
measurements (searches).
|
The Carina Project. VI. The helium burning variable stars | We present new optical (BVI) time-series data for the evolved variable stars
in the Carina dwarf spheroidal galaxy. The quality of the data and the
observing strategy allowed us to identify 14 new variable stars. Eight out of
the 14 are RR Lyrae (RRL) stars, four are Anomalous Cepheids (ACs) and two are
geometrical variables. Comparison of the period distribution for the entire
sample of RRLs with similar distributions in nearby dSphs and in the Large
Magellanic Cloud indicates that the old stellar populations in these systems
share similar properties. This finding is also supported by the RRL
distribution in the Bailey diagram. On the other hand, the period distribution
and the Bailey diagram of ACs display significant differences among the above
stellar systems. This evidence suggests that the properties of intermediate-age
stellar populations might be affected both by environmental effects and
structural parameters. We use the BV Period--Wesenheit (PW) relation of RRLs
together with evolutionary prescriptions and find a true distance modulus of
20.09+/-0.07(intrinsic)+/-0.1(statistical) mag that agrees quite well with
similar estimates available in the literature. We identified four peculiar
variables. Taking into account their position in the Bailey diagram and in the
BV PW relation, two of them (V14, V149) appear to be candidate ACs, while two
(V158, V182) might be peculiar RRLs. In particular, the variable V158 has a
period and a V-band amplitude very similar to the low-mass RRL
---RRLR-02792---recently identified by Pietrzynski at al. (2012) in the
Galactic bulge.
|
SDSS DR7 superclusters. Principal component analysis | We apply the principal component analysis and Spearman's correlation test to
study the properties of superclusters drawn from the SDSS DR7. We analyse
possible selection effects in the supercluster catalogue, study the physical
and morphological properties of superclusters, find their possible subsets, and
determine scaling relations for superclusters. We show that the parameters of
superclusters do not correlate with their distance. The correlations between
the physical and morphological properties of superclusters are strong.
Superclusters can be divided into two populations according to their total
luminosity. High-luminosity superclusters form two sets, more elongated systems
with the shape parameter K_1/K_2 < 0.5 and less elongated ones with K_1/K_2 >
0.5. The first two principal components account for more than 90% of the
variance in the supercluster parameters and define the fundamental plane, which
characterises the physical and morphological properties of superclusters. We
use principal component analysis to derive scaling relations for superclusters,
in which we combine the physical and morphological parameters of superclusters.
Structure formation simulations for different cosmologies, and more data about
the local and high redshift superclusters are needed to understand better the
evolution and the properties of superclusters.
|
An nl-model with radiative transfer for hydrogen recombination line
masers | Atomic hydrogen masers occur in recombination plasmas in sufficiently dense
HII regions. These hydrogen recombination line (HRL) masers have been observed
in a handful of objects to date and the analysis of the atomic physics involved
has been rudimentary. In this work a new model of HRL masers is presented which
uses an nl-model to describe the atomic populations interacting with free-free
radiation from the plasma, and an escape probability framework to deal with
radiative transfer effects. The importance of including the collisions between
angular momentum quantum states and the free-free emission in models of HRL
masers is demonstrated. The model is used to describe the general behaviour of
radiative transfer of HRLs and to investigate the conditions under which HRL
masers form. The model results show good agreement with observations collected
over a broad range of frequencies. Theoretical predictions are made regarding
the ratio of recombination lines from the same upper quantum level for these
objects.
|
The scaling relations of early-type dwarf galaxies across a range of
environments | We present the results of a Keck-ESI study of dwarf galaxies across a range
of environment: the Perseus Cluster, the Virgo Cluster, the NGC 1407 group, and
the NGC 1023 group. Eighteen dEs are targeted for spectroscopy, three for the
first time. We confirm cluster membership for one Virgo dE, and group
membership for one dE in the NGC 1023 group, and one dE in the NGC 1407 group
for the first time. Regardless of environment, the dEs follow the same
size-magnitude and $\sigma$-luminosity relation. Two of the Virgo dwarfs, VCC
1199 and VCC 1627, have among the highest central velocity dispersions
($\sigma_{0}$ = 58.4 km s$^{-1}$ and 49.2 km s$^{-1}$) measured for dwarfs of
their luminosity ($M_{R}\approx -17$). Given their small sizes ($R_{e} < 300$
pc) and large central velocity dispersions, we classify these two dwarfs as
compact ellipticals rather than dEs. Group dEs typically have higher mean
dynamical-to-stellar mass ratios than the cluster dEs, with $M_{dyn}/M_{\star}
= 5.1\pm0.6$ for the group dwarfs, vs. $M_{dyn}/M_{\star} = 2.2\pm0.5$ for the
cluster sample, which includes two cEs. We also search for trends in
$M_{dyn}/M_{\star}$ vs. distance from M87 for the Virgo Cluster population, and
find no preference for dwarfs with high values of $M_{dyn}/M_{\star}$ to reside
in the cluster outskirts vs. centre.
|
Joint Bayesian estimation of tensor and lensing B-modes in the power
spectrum of CMB polarization data | We investigate the performance of a simple Bayesian fitting approach to
correct the cosmic microwave background (CMB) B-mode polarization for
gravitational lensing effects in the recovered probability distribution of the
tensor-to-scalar ratio. We perform a two-dimensional power spectrum fit of the
amplitude of the primordial B-modes (tensor-to-scalar ratio, $r$) and the
amplitude of the lensing B-modes (parameter $A_{lens}$), jointly with the
estimation of the astrophysical foregrounds including both synchrotron and
thermal dust emissions. Using this Bayesian framework, we forecast the ability
of the proposed CMB space mission LiteBIRD to constrain $r$ in the presence of
realistic lensing and foreground contributions. We compute the joint posterior
distribution of $r$ and $A_{lens}$, which we improve by adopting a prior on
$A_{lens}$ taken from the South Pole Telescope (SPT) measurement. As it applies
to the power spectrum, this approach cannot mitigate the uncertainty on $r$
that is due to E-mode cosmic variance transferred to B-modes by lensing, unlike
standard delensing techniques that are performed on maps. However, the method
allows to correct for the bias on $r$ induced by lensing, at the expense of a
larger uncertainty due to the increased volume of the parameter space. We
quantify, for different values of the tensor-to-scalar ratio, the trade-off
between bias correction and increase of uncertainty on $r$. For LiteBIRD
simulations, which include foregrounds and lensing contamination, we find that
correcting the foreground-cleaned CMB B-mode power spectrum for the lensing
bias, not the lensing cosmic variance, still guarantees a $3\sigma$ detection
of $r=5\times 10^{-3}$. The significance of the detection is increased to
$6\sigma$ when the current SPT prior on $A_{lens}$ is adopted.
|
Return to [Log-]Normalcy: Rethinking Quenching, The Star Formation Main
Sequence, and Perhaps Much More | Knowledge of galaxy evolution rests on cross-sectional observations of
different objects at different times. Understanding of galaxy evolution rests
on longitudinal interpretations of how these data relate to individual objects
moving through time. The connection between the two is often assumed to be
clear, but we use a simple "physics-free" model to show that it is not, and
that exploring its nuances can yield new insights. Comprising nothing more than
$2094$ loosely constrained lognormal star formation histories (SFHs), the model
faithfully reproduces the following data it was not designed to match: stellar
mass functions at $z\leq8$; the slope of the star formation rate/stellar mass
relation (the SF "Main Sequence") at $z\leq6$; the mean ${\rm sSFR}(\equiv{\rm
SFR}/M_*)$ of low-mass galaxies at $z\leq7$; "fast-" and "slow-track"
quenching; downsizing; and a correlation between formation timescale and ${\rm
sSFR}(M_*; t)$ similar to results from simulations that provides a natural
connection to bulge growth. We take these findings---which suggest that
quenching is the natural downturn of all SFHs affecting galaxies at rates/times
correlated with their densities---to mean that: (1) models in which galaxies
are diversified on Hubble timescales by something like initial conditions rival
the dominant grow-and-quench framework as good descriptions of the data; or (2)
absent spatial information, many metrics of galaxy evolution are too
undiscriminating---if not inherently misleading---to confirm a unique
explanation. We outline future tests of our model but stress that, even if
ultimately incorrect, it illustrates how exploring different paradigms can aid
learning and, we hope, more detailed modeling efforts.
|
The evolution of the barred galaxy population in the TNG50 simulation | We use the magnetic-hydrodynamical simulation TNG50 to study the evolution of
barred massive disc galaxies. Massive spiral galaxies are already present as
early as $z=4$, and bar formation takes place already at those early times. The
bars grow longer and stronger as the host galaxies evolve, with the bar sizes
increasing at a pace similar to that of the disc scale lengths. The bar
fraction mildly evolves with redshift for galaxies with
$M_{*}\geq10^{10}M\odot$, being greater than $\sim40\%$ at $0.5<z<3$ and
$\sim30\%$ at $z=0$. When bars larger than a given physical size ($\geq 2\,\rm
kpc$) or the angular resolution limit of twice the I-band angular PSF FWHM of
the HST are considered, the bar fraction dramatically decreases with increasing
redshift, reconciling the theoretical predictions with observational data. We
find that barred galaxies have an older stellar population, lower gas fractions
and star formation rates than unbarred galaxies. In most cases, the discs of
barred galaxies assembled earlier and faster than the discs of unbarred
galaxies. We also find that barred galaxies are typical in haloes with larger
concentrations and smaller spin parameters than unbarred galaxies. Furthermore,
the inner regions of barred galaxies are more baryon-dominated than those of
unbarred galaxies but have comparable global stellar mass fractions. Our
findings suggest that the bar population could be used as a potential tracer of
the buildup of disc galaxies and their host haloes. With this paper, we release
a catalogue of barred galaxies in TNG50 at $6$ redshifts between $z=4$ and
$z=0$.
|
Dark matter haloes determine the masses of supermassive black holes | The energy and momentum deposited by the radiation from accretion onto the
supermassive black holes (BHs) that reside at the centres of virtually all
galaxies can halt or even reverse gas inflow, providing a natural mechanism for
supermassive BHs to regulate their growth and to couple their properties to
those of their host galaxies. However, it remains unclear whether this
self-regulation occurs on the scale at which the BH is gravitationally
dominant, on that of the stellar bulge, the galaxy, or that of the entire dark
matter halo. To answer this question, we use self-consistent simulations of the
co-evolution of the BH and galaxy populations that reproduce the observed
correlations between the masses of the BHs and the properties of their host
galaxies. We first confirm unambiguously that the BHs regulate their growth:
the amount of energy that the BHs inject into their surroundings remains
unchanged when the fraction of the accreted rest mass energy that is injected,
is varied by four orders of magnitude. The BHs simply adjust their masses so as
to inject the same amount of energy. We then use simulations with artificially
reduced star formation rates to demonstrate explicitly that BH mass is not set
by the stellar mass. Instead, we find that it is determined by the mass of the
dark matter halo with a secondary dependence on the halo concentration, of the
form that would be expected if the halo binding energy were the fundamental
property that controls the mass of the BH. We predict that the logarithmic
slope of the relation between dark matter halo mass and black hole mass is
1.55+/-0.05 and that the scatter around the mean relation in part reflects the
scatter in the halo concentration-mass relation.
|
Low-redshift 21cm Cosmology in Canada | Line-intensity mapping of the 21cm line is a powerful probe of large scale
structure at z<6, tracing large-scale structure via neutral hydrogen content
that is found within galaxies. In principle, it enables cost-efficient surveys
of the matter distribution up to z~6, unlocking orders of magnitude more modes
for observational cosmology. Canada has been a traditional leader in this
field, having led the first detections of the cosmological 21cm signal via
cross-correlations with optical galaxy surveys and having constructed the
Canadian Hydrogen Intensity Mapping Experiment (CHIME). The field is now
entering a new era where data is abundant, allowing studies in how to overcome
systematics to be tackled in an empirical, head-on fashion. In the next few
years, this will produce the first detection of the 21cm auto power spectrum,
which will pave the way towards a large suite of scientific possibilities.
These potentially include precision measurements on the dark energy equation of
state and other LCDM parameters, constraints on how HI mass traces dark matter,
a detection of neutrino effects on large-scale structure, and the use of 21cm
lensing to further constrain cosmology. To turn these promising directions into
reality, we recommend a sustained program of investment in 21cm cosmology,
starting with funding for the Canadian Hydrogen Observatory and Radio transient
Detector (CHORD), followed by small-scale development efforts targeting
next-generation hardware and sustained support for theory and technical staff
support. Additionally, Canada should invest in complementary line-intensity
mapping efforts (such as with CO or [CII] lines) and maintain participation in
next-generation international efforts such as the Packed Ultra-wideband Mapping
Array (PUMA) and the Square Kilometre Array (SKA).
|
New Bounds for Axions and Axion-Like Particles with keV-GeV Masses | We give updated constraints on hypothetical light bosons with a two-photon
coupling such as axions or axion-like particles (ALPs). We focus on masses and
lifetimes where decays happen near big bang nucleosynthesis (BBN), thus
altering the baryon-to-photon ratio and number of relativistic degrees of
freedom between the BBN epoch and the cosmic microwave background (CMB) last
scattering epoch, in particular such that $N_{\rm eff}^{\rm CMB} < N_{\rm
eff}^{\rm BBN}$ and $\eta^{\rm CMB} < \eta^{\rm BBN}$. New constraints
presented here come from Planck measurements of the CMB power spectrum combined
with the latest inferences of primordial $^4$He and D/H abundances. We find
that a previously allowed region in parameter space near $m=1\,\rm MeV$ and
$\tau=100\,\rm ms$, consistent with a QCD axion arising from a symmetry
breaking near the electroweak scale, is now ruled out at $>3\sigma$ by the
combination of CMB+D/H measurements if only ALPs and three thermalized neutrino
species contribute to $N_{\rm eff}$. The bound relaxes if there are additional
light degrees of freedom present which, in this scenario, have their
contribution limited to $\Delta N_{\rm eff}=1.1\pm0.3$. We give forecasts
showing that a number of experiments are expected to reach the sensitivity
needed to further test this region, such as Stage-IV CMB and SUPER-KEKB, the
latter a direct test insensitive to any extra degrees of freedom.
|
The Evolution of the Intracluster Medium Metallicity in
Sunyaev-Zel'dovich-Selected Galaxy Clusters at 0 < z < 1.5 | We present the results of an X-ray spectral analysis of 153 galaxy clusters
observed with the Chandra, XMM-Newton, and Suzaku space telescopes. These
clusters, which span 0 < z < 1.5, were drawn from a larger, mass-selected
sample of galaxy clusters discovered in the 2500 square degree South Pole
Telescope Sunyaev Zel'dovich (SPT-SZ) survey. With a total combined exposure
time of 9.1 Ms, these data yield the strongest constraints to date on the
evolution of the metal content of the intracluster medium (ICM). We find no
evidence for strong evolution in the global (r<R500) ICM metallicity (dZ/dz =
-0.06 +/- 0.04 Zsun), with a mean value at z=0.6 of <Z> = 0.23 +/- 0.01 Zsun
and a scatter of 0.08 +/- 0.01 Zsun. These results imply that >60% of the
metals in the ICM were already in place at z=1 (at 95% confidence), consistent
with the picture of an early (z>1) enrichment. We find, in agreement with
previous works, a significantly higher mean value for the metallicity in the
centers of cool core clusters versus non-cool core clusters. We find weak
evidence for evolution in the central metallicity of cool core clusters (dZ/dz
= -0.21 +/- 0.11 Zsun), which is sufficient to account for this enhanced
central metallicity over the past ~10 Gyr. We find no evidence for metallicity
evolution outside of the core (dZ/dz = -0.03 +/- 0.06 Zsun), and no significant
difference in the core-excised metallicity between cool core and non-cool core
clusters. This suggests that strong radio-mode AGN feedback does not
significantly alter the distribution of metals at r>0.15R500. Given the
limitations of current-generation X-ray telescopes in constraining the ICM
metallicity at z>1, significant improvements on this work will likely require
next-generation X-ray missions.
|
Validating estimates of the growth rate of structure with modified
gravity simulations | We perform a validation of estimates of the growth rate of structure,
described by the parameter combination $f\sigma_8$, in modified gravity
cosmologies. We consider an analysis pipeline based on the redshift-space
distortion modelling of the clustering wedges statistic of the galaxy
correlation function and apply it to mock catalogues of $\Lambda{\rm CDM}$ and
the normal branch of DGP cosmologies. We employ a halo occupation distribution
approach to construct our mocks, which we ensure resemble the CMASS sample from
BOSS in terms of the total galaxy number density and large scale amplitude of
the power spectrum monopole. We show that the clustering wedges model
successfully recovers the true growth rate difference between DGP and
$\Lambda{\rm CDM}$, even for cases with over 40\% enhancement in $f\sigma_8$
compared to $\Lambda{\rm CDM}$. The unbiased performance of the clustering
wedges model allows us to use the growth rate values estimated from the BOSS
DR12 data to constrain the cross-over scale $r_c$ of DGP gravity to
$\left[r_cH_0\right]^{-1} < 0.97$ ($2\sigma$) or $r_c > 3090\ {\rm Mpc}/h$,
cutting into the interesting region of parameter space with $r_c \sim H_0^{-1}$
using constraints from the growth of structure alone.
|
Sensitivity analysis of a galaxy formation model | We present the first application of a variance-based sensitivity analysis
(SA) to a model that aims to predict the evolution and properties of the whole
galaxy population. SA is a well-established technique in other quantitative
sciences, but is a relatively novel tool for the evaluation of astrophysical
models. We perform a multi-parameter exploration of the GALFORM semi-analytic
galaxy formation model, to compute how sensitive the present-day K-band
luminosity function is to varying different model parameters. The parameter
space is scanned using a low-discrepancy sampling technique proposed by
Saltelli. We first demonstrate the usefulness of the SA approach by varying
just two model parameters, one which controls supernova feedback and the other
the heating of gas by AGN. The SA analysis matches our physical intuition
regarding how these parameters affect the predictions for different parts of
the galaxy luminosity function. We then use SA to compute Sobol' sensitivity
indices varying seven model parameters, connecting the variance in the model
output to the variance in the input parameters. The sensitivity is computed in
luminosity bins, allowing us to probe the origin of the model predictions in
detail. We discover that the SA correctly identifies the least- and most
important parameters. Moreover, the SA also captures the combined responses of
varying multiple parameters at the same time. Our study marks a much needed
step away from the traditional "one-at-a-time" parameter variation often used
in this area and improves the transparency of multi-parameter models of galaxy
formation.
|
Neutrino Masses, Dark Energy and the Gravitational Lensing of
Pregalactic HI | We study the constraints which the next generation of radio telescopes could
place on the mass and number of neutrino species by studying the gravitational
lensing of high redshift 21 cm emission in combination with wide-angle surveys
of galaxy lensing. We use simple characterizations of reionization history and
of proposed telescope designs to forecast the constraints and detectability
threshold for neutrinos. It is found that the degeneracy between neutrino
parameters and dark energy parameters is significantly reduced by incorporating
21 cm lensing. The combination of galaxy and 21 cm lensing could constrain the
sum of the neutrino masses to within ~ 0.04 eV and the number of species to
within ~ 0.1. This is an improvement of a factor of 2.6 in mass and 1.3 in
number over a galaxy lensing survey alone. This includes marginalizing over an
11 parameter cosmological model with a two parameter model for the dark energy
equation of state. If the dark energy equation of state is held fixed at w =
p/\rho=-1 the constraints improve to ~0.03 eV and 0.04. These forecasted errors
depend critically on the fraction of sky that can be surveyed in redshifted 21
cm emission (25% is assumed here) and the redshift of reionization ($z=7$ is
assumed here). It is also found that neutrinos with masses too small to be
detected in the data could none the less cause a significant bias in the
measured dark energy equation of state.
|
Green's function method for handling radiative effects on false vacuum
decay | We introduce a Green's function method for handling radiative effects on
false vacuum decay. In addition to the usual thin-wall approximation, we
achieve further simplification by treating the bubble wall in the planar limit.
As an application, we take the $\lambda\phi^4$ theory, extended with $N$
additional heavier scalars, wherein we calculate analytically both the
functional determinant of the quadratic fluctuations about the classical
soliton configuration and the first correction to the soliton configuration
itself.
|
Numerical Investigation of Dynamical and Morphological Trends in
Relativistic Jets | Active galactic nuclei (AGN) show a range of morphologies and dynamical
properties, which are determined not only by parameters intrinsic to the
central engine but also their interaction with the surrounding environment. We
investigate the connection of kiloparsec scale AGN jet properties to their
intrinsic parameters and surroundings. This is done using a suite of 40
relativistic hydrodynamic simulations spanning a wide range of engine
luminosities and opening angles. We explore AGN jet propagation with different
ambient density profiles, including $r^{-2}$ (self-similar solution) and
$r^{-1}$, which is more relevant for AGN host environments. The Fanaroff-Riley
(FR) morphological dichotomy arises naturally in our models. Jets with low
energy density compared to the ambient medium produce a center-brightened
emissivity distribution, while emissivity from relatively higher energy density
jets is dominated by a terminal bright spot. We observe recollimation shocks in
our simulations that can generate bright spots along the spine of the jet,
providing a possible explanation for "knots" observed in AGN jets. We
additionally find a scaling relation between the number of knots and the
jet-head-to-surroundings energy density ratio. This scaling relation is
generally consistent with the observations of the jets in M87 and Cygnus A. Our
model also correctly predicts M87 as FR I and Cygnus A as FR II. Our model can
be used to relate jet dynamical parameters such as jet head velocity, jet
opening angle, and external pressure to jet power and ambient density
estimates.
|
Less is more: How cosmic voids can shed light on dark energy | We showed how the shape of cosmic voids can be used to distinguish between
different models of dark energy using galaxy positions.
|
A new methodology to test galaxy formation models using the dependence
of clustering on stellar mass | We present predictions for the two-point correlation function of galaxy
clustering as a function of stellar mass, computed using two new versions of
the GALFORM semi-analytic galaxy formation model. These models make use of a
high resolution, large volume N-body simulation, set in the WMAP7 cosmology.
One model uses a universal stellar initial mass function (IMF), while the other
assumes different IMFs for quiescent star formation and bursts. Particular
consideration is given to how the assumptions required to estimate the stellar
masses of observed galaxies (such as the choice of IMF, stellar population
synthesis model and dust extinction) influence the perceived dependence of
galaxy clustering on stellar mass. Broad-band spectral energy distribution
fitting is carried out to estimate stellar masses for the model galaxies in the
same manner as in observational studies. We show clear differences between the
clustering signals computed using the true and estimated model stellar masses.
As such, we highlight the importance of applying our methodology to compare
theoretical models to observations. We introduce an alternative scheme for the
calculation of the merger timescales for satellite galaxies in GALFORM, which
takes into account the dark matter subhalo information from the simulation.
This reduces the amplitude of small-scale clustering. The new merger scheme
offers improved or similar agreement with observational clustering
measurements, over the redshift range 0 < z < 0.7. We find reasonable agreement
with clustering measurements from GAMA, but find larger discrepancies for some
stellar mass ranges and separation scales with respect to measurements from
SDSS and VIPERS, depending on the GALFORM model used.
|
VLTI/AMBER observations of the Seyfert nucleus of NGC 3783 | Context. The putative tori surrounding the accretion disks of active galactic
nuclei (AGNs) play a fundamental role in the unification scheme of AGNs.
Infrared long-baseline interferometry allows us to study the inner dust
distribution in AGNs with unprecedented spatial resolution over a wide infrared
wavelength range.
Aims. Near- and mid-infrared interferometry is used to investigate the
milli-arcsecond-scale dust distribution in the type 1.5 Seyfert nucleus of NGC
3783.
Methods. We observed NGC 3783 with the VLTI/AMBER instrument in the K-band
and compared our observations with models.
Results. From the K-band observations, we derive a ring-fit torus radius of
0.74 +/- 0.23 mas or 0.16 +/- 0.05 pc. We compare this size with infrared
interferometric observations of other AGNs and UV/optical-infrared
reverberation measurements. For the interpretation of our observations, we
simultaneously model our near- and mid-infrared visibilities and the SED with a
temperature/density-gradient model including an additional inner hot 1400 K
ring component.
|
The evolution of a supermassive retrograde binary embedded in an
accretion disk | In this note we discuss the main results of a study of a massive binary with
unequal mass ratio, q, embedded in an accretion disk, with its orbital rotation
being opposed to that of the disk. When the mass ratio is sufficiently large, a
gap opens in the disk, but the mechanism of gap formation is very different
from the prograde case. Inward migration occurs on a timescale of t_ev ~
M_p/(dot M), where M_p is the mass of the less massive component (the
perturber), and dot M is the accretion rate. When q<< 1, the accretion takes
place mostly onto the more massive component, with the accretion rate onto the
perturber being smaller than, or of order of, q^(1/3)M. However, this rate
increases when supermassive binary black holes are considered and gravitational
wave emission is important. We estimate a typical duration of time for which
the accretion onto the perturber and gravitational waves could be detected.
|
Cas A and the Crab Were Not Stellar Binaries At Death | The majority of massive stars are in binaries, which implies that many core
collapse supernovae (ccSNe) should be binaries at the time of the explosion.
Here we show that the three most recent, local (visual) SNe (the Crab, CasA and
SN1987A) were not binaries, with limits on the initial mass ratios of
q=M2/M1<0.1. No quantitative limits have previously been set for CasA and the
Crab, while for SN1987A we merely updated existing limits in view of new
estimates of the dust content. The lack of stellar companions to these three
ccSNe implies a 90% confidence upper limit on the q>0.1 binary fraction at
death of fb<44%. In a passively evolving binary model (meaning no binary
interactions), with a flat mass ratio distribution and a Salpeter IMF, the
resulting 90% confidence upper limit on the initial binary fraction of F<63% is
in considerable tension with observed massive binary statistics. Allowing a
significant fraction fM~25% of stellar binaries to merge reduces the tension,
with F<63/(1-fM)~81%, but allowing for the significant fraction in higher order
systems (triples, etc.) reintroduces the tension. That CasA was not a stellar
binary at death also shows that a massive binary companion is not necessary for
producing a Type IIb SNe. Much larger surveys for binary companions to Galactic
SNe will become feasible with the release of the full Gaia proper motion and
parallax catalogs, providing a powerful probe of the statistics of such
binaries and their role in massive star evolution, neutron star velocity
distributions and runaway stars.
|
Nitrogen isotope fractionation in protoplanetary disks | Aims: The two stable isotopes of nitrogen, 14N and 15N, exhibit a range of
abundance ratios both inside and outside the solar system. The elemental ratio
in the solar neighborhood is 440. Recent ALMA observations showed HCN/HC15N
ratios from 83 to 156 in six T Tauri and Herbig disks and a CN/C15 N ratio of
323 +/- 30 in one T Tauri star. We aim to determine the dominant mechanism
responsible for these enhancements of 15N: low-temperature exchange reactions
or isotope-selective photodissociation of N2.
Methods: Using the thermochemical code DALI, we model the nitrogen isotope
chemistry in circumstellar disks with a 2D axisymmetric geometry. Our chemical
network is the first to include both fractionation mechanisms for nitrogen. The
model produces abundance profiles and isotope ratios for several key N-bearing
species. We study how these isotope ratios depend on various disk parameters.
Results: The formation of CN and HCN is closely coupled to the vibrational
excitation of H2 in the UV-irradiated surface layers of the disk. Isotope
fractionation is completely dominated by isotope-selective photodissociation of
N2. The column density ratio of HCN over HC15N in the disk's inner 100 au does
not depend strongly on the disk mass, the flaring angle or the stellar
spectrum, but it is sensitive to the grain size distribution. For larger
grains, self-shielding of N2 becomes more important relative to dust
extinction, leading to stronger isotope fractionation. Between disk radii of
~50 and 200 au, the models predict HCN/HC15N and CN/C15N abundance ratios
consistent with observations of disks and comets. The HCN/HC15N and CN/C15N
column density ratios in the models are a factor of 2-3 higher than those
inferred from the ALMA observations.
|
Why is the Milky Way X-factor Constant? | The CO-H2 conversion factor (Xco; otherwise known as the X-factor) is
observed to be remarkably constant in the Milky Way and in the Local Group
(aside from the SMC). To date, our understanding of why Xco should be so
constant remains poor. Using a combination of extremely high resolution (~ 1
pc) galaxy evolution simulations and molecular line radiative transfer
calculations, we suggest that Xco displays a narrow range of values in the
Galaxy due to the fact that molecular clouds share very similar physical
properties. In our models, this is itself a consequence of stellar feedback
competing against gravitational collapse. GMCs whose lifetimes are regulated by
radiative feedback show a narrow range of surface densities, temperatures and
velocity dispersions with values comparable to those seen in the Milky Way. As
a result, the X-factors from these clouds show reasonable correspondence with
observed data from the Local Group, and a relatively narrow range. On the other
hand, feedback-free clouds collapse to surface densities that are larger than
those seen in the Galaxy, and hence result in X-factors that are systematically
too large compared to the Milky Way's. We conclude that radiative feedback
within GMCs can generate cloud properties similar to those observed in the
Galaxy, and hence a roughly constant Milky Way X-factor in normal, quiescent
clouds.
|
Review of the theoretical and experimental status of dark matter
identification with cosmic-ray antideuterons | Recent years have seen increased theoretical and experimental effort towards
the first-ever detection of cosmic-ray antideuterons, in particular as an
indirect signature of dark matter annihilation or decay. In contrast to
indirect dark matter searches using positrons, antiprotons, or gamma-rays,
which suffer from relatively high and uncertain astrophysical backgrounds,
searches with antideuterons benefit from very suppressed conventional
backgrounds, offering a potential breakthrough in unexplored phase space for
dark matter. This article is based on the first dedicated cosmic-ray
antideuteron workshop, which was held at UCLA in June 2014. It reviews broad
classes of dark matter candidates that result in detectable cosmic-ray
antideuteron fluxes, as well as the status and prospects of current
experimental searches. The coalescence model of antideuteron production and the
influence of antideuteron measurements at particle colliders are discussed.
This is followed by a review of the modeling of antideuteron propagation
through the magnetic fields, plasma currents, and molecular material of our
Galaxy, the solar system, the Earth's geomagnetic field, and the atmosphere.
Finally, the three ongoing or planned experiments that are sensitive to
cosmic-ray antideuterons, BESS, AMS-02, and GAPS, are detailed. As cosmic-ray
antideuteron detection is a rare event search, multiple experiments with
orthogonal techniques and backgrounds are essential. Many theoretical and
experimental groups have contributed to these studies over the last decade,
this review aims to provide the first coherent discussion of the relevant dark
matter theories that antideuterons probe, the challenges to predictions and
interpretations of antideuteron signals, and the experimental efforts toward
cosmic antideuteron detection.
|
Notes on natural inflation | In the so-called natural inflation, an axion-like inflaton is assumed to have
a cosine-type periodic potential. This is not the case in a very simple model
in which the axion-like inflaton is coupled to an SU(N) (or other) pure
Yang-Mills, at least in the large N limit as pointed out by Witten. It has a
multi-valued potential, which is effectively quadratic, i.e., there is only a
mass term in the large N limit. Thanks to this property, chaotic inflation can
be realized more naturally with the decay constant of the axion-like inflaton
less than the Planck scale. We demonstrate these points explicitly by using
softly broken ${\mathcal N}=1$ Super-Yang-Mills which allows us to treat finite
N. This analysis also suggests that moderately large gauge groups such as
$E_{8}$ are good enough with a Planck scale decay constant.
|
Photon sector analysis of Super and Lorentz symmetry breaking: effective
photon mass, bi-refringence and dissipation | Within the Standard Model Extension (SME), we expand our previous findings on
four classes of violations of Super-Symmetry (SuSy) and Lorentz Symmetry
(LoSy), differing in the handedness of the Charge conjugation-Parity-Time
reversal (CPT) symmetry and in whether considering the impact of photinos on
photon propagation. The violations, occurring at the early universe high
energies, show visible traces at present in the Dispersion Relations (DRs). For
the CPT-odd classes ($V_{\mu}$ breaking vector) associated with the
Carroll-Field-Jackiw (CFJ) model, the DRs and the Lagrangian show for the
photon an effective mass, gauge invariant, proportional to $|{\vec V}|$. The
group velocity exhibits a classic dependency on the inverse of the frequency
squared. For the CPT-even classes ($k_{F}$ breaking tensor), when the photino
is considered, the DRs display also a massive behaviour inversely proportional
to a coefficient in the Lagrangian and to a term linearly dependent on $k_{F}$.
All DRs display an angular dependence and lack LoSy invariance. In describing
our results, we also point out the following properties: i) the appearance of
complex or simply imaginary frequencies and super-luminal speeds and ii) the
emergence of bi-refringence. Finally, we point out the circumstances for which
SuSy and LoSy breakings, possibly in presence of an external field, lead to the
non-conservation of the photon energy-momentum tensor. We do so for both CPT
sectors.
|
SED Analysis of 13 Spectroscopically Confirmed Galaxies at z$\simeq$6 to
Constrain UV-Slope, Model Dust Attenuation and Escape Fractions | The reionization of the hydrogen in the Universe is thought to have completed
by redshift $z\simeq5.5-6$. To probe this era, galaxy observations in the
Subaru Deep Field (SDF) have identified more than 100 galaxies at $z\simeq6$,
many spectroscopically confirmed through follow-up observations. We model the
spectral energy distributions (SEDs) of 13 SDF galaxies with the CIGALE and
Dense Basis codes using available optical/IR data. Modeling deep IR photometry
has the potential to constrain the galaxy's Lyman continuum (LyC) escape
fraction (\fesc). We use the modeled nebular emission lines and find that the
implied escape fractions ranges from \textbf{0 to 0.8 with a median of
$\sim$0.35 for Dense Basis and $\sim$0.55 for CIGALE.} Significant
uncertainties in the data exist, so that fitting results in a large range of
\fesc\ for individual objects. The implied median \fesc-values may be high
enough for galaxies to finish reionization by $z\sim6$. Furthermore, we find no
strong trends between the UV-slope $\beta$ or \EBminV with model \fesc. If
true, the lack of trends suggest that other factors besides nebular emission or
dust extinction could have led to LyC escaping, such as the presence of holes
in the ISM with sufficiently wide opening angles from outflows of supernovae
and/or weak AGN, resulting in a range of implied \fesc-values depending on the
viewing angle of each galaxy. The current \textit{HST, Spitzer} and
ground-based photometric and model errors for the galaxies remain large, so IR
spectroscopic observations with the \textit{James Webb Space Telescope} are
needed to constrain this possibility.
|
Deriving physical parameters of unresolved star clusters. II. The
degeneracies of age, mass, extinction, and metallicity | This paper is the second of a series that investigates the stochasticity and
degeneracy problems that hinder the derivation of the age, mass, extinction,
and metallicity of unresolved star clusters in external galaxies when
broad-band photometry is used. While Paper I concentrated on deriving age,
mass, and extinction of star clusters for one fixed metallicity, we here derive
these parameters in case when metallicity is let free to vary. The results were
obtained using several different filter systems ($UBVRI$, $UBVRIJHK$,
GALEX+$UBVRI$), which allowed to optimally reduce the different degeneracies
between the cluster physical parameters. The age, mass, and extinction of a
sample of artificial star clusters were derived by comparing their broad-band
integrated magnitudes with the magnitudes of a large grid of cluster models
with various metallicities. A large collection of artificial clusters was
studied to model the different degeneracies in the age, mass, extinction, and
metallicity parameter space when stochasticity is taken into account in the
cluster models. We show that, without prior knowledge on the metallicity, the
optical bands ($UBVRI$) fail to allow a correct derivation of the age, mass,
and extinction because of the strong degeneracies between models of different
metallicities. Adding near-infrared information ($UBVRI$+$JHK$) slightly helps
in improving the parameter derivation, except for the metallicity. Adding
ultraviolet data (GALEX+$UBVRI$) helps significantly in deriving these
parameters and allows constraining the metallicity when the photometric errors
have a Gaussian distribution with standard deviations 0.05 mag for $UBVRI$ and
0.15 mag for the GALEX bands.
|
Constraining Velocity-dependent Self-Interacting Dark Matter with the
Milky Way's dwarf spheroidal galaxies | The observed anti-correlation between the central dark matter (DM) densities
of the bright Milky Way (MW) dwarf spheroidal galaxies (dSphs) and their
orbital pericenter distances poses a potential signature of self-interacting
dark matter (SIDM). In this work we investigate this possibility by analysing
the range of SIDM scattering cross section per unit mass, $\sigma/m_{\chi}$,
able to explain such anti-correlation. We simulate the orbital evolution of
dSphs subhaloes around the MW assuming an analytical form for the gravitational
potential, adopting the proper motions from the Gaia mission and including a
consistent characterization of gravitational tidal stripping. The evolution of
the subhaloes density profile is modelled using the gravothermal fluid
formalism, where DM particle collisions induce thermal conduction that depends
on $\sigma/m_{\chi}$. We find that models of dSphs, such as Carina and Fornax,
reproduce the observed central DM densities with fixed $\sigma/m_{\chi}$
ranging between $30$ and $50$ cm$^{2}$g$^{-1}$, whereas other dSphs prefer
larger values ranging between $70$ and $100$ cm$^{2}$g$^{-1}$. These cross
sections correlate with the average collision velocity of DM particles within
each subhalo's core, so that systems modelled with large cross sections have
lower collision velocities. We fit the cross section-velocity correlation with
a SIDM particle model, where a DM particle of mass $m_{\chi}=0.648\pm 0.154$
GeV interacts under the exchange of a light mediator of mass $m_{\phi}=0.636\pm
0.055$ MeV, with the self-interactions being described by a Yukawa potential.
The outcome is a cross section-velocity relation that explains the diverse DM
profiles of MW dSph satellites and is consistent with observational constraints
on larger scales.
|
Gaia, counting down to launch | In this contribution I provide an overview of the the European Space Agency's
Gaia mission just ahead of its launch scheduled for November 2013.
|
The influence of Galactic aberration on precession parameters determined
from VLBI observations | The influence of proper motions of sources due to Galactic aberration on
precession models based on VLBI data is determined. Comparisons of the linear
trends in the coordinates of the celestial pole obtained with and without
taking into account Galactic aberration indicate that this effect can reach 20
$\mu$as per century, which is important for modern precession models. It is
also shown that correcting for Galactic aberration influences the derived
parameters of low-frequency nutation terms. It is therefore necessary to
correct for Galactic aberration in the reduction of modern astrometric
observations.
|
Identifying Multiple Populations in M71 Using CN | We have observed the CN features at $\sim$3800 ~\AA~ and 4120~ \AA~ as well
as the CH band at $\sim$4300~\AA~ for 145 evolved stars in the Galactic
globular cluster M71 using the multi-object spectrograph, Hydra, on the
WIYN-3.5m telescope. We use these measurements to create two $\delta$CN indices
finding that both distributions are best fit by two populations, a CN-enhanced
and CN-normal. We find that 42 $\pm$ 4\% of the RGB stars in our sample are
CN-enhanced. The percentage of CN-enhanced is 40 $\pm$ 13\% for the AGB and 33
$\pm$ 9\% for the HB stars, which suggests there are no missing second
generation stars at these stages of stellar evolution. The two generations also
separate in magnitude and color on the HB, which allows us to find the
difference in He abundance between the two populations by fitting appropriate
ZAHBs. The broad range of distances from the cluster's center covered by our
sample allows us to study the dependence of the ratio of the number of first to
second population stars on the distance from the cluster's center, and we find
that this ratio does not vary radially and that the two populations are
spatially mixed. Finally, we compare our identification of multiple populations
with the classification based on the Na-O anti-correlation and the HST UV
photometry, and we find good agreement with both methods.
|
Probing Cosmic Strings with Satellite CMB measurements | We study the problem of searching for cosmic string signal patterns in the
present high resolution and high sensitivity observations of the Cosmic
Microwave Background (CMB). This article discusses a technique capable of
recognizing Kaiser-Stebbins effect signatures in total intensity anisotropy
maps, and shows that the biggest factor that produces confusion is represented
by the acoustic oscillation features of the scale comparable to the size of
horizon at recombination. Simulations show that the distribution of null
signals for pure Gaussian maps converges to a $\chi^2$ distribution, with
detectability threshold corresponding to a string induced step signal with an
amplitude of about 100 $\muK$ which corresponds to a limit of roughly $G\mu <
1.5\times 10^{-6}$. We study the statistics of spurious detections caused by
extra-Galactic and Galactic foregrounds. For diffuse Galactic foregrounds,
which represents the dominant source of contamination, we derive sky masks
outlining the available region of the sky where the Galactic confusion is
sub-dominant, specializing our analysis to the case represented by the
frequency coverage and nominal sensitivity and resolution of the Planck
experiment.
|
Galaxy Interactions in Compact Groups II: abundance and kinematic
anomalies in HCG 91c | Galaxies in Hickson Compact Group 91 (HCG 91) were observed with the WiFeS
integral field spectrograph as part of our ongoing campaign targeting the
ionized gas physics and kinematics inside star forming members of compact
groups. Here, we report the discovery of HII regions with abundance and
kinematic offsets in the otherwise unremarkable star forming spiral HCG 91c.
The optical emission line analysis of this galaxy reveals that at least three
HII regions harbor an oxygen abundance ~0.15 dex lower than expected from their
immediate surroundings and from the abundance gradient present in the inner
regions of HCG 91c. The same star forming regions are also associated with a
small kinematic offset in the form of a lag of 5-10 km/s with respect to the
local circular rotation of the gas. HI observations of HCG 91 from the Very
Large Array and broadband optical images from Pan-STARRS suggest that HCG 91c
is caught early in its interaction with the other members of HCG 91. We discuss
different scenarios to explain the origin of the peculiar star forming regions
detected with WiFeS, and show that evidence point towards infalling and
collapsing extra-planar gas clouds at the disk-halo interface, possibly as a
consequence of long-range gravitational perturbations of HCG 91c from the other
group members. As such, HCG 91c provides evidence that some of the
perturbations possibly associated with the early phase of galaxy evolution in
compact groups impact the star forming disk locally, and on sub-kpc scales.
|
Low-l CMB Power Loss in String Inflation | The lack of power on large scales (l < 40) might have been observed by the
PLANCK satellite. We argue that this putative feature can be explained by a
phase of fast roll at the onset of inflation. We show that in the context of
single field models what is required is an asymmetric inflection point model of
which fibre inflation is a string motivated example. We study the ability of
fibre inflation to generate a suppression of the CMB 2-point function power at
low l, finding that the potential derived from string loops is not steep enough
for this purpose. We introduce a steeper contribution to the potential, that
dominates away from the inflationary region, and show that if properly tuned it
can indeed lead to a spectrum with lack of power at large scales.
|
Possible Accretion Disk Origin of the Emission Variability of a Blazar
Jet | We analyze X-ray light curves of the blazar Mrk 421 obtained from the Soft
X-ray Imaging Telescope and the Large Area X-Ray Proportional Counter
instrument onboard the Indian space telescope $AstroSat$ and archival
observations from $Swift$. We show that the X-ray power spectral density (PSD)
is a piece-wise power-law with a break, i.e., the index becomes more negative
below a characteristic "break-timescale". Galactic black hole X-ray binaries
and Seyfert galaxies exhibit a similar characteristic timescale in their X-ray
variability that is proportional to their respective black hole mass. X-rays in
these objects are produced in the accretion disk or corona. Hence, such a
timescale is believed to be linked to the properties of the accretion flow. Any
relation observed between events in the accretion disk and those in the jet can
be used to characterize the disk-jet connection. However, evidence of such link
have been scarce and indirect. Mrk 421 is a BL Lac object which has a prominent
jet pointed towards us and a weak disk emission, and it is assumed that most of
its X-rays are generated in the jet. Hence, existence of the break in its X-ray
PSD may indicate that changes in the accretion disk, which may be the source of
the break timescale are translating into the jet, where the X-rays are
produced.
|
Joint anisotropy and source count constraints on the contribution of
blazars to the diffuse gamma-ray background | We place new constraints on the contribution of blazars to the large-scale
isotropic gamma-ray background (IGRB) by jointly analyzing the measured source
count distribution (logN-logS) of blazars and the measured intensity and
anisotropy of the IGRB. We find that these measurements point to a consistent
scenario in which unresolved blazars make less than 20% of the IGRB intensity
at 1-10 GeV while accounting for the majority of the measured anisotropy in
that energy band. These results indicate that the remaining fraction of the
IGRB intensity is made by a component with a low level of intrinsic anisotropy.
We determine upper limits on the anisotropy from non-blazar sources, adopting
the best-fit parameters of the measured source count distribution to calculate
the unresolved blazar anisotropy. In addition, we show that the anisotropy
measurement excludes some recently proposed models of the unresolved blazar
population.
|
A method to determine distances to molecular clouds using near-IR
photometry | Aims: We aim to develop a method to determine distances to molecular clouds
using JHK near-infrared photometry. Methods: The method is based on a technique
that aids spectral classification of stars lying towards the fields containing
the clouds into main sequence and giants. In this technique, the observed (J-H)
and (H-K_s) colours are dereddened simultaneously using trial values of A V and
a normal interstellar extinction law. The best fit of the dereddened colours to
the intrinsic colours giving a minimum value of Chi^2 then yields the
corresponding spectral type and A_V for the star. The main sequence stars, thus
classified, are then utilized in an A V versus distance plot to bracket the
cloud distances. Results: We applied the method to four clouds, L1517,
Chamaeleon I, Lupus 3 and NGC 7023 and estimated their distances as 167+-30,
151+-28, 157+-29 and 408+-76 pc respectively, which are in good agreement with
the previous distance estimations available in the literature
|
Molecules with ALMA at Planet-forming Scales (MAPS) XI: CN and HCN as
Tracers of Photochemistry in Disks | UV photochemistry in the surface layers of protoplanetary disks dramatically
alters their composition relative to previous stages of star formation. The
abundance ratio CN/HCN has long been proposed to trace the UV field in various
astrophysical objects, however to date the relationship between CN, HCN, and
the UV field in disks remains ambiguous. As part of the ALMA Large Program MAPS
(Molecules with ALMA at Planet-forming Scales), we present observations of CN
N=1-0 transitions at 0.3'' resolution towards five disk systems. All disks show
bright CN emission within $\sim$50-150 au, along with a diffuse emission shelf
extending up to 600 au. In all sources we find that the CN/HCN column density
ratio increases with disk radius from about unity to 100, likely tracing
increased UV penetration that enhances selective HCN photodissociation in the
outer disk. Additionally, multiple millimeter dust gaps and rings coincide with
peaks and troughs, respectively, in the CN/HCN ratio, implying that some
millimeter substructures are accompanied by changes to the UV penetration in
more elevated disk layers. That the CN/HCN ratio is generally high (>1) points
to a robust photochemistry shaping disk chemical compositions, and also means
that CN is the dominant carrier of the prebiotically interesting nitrile group
at most disk radii. We also find that the local column densities of CN and HCN
are positively correlated despite emitting from vertically stratified disk
regions, indicating that different disk layers are chemically linked. This
paper is part of the MAPS special issue of the Astrophysical Journal
Supplement.
|
First Direct-Detection Constraints on eV-Scale Hidden-Photon Dark Matter
with DAMIC at SNOLAB | We present direct detection constraints on the absorption of hidden-photon
dark matter with particle masses in the range 1.2-30 eV$c^{-2}$ with the DAMIC
experiment at SNOLAB. Under the assumption that the local dark matter is
entirely constituted of hidden photons, the sensitivity to the kinetic mixing
parameter $\kappa$ is competitive with constraints from solar emission,
reaching a minimum value of 2.2$\times$$10^{-14}$ at 17 eV$c^{-2}$. These
results are the most stringent direct detection constraints on hidden-photon
dark matter in the galactic halo with masses 3-12 eV$c^{-2}$ and the first
demonstration of direct experimental sensitivity to ionization signals $<$12 eV
from dark matter interactions.
|
Star formation within globular clusters:discrete multiple bursts and
top-light mass functions | The observed discrete multiple stellar populations and internal abundance
spreads in r- and s-process elements within globular clusters (GCs) have been
suggested to be explained self-consistently by discrete star formation events
over a longer timescale (10^8 yr). We here investigate whether such star
formation is really possible within GCs using numerical simulations that
include effects of dynamical interaction between individual stars and the
accumulated gas ("star-gas interaction") on star formation. The principal
results are as follows. Small gas clouds with densities larger than $10^{10}$
atoms cm^{-3} corresponding to first stellar cores can be developed from gas
without turbulence. Consequently, new stars can be formed from the gas with
high star formation efficiencies (>0.5) in a bursty manner. However, star
formation can be suppressed when the gas mass fractions within GCs (f_g) are
less than a threshold value (f_g, th). This f_g, th is larger for GCs with
lower masses and larger gas disks. Star-gas interaction and gravitational
potentials of GCs can combine to suppress the formation of massive stars (i.e.,
"top-light" stellar initial mass function). Formation of He-rich stars directly
from gas of massive AGB stars is possible in massive GCs due to low f_g, th
(<0.01). Short bursty star formation only for f_g>f_g, th can be partly
responsible for discrete multiple star formation events within GCs.
|
Molecular gas in the Herschel-selected strongly lensed submillimeter
galaxies at z~2-4 as probed by multi-J CO lines | (abridged) We present the IRAM-30m observations of multiple-J CO and CI line
emission in a sample of redshift ~2-4 Herschel-ATLAS SMGs. A non-negligible
effect of differential lensing is found for the CO emission lines, which could
have caused significant underestimations of the linewidths, hence of the
dynamical masses. The CO SLEDs are found to be similar to those of the local
starburst-dominated ULIRGs and of the previously studied SMGs. After correcting
for lensing amplification, we derived the global properties of the bulk of
molecular gas in the SMGs using non-LTE radiative transfer modelling. The gas
thermal pressure is found to be correlated with star formation efficiency.
Further decomposing the CO SLEDs into two excitation components, we find a
low-excitation component, which is less correlated with star formation, and a
high-excitation one which is tightly related to the on-going star-forming
activity. Additionally, tight linear correlations between the FIR and CO line
luminosities have been confirmed for the $J \ge 5$ CO lines, implying that
these CO lines are good tracers of star formation. The [CI](2-1) lines follow
the tight linear correlation between the luminosities of the [CI](2-1) and the
CO(1-0) line found in local starbursts, indicating that CI lines could serve as
good total molecular gas mass tracers for high-redshift SMGs. The total mass of
the molecular gas reservoir, $(1-30) \times 10^{10} M_\odot$, suggests a
typical molecular gas depletion time ~20-100 Myr and a gas to dust mass ratio
${\delta}_{\rm GDR}$~30-100. The ratio between CO line luminosity and the dust
mass appears to be slowly increasing with redshift for the SMGs, which need to
be further confirmed. Finally, through comparing the linewidth of CO and H2O
lines, we find that they agree well in almost all our SMGs, confirming that the
emitting regions are co-spatially located.
|
A synchrotron self-Compton -- disk reprocessing model for optical/X-ray
correlation in black hole X-ray binaries | Physical picture of the emission mechanisms operating in the X-ray binaries
was put under question by the simultaneous optical/X-ray observations with high
time resolution. The light curves of the two energy bands appeared to be
connected and the cross-correlation functions observed in three black hole
binaries exhibited a complicated shape. They show a dip of the optical emission
a few seconds before the X-ray peak and the optical flare just after the X-ray
peak. This behavior could not be explained in terms of standard optical
emission candidates (e.g., emission from the cold accretion disk or a jet). We
propose a novel model, which explains the broadband optical to the X-ray
spectra and the variability properties. We suggest that the optical emission
consists of two components: synchrotron radiation from the non-thermal
electrons in the hot accretion flow and the emission produced by reprocessing
of the X-rays in the outer part of the accretion disk. The first component is
anti-correlated with the X-rays, while the second one is correlated, but
delayed and smeared relative to the X-rays. The interplay of the components
explains the complex shape of the cross-correlation function, the features in
the optical power spectral density as well as the time lags.
|
Chandra and XMM-Newton X-ray observations of AWM 7 - I: Investigating
X-ray surface brightness fluctuations | We investigate the levels of small scale structure in surface brightness
images of the core of the X-ray bright cool-core galaxy cluster AWM 7. After
subtraction of a model of the smooth cluster emission, we find a number of
approximately radial surface brightness depressions which are not present in
simulated images and are seen in both the Chandra and XMM-Newton data. The
depressions are most strongly seen in the south of the cluster and have a
magnitude of around 4 per cent in surface brightness. We see these features in
both an energy band sensitive to the density (0.6 to 5 keV) and a band more
sensitive to the pressure (3.5 to 7.5 keV). Histograms of surface brightness in
the data, when compared to realisations of a smooth model, reveal stronger
surface brightness variations. We use the Delta-variance technique to
characterise the magnitude of the fluctuations as a function of length scale.
We find that the spectrum in the 0.6 to 5 keV band is flatter than expected for
Kolmogorov index fluctuations. If characterised by a power spectrum, on large
scales it would have an index around -1.7, rather than -3.7. The implied 3D
density fluctuations have a standard deviation of around 4 per cent. The
implied 3D pressure variations are at most 4 per cent. Most of the longer-scale
power in the density spectrum is contributed by the southern half of the
cluster, where the depressions are seen. The density variations implied by the
spectrum of the northern sector have a standard deviation of about 2 per cent.
|
Tidal Debris as a Dark Matter Probe | Tidal debris streams from galaxy satellites can provide insight into the dark
matter distribution in halos. This is because we have more information about
stars in a debris structure than about a purely random population of stars: we
know that in the past they were all bound to the same dwarf galaxy; and we know
that they form a dynamically cold population moving on similar orbits. They
also probe a different region of the matter distribution in a galaxy than many
other methods of mass determination, as their orbits take them far beyond the
typical extent of those for the bulk of stars. Although conclusive results from
this information have yet to be obtained, significant progress has been made in
developing the methodologies for determining both the global mass distribution
of the Milky Way's dark matter halo and the amount of dark matter substructure
within it. Methods for measuring the halo shape are divided into "predictive
methods," which predict the tidal debris properties from the progenitor
satellite's mass and orbit, given an assumed parent galaxy mass distribution;
and "fundamental methods," which exploit properties fundamental to the nature
of tidal debris as global potential constraints. Methods for quantifying the
prevalence of dark matter subhalos within halos through the analysis of the
gaps left in tidal streams after these substructures pass through them are
reviewed.
|
A Multiple Dry Merger at z=0.18: Witnessing The Assembly of a Massive
Elliptical Galaxy | Mergers of gas-poor galaxies, so-called dry mergers, may play a fundamental
role in the assembly of the most massive galaxies, and therefore, in galaxy
formation theories. Using the SDSS, we have serendipitously discovered a rare
system in the observational and theoretical context, possibly a quintuple dry
merger at low redshift. As a follow-up, we have obtained NOT long-slit spectra
of the group, in order to measure the individual redshifts and gain insight
into its merger fate. Our results show an isolated, low-redshift galaxy group
consisting of massive, quiescent, early-type galaxies, composed of two clumps
(possibly themselves in the process of merging), which we estimate will
hypothetically merge in roughly less than a Gyr. With the possible exception of
the high line-of-sight velocity dispersion, the overall properties of the
system may be comparable to a compact Shakhbazyan group. However, when the
small projected separations and relative mass ratios of the galaxies are taken
into account in cosmological simulations, we find that this system is rather
unique. We hypothesize that this group is a dry merger, whose fate will result
in the assembly of an isolated, massive elliptical galaxy at low redshift.
|
Cosmological constraints on the neutrino mass including systematic
uncertainties | When combining cosmological and oscillations results to constrain the
neutrino sector, the question of the propagation of systematic uncertainties is
often raised. We address this issue in the context of the derivation of an
upper bound on the sum of the neutrino masses ($\Sigma m_\nu$) with recent
cosmological data. This work is performed within the
${{\mathrm{\Lambda{CDM}}}}$ model extended to $\Sigma m_\nu$, for which we
advocate the use of three mass-degenerate neutrinos. We focus on the study of
systematic uncertainties linked to the foregrounds modelling in CMB data
analysis, and on the impact of the present knowledge of the reionisation
optical depth. This is done through the use of different likelihoods built from
Planck data. Limits on $\Sigma m_\nu$ are derived with various combinations of
data, including the latest Baryon Acoustic Oscillations (BAO) and Type Ia
Supernovae (SN) results. We also discuss the impact of the preference for
current CMB data for amplitudes of the gravitational lensing distortions higher
than expected within the ${{\mathrm{\Lambda{CDM}}}}$ model, and add the Planck
CMB lensing. We then derive a robust upper limit: $\Sigma m_\nu< 0.17\hbox{ eV
at }95\% \hbox{CL}$, including 0.01 eV of foreground systematics. We also
discuss the neutrino mass repartition and show that today's data do not allow
one to disentangle normal from inverted hierarchy. The impact on the other
cosmological parameters is also reported, for different assumptions on the
neutrino mass repartition, and different high and low multipole CMB
likelihoods.
|
Optical Confirmation and Redshift Estimation of the Planck Cluster
Candidates overlapping the Pan-STARRS Survey | We report results of a study of Planck Sunyaev-Zel'dovich effect (SZE)
selected galaxy cluster candidates using the Panoramic Survey Telescope & Rapid
Response System (Pan-STARRS) imaging data. We first examine 150 Planck
confirmed galaxy clusters with spectroscopic redshifts to test our algorithm
for identifying optical counterparts and measuring their redshifts; our
redshifts have a typical accuracy of $\sigma_{z/(1+z)} \sim 0.022$ for this
sample. Using 60 random sky locations, we estimate that our chance of
contamination through a random superposition is ~ 3 per cent. We then examine
an additional 237 Planck galaxy cluster candidates that have no redshift in the
source catalogue. Of these 237 unconfirmed cluster candidates we are able to
confirm 60 galaxy clusters and measure their redshifts. A further 83 candidates
are so heavily contaminated by stars due to their location near the Galactic
plane that we do not attempt to identify counterparts. For the remaining 94
candidates we find no optical counterpart but use the depth of the Pan-STARRS1
data to estimate a redshift lower limit $z_{\text{lim}(10^{15})}$ beyond which
we would not have expected to detect enough galaxies for confirmation. Scaling
from the already published Planck sample, we expect that $\sim$12 of these
unconfirmed candidates may be real clusters.
|
The high-redshift gamma-ray burst GRB140515A | High-redshift gamma-ray bursts have several advantages for the study of the
distant universe, providing unique information about the structure and
properties of the galaxies in which they exploded. Spectroscopic identification
with large ground-based telescopes has improved our knowledge of the class of
such distant events. We present the multi-wavelength analysis of the high-$z$
Swift gamma-ray burst GRB140515A ($z = 6.327$). The best estimate of the
neutral hydrogen fraction of the intergalactic medium (IGM) towards the burst
is $x_{HI} \leq 0.002$. The spectral absorption lines detected for this event
are the weakest lines ever observed in gamma-ray burst afterglows, suggesting
that GRB140515A exploded in a very low density environment. Its circum-burst
medium is characterised by an average extinction (A$_{\rm V} \sim 0.1$) that
seems to be typical of $z \ge 6$ events. The observed multi-band light curves
are explained either with a very flat injected spectrum ($p = 1.7$) or with a
multi-component emission ($p = 2.1$). In the second case a long-lasting central
engine activity is needed in order to explain the late time X-ray emission. The
possible origin of GRB140515A from a Pop III (or from a Pop II stars with local
environment enriched by Pop III) massive star is unlikely.
|
Tuned MSSM Higgses as an inflaton | We consider the possibility that the vacuum energy density of the MSSM
(Minimal Supersymmetric Standard Model) flat direction condensate involving the
Higgses H_1 and H_2 is responsible for inflation. We also discuss how the
finely tuned Higgs potential at high vacuum expectation values can realize {\it
cosmologically} flat direction along which it can generate the observed density
perturbations, and after the end of inflation -- the coherent oscillations of
the Higgses reheat the universe with all the observed degrees of freedom,
without causing any problem for the electroweak phase transition.
|
Photoinduced polycyclic aromatic hydrocarbon dehydrogenation: Molecular
hydrogen formation in dense PDRs | The physical and chemical conditions in photodissociation regions (PDRs) are
largely determined by the influence of far ultraviolet radiation. Far-UV
photons can efficiently dissociate molecular hydrogen, a process that must be
balanced at the HI/H2 interface of the PDR. Given that reactions involving
hydrogen atoms in the gas phase are highly inefficient under interstellar
conditions, H2 formation models mostly rely on catalytic reactions on the
surface of dust grains. Additionally, molecular hydrogen formation in
polycyclic aromatic hydrocarbons (PAHs) through the Eley-Rideal mechanism has
been considered as well, although it has been found to have low efficiency in
PDR fronts. In a previous work, we have described the possibility of efficient
H2 release from medium to large sized PAHs upon photodissociation, with the
exact branching between H-/H2-loss reactions being molecule dependent. Here we
investigate the astrophysical relevance of this process, by using a model for
the photofragmentation of PAHs under interstellar conditions. We focus on three
PAHs cations (coronene, ovalene and circumcoronene), which represent three
possibilities in the branching of atomic and molecular hydrogen losses. We find
that, for ovalene (H2-loss dominated) the rate coefficient for H2 formation
reaches values of the same order as H2 formation in dust grains. This result
suggests that this hitherto disregarded mechanism can account, at least partly,
for the high level of molecular hydrogen formation in dense PDRs.
|
Radio Galaxy Zoo: A Search for Hybrid Morphology Radio Galaxies | Hybrid morphology radio sources are a rare type of radio galaxy that display
different Fanaroff-Riley classes on opposite sides of their nuclei. To enhance
the statistical analysis of hybrid morphology radio sources, we embarked on a
large-scale search of these sources within the international citizen science
project, Radio Galaxy Zoo (RGZ). Here, we present 25 new candidate hybrid
morphology radio galaxies. Our selected candidates are moderate power radio
galaxies (L_median = 4.7x10^{24} W/(Hz sr) at redshifts 0.14<z<1.0. Hosts of
nine candidates have spectroscopic observations, of which six are classified as
quasars, one as high- and two as low-excitation galaxies. Two candidate HyMoRS
are giant (>1Mpc) radio galaxies, one resides at a centre of a galaxy cluster,
and one is hosted by a rare green bean galaxy. Although the origin of the
hybrid morphology radio galaxies is still unclear, this type of radio source
starts depicting itself as a rather diverse class. We discuss hybrid radio
morphology formation in terms of the radio source environment (nurture) and
intrinsically occurring phenomena (nature; activity cessation and
amplification), showing that these peculiar radio galaxies can be formed by
both mechanisms. While high angular resolution follow-up observations are still
necessary to confirm our candidates, we demonstrate the efficacy of the Radio
Galaxy Zoo in the pre-selection of these sources from all-sky radio surveys,
and report the reliability of citizen scientists in identifying and classifying
complex radio sources.
|
NIHAO III: The constant disc gas mass conspiracy | We show that the cool gas masses of galactic discs reach a steady state that
lasts many Gyr after their last major merger in cosmological hydrodynamic
simulations. The mass of disc gas, M$_{\rm gas}$, depends upon a galaxy halo's
spin and virial mass, but not upon stellar feedback. Halos with low spin have
high star formation efficiency and lower disc gas mass. Similarly, lower
stellar feedback leads to more star formation so the gas mass ends up nearly
the same irregardless of stellar feedback strength. Even considering spin, the
M$_{\rm gas}$ relation with halo mass, M$_{200}$ only shows a factor of 3
scatter. The M$_{\rm gas}$--M$_{200}$ relation show a break at
M$_{200}$=$2\times10^{11}$ M$_\odot$ that corresponds to an observed break in
the M$_{\rm gas}$--M$_\star$ relation. The constant disc mass stems from a
shared halo gas density profile in all the simulated galaxies. In their outer
regions, the profiles are isothermal. Where the profile rises above $n=10^{-3}$
cm$^{-3}$, the gas readily cools and the profile steepens. Inside the disc,
rotation supports gas with a flatter density profile except where supernova
explosions disrupt the disc. Energy injection from stellar feedback also
provides pressure support to the halo gas to prevent runaway cooling flows. The
resulting constant gas mass makes simpler models for galaxy formation possible,
either using a "bathtub" model for star formation rates or when modeling
chemical evolution.
|
A MODEST review | We present an account of the state of the art in the fields explored by the
research community invested in 'Modeling and Observing DEnse STellar systems'.
For this purpose, we take as a basis the activities of the MODEST-17
conference, which was held at Charles University, Prague, in September 2017.
Reviewed topics include recent advances in fundamental stellar dynamics,
numerical methods for the solution of the gravitational N-body problem,
formation and evolution of young and old star clusters and galactic nuclei,
their elusive stellar populations, planetary systems, and exotic compact
objects, with timely attention to black holes of different classes of mass and
their role as sources of gravitational waves.
Such a breadth of topics reflects the growing role played by collisional
stellar dynamics in numerous areas of modern astrophysics. Indeed, in the next
decade, many revolutionary instruments will enable the derivation of positions
and velocities of individual stars in the Milky Way and its satellites and will
detect signals from a range of astrophysical sources in different portions of
the electromagnetic and gravitational spectrum, with an unprecedented
sensitivity. On the one hand, this wealth of data will allow us to address a
number of long-standing open questions in star cluster studies; on the other
hand, many unexpected properties of these systems will come to light,
stimulating further progress of our understanding of their formation and
evolution.
|
Structural Analogs of the Milky Way Galaxy: Stellar Populations in the
Boxy Bulges of NGC 4565 and NGC 5746 | We present NGC 4565 and NGC 5746 as structural analogs of our Milky Way. All
three are giant, SBb - SBbc galaxies with two pseudobulges, i. e., a compact,
disky, star-forming pseudobulge embedded in a vertically thick, "red and dead",
boxy pseudobulge that really is a bar seen almost end-on. The stars in the boxy
bulge of our Milky Way are old and enhanced in alpha elements, indicating that
star formation finished within ~ 1 Gyr of when it started. Here, we present
Hobby-Eberly Telescope spectroscopy of the boxy pseudobulges of NGC 4565 and
NGC 5746 and show that they also are made of old and alpha-element-enhanced
stars. Evidently it is not rare that the formation of stars that now live in
bars finished quickly and early, even in galaxies of intermediate Hubble types
whose disks still form stars now. Comparison of structural component parameters
leads us to suggest that NGC 4565 and NGC 5746 are suitable analogs of the
Milky Way, because they show signatures of similar evolution processes.
|
SPT-CL J2032-5627: a new Southern double relic cluster observed with
ASKAP | We present a radio and X-ray analysis of the galaxy cluster SPT-CL
J2032-5627. Investigation of public data from the Australian Square Kilometre
Array Pathfinder (ASKAP) at 943 MHz shows two previously undetected radio
relics at either side of the cluster. For both relic sources we utilise
archival Australia Telescope Compact Array (ATCA) data at 5.5 GHz in
conjunction with the new ASKAP data to determine that both have steep
integrated radio spectra ($\alpha_\mathrm{SE} = -1.52 \pm 0.10$ and
$\alpha_\mathrm{NW,full} = -1.18 \pm 0.10$ for the southeast and northwest
relic sources, respectively). No shock is seen in XMM-Newton observations,
however, the southeast relic is preceded by a cold front in the X-ray emitting
intra-cluster medium. We suggest the lack of a detectable shock may be due to
instrumental limitations, comparing the situation to the southeast relic in
Abell 3667. We compare the relics to the population of double relic sources and
find they are located below the current power-mass ($P$-$M$) scaling relation.
We present an analysis of the low-surface brightness sensitivity of ASKAP and
the ATCA, the excellent sensitivity of both allow the ability to find
heretofore undetected diffuse sources, suggesting these low-power radio relics
will become more prevalent in upcoming large-area radio surveys such as the
Evolutionary Map of the Universe (EMU).
|
Axions, Inflation and the Anthropic Principle | The QCD axion is the leading solution to the strong-CP problem, a dark matter
candidate, and a possible result of string theory compactifications. However,
for axions produced before inflation, symmetry-breaking scales of $f_a \gtrsim
10^{12}$ GeV (which are favored in string-theoretic axion models) are ruled out
by cosmological constraints unless both the axion misalignment angle $\theta_0$
and the inflationary Hubble scale $H_I$ are extremely fine-tuned. We show that
attempting to accommodate a high-$f_a$ axion in inflationary cosmology leads to
a fine-tuning problem that is worse than the strong-CP problem the axion was
originally invented to solve. We also show that this problem remains unresolved
by anthropic selection arguments commonly applied to the high-$f_a$ axion
scenario.
|
The Problem of Inertia in Friedmann Universes | In this paper we study the origin of inertia in a curved spacetime,
particularly the spatially flat, open and closed Friedmann universes. This is
done using Sciama's law of inertial induction, which is based on Mach's
principle, and expresses the analogy between the retarded far fields of
electrodynamics and those of gravitation. After obtaining covariant expressions
for electromagnetic fields due to an accelerating point charge in Friedmann
models, we adopt Sciama's law to obtain the inertial force on an accelerating
mass $m$ by integrating over the contributions from all the matter in the
universe. The resulting inertial force has the form $F = -kma$, where $k < 1 $
depends on the choice of the cosmological parameters such as $\Omega_{M}$,
$\Omega_{\Lambda}$, and $\Omega_{R}$ and is also red-shift dependent.
|
Morphology and Size Differences between Local & High Redshift Luminous
Infrared Galaxies | We show that the star-forming regions in high-redshift luminous and
ultraluminous infrared galaxies (LIRGs and ULIRGs) and submillimeter galaxies
(SMGs) have similar physical scales to those in local normal star-forming
galaxies. To first order, their higher infrared (IR) luminosities result from
higher luminosity surface density. We also find a good correlation between the
IR luminosity and IR luminosity surface density in starburst galaxies across
over five orders of magnitude of IR luminosity from local normal galaxies to z
~ 2 SMGs. The intensely star-forming regions of local ULIRGs are significantly
smaller than those in their high-redshift counterparts and hence diverge
significantly from this correlation, indicating that the ULIRGs found locally
are a different population from the high-redshift ULIRGs and SMGs. Based on
this relationship, we suggest that luminosity surface density should serve as a
more accurate indicator for the IR emitting environment, and hence the
observable properties, of star-forming galaxies than their IR luminosity. We
demonstrate this approach by showing that ULIRGs at z ~ 1 and a lensed galaxy
at z ~ 2.5 exhibit aromatic features agreeing with local LIRGs that are an
order of magnitude less luminous, but have similar IR luminosity surface
density. A consequence of this relationship is that the aromatic emission
strength in star-forming galaxies will appear to increase at z > 1 for a given
IR luminosity compared to their local counterparts.
|
A new smooth-$k$ space filter approach to calculate halo abundances | We propose a new filter, a smooth-$k$ space filter, to use in the
Press-Schechter approach to model the dark matter halo mass function which
overcomes shortcomings of other filters. We test this against the mass function
measured in N-body simulations. We find that the commonly used sharp-$k$ filter
fails to reproduce the behaviour of the halo mass function at low masses
measured from simulations of models with a sharp truncation in the linear power
spectrum. We show that the predictions with our new filter agree with the
simulation results over a wider range of halo masses for both damped and
undamped power spectra than is the case with the sharp-$k$ and real-space
top-hat filters.
|
Measurements of the diffuse Galactic synchrotron spectral index and
curvature from MeerKLASS pilot data | 21cm intensity mapping experiments are bringing an influx of high spectral
resolution observational data in the $\sim100$ MHz $- 1$ GHz regime. We use
pilot $971-1075$ MHz data from MeerKAT in single-dish mode, recently used to
test the calibration and data reduction scheme of the upcoming MeerKLASS
survey, to probe the spectral index of diffuse synchrotron emission below 1 GHz
within $145^{\circ} < \alpha < 180^{\circ}$, $-1^{\circ} < \delta < 8^{\circ}$.
Through comparisons with data from the OVRO Long Wavelength Array and the Maipu
and MU surveys, we find an average spectral index of $-2.75 < \beta < -2.71$
between 45 and 1055 MHz. By fitting for spectral curvature with a spectral
index of the form $\beta + c \, {\rm{ln}}(\nu / 73~{\rm MHz})$, we measure
$\beta = -2.55 \pm 0.13$ and $c = -0.12 \pm 0.05$ within our target field. Our
results are in good agreement (within $1\sigma$) with existing measurements
from experiments such as ARCADE2 and EDGES. These results show the calibration
accuracy of current data and demonstrate that MeerKLASS will also be capable of
achieving a secondary science goal of probing the interstellar medium.
|
Possible Evolution of Supermassive Black Holes from FRI quasars | We explore the question of the rapid buildup of black hole mass in the early
universe employing a growing black hole mass-based determination of both jet
and disk powers predicted in recent theoretical work on black hole accretion
and jet formation. Despite simplified, even artificial assumptions about
accretion and mergers, we identify an interesting low probability channel for
the growth of one billion solar mass black holes within hundreds of millions of
years of the Big Bang without appealing to super Eddington accretion. This
result is made more compelling by the recognition of a connection between this
channel and an end product involving active galaxies with FRI radio morphology
but weaker jet powers in mildly sub-Eddington accretion regimes. While FRI
quasars have already been shown to occupy a small region of the available
parameter space for black hole feedback in the paradigm, we further suggest
that the observational dearth of FRI quasars is also related to their
connection to the most massive black hole growth due to both these FRIs high
redshifts and relative weakness. Our results also allow us to construct the AGN
luminosity function at high redshift, that agree with recent studies. In short,
we produce a connection between the unexplained paucity of a given family of
active galactic nuclei and the rapid growth of supermassive black holes, two
heretofore seemingly unrelated aspects of the physics of active galactic
nuclei.
|
Canonical transformations and squeezing formalism in cosmology | Canonical transformations are ubiquitous in Hamiltonian mechanics, since they
not only describe the fundamental invariance of the theory under phase-space
reparameterisations, but also generate the dynamics of the system. In the first
part of this work we study the symplectic structure associated with linear
canonical transformations. After reviewing salient mathematical properties of
the symplectic group in a pedagogical way, we introduce the squeezing
formalism, and show how any linear dynamics can be cast in terms of an
invariant representation. In the second part, we apply these results to the
case of cosmological perturbations, and focus on scalar field fluctuations
during inflation. We show that different canonical variables select out
different vacuum states, and that this leaves an ambiguity in observational
predictions if initial conditions are set at a finite time in the past. We also
discuss how the effectiveness of the quantum-to-classical transition of
cosmological perturbations depends on the set of canonical variables used to
describe them.
|
Optical Variability of the Dwarf AGN NGC 4395 from the Transiting
Exoplanet Survey Satellite | We present optical light curves from the Transiting Exoplanet Survey
Satellite (TESS) for the archetypical dwarf active galactic nucleus (AGN) in
the nearby galaxy NGC 4395 hosting a $\sim 10^5\,M_\odot$ supermassive black
hole (SMBH). Significant variability is detected on timescales from weeks to
hours before reaching the background noise level. The $\sim$month-long, 30
minute-cadence, high-precision TESS light curve can be well fit by a simple
damped random walk (DRW) model, with the damping timescale $\tau_{\rm DRW}$
constrained to be $2.3_{-0.7}^{+1.8}$~days ($1\sigma$). NGC 4395 lies almost
exactly on the extrapolation of the $\tau_{\rm DRW}-M_{\rm BH}$ relation
measured for AGNs with BH masses that are more than three orders of magnitude
larger. The optical variability periodogram can be well fit by a broken power
law with the high-frequency slope ($-1.88\pm0.15$) and the characteristic
timescale ($\tau_{\rm br}\equiv 1/(2\pi f_{\rm br})=1.4_{-0.5}^{+1.9}\,$days)
consistent with the DRW model within 1$\sigma$. This work demonstrates the
power of TESS light curves in identifying low-mass accreting SMBHs with optical
variability, and a potential global $\tau_{\rm DRW}-M_{\rm BH}$ relation that
can be used to estimate SMBH masses with optical variability measurements.
|
Small Bites: Star formation recipes in extreme dwarfs | We study the relationship between the gas column density (Sigma_HI) and the
star formation rate surface density (Sigma_SFR) for a sample of extremely small
(M_B ~ -13, Delta V_50 ~ 30 km/s) dwarf irregular galaxies. We find a clear
stochasticity in the relation between the gas column density and star
formation. All gas with Sigma_HI >~ 10 M_sun/pc^2 has some ongoing star
formation, but the fraction of gas with ongoing star formation decreases as the
gas column density decreases, and falls to about 50% at Sigma_HI ~ 3
M_sun/pc^2. Further, even for the most dense gas, the star formation efficiency
is at least a factor of ~ 2 smaller than typical of star forming regions in
spirals. We also find that the ratio of H-alpha emission to FUV emission
increases with increasing gas column density. This is unlikely to be due to
increasing dust extinction because the required dust to gas ratios are too
high. We suggest instead that this correlation arises because massive (i.e.
H-alpha producing) stars are formed preferentially in regions with high gas
density.
|
The Properties of Radio and Mid-infrared Detected Galaxies and the
Effect of Environment on the Co-evolution of AGN and Star Formation at $z
\sim 1$ | In this study we investigate 179 radio-IR galaxies drawn from a sample of
spectroscopically-confirmed galaxies that are detected in radio and
mid-infrared (MIR) in the redshift range of $0.55 \leq z \leq 1.30$ in the
Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey.
We constrain the Active Galactic Nuclei (AGN) contribution in the total IR
luminosity (f$_{\text{AGN}}$), and estimate the AGN luminosity
(L$_{\text{AGN}}$) and the star formation rate (SFR) using the CIGALE Spectral
Energy Distribution (SED) fitting routine. Based on the f$_{\text{AGN}}$ and
radio luminosity, radio-IR galaxies are split into: galaxies that host either
high or low f$_{\text{AGN}}$ AGN (high-/low-f$_{\text{AGN}}$), and star forming
galaxies with little to no AGN activity (SFGs). We study the colour, stellar
mass, radio luminosity, L$_{\text{AGN}}$ and SFR properties of the three
radio-IR sub-samples, comparing to a spec-IR sample drawn from
spectroscopically-confirmed galaxies that are also detected in MIR. No
significant difference between radio luminosity of these sub-samples was found,
which could be due to the combined contribution of radio emission from AGN and
star formation. We find a positive relationship between L$_{\text{AGN}}$and
specific SFR (sSFR) for both AGN sub-samples, strongly suggesting a
co-evolution scenario of AGN and SF in these galaxies. A toy model is designed
to demonstrate this co-evolution scenario, where we find that, in almost all
cases, a rapid quenching timescale is required, which we argue is a signature
of AGN quenching. The environmental preference for intermediate/infall regions
of clusters/groups remains across the co-evolution scenario, which suggests
that galaxies might be in an orbital motion around the cluster/group during the
scenario.
|
Application of beyond $\delta N$ formalism -- Varying sound speed | We focus on the evolution of curvature perturbation on superhorizon scales by
adopting the spatial gradient expansion and show that the nonlinear theory,
called the beyond $\delta N$-formalism as the next-leading order in the
expansion. As one application of our formalism for a single scalar field, we
investigate the case of varying sound speed. In our formalism, we can deal with
the time evolution in contrast to $\delta N$-formalism, where curvature
perturbations remain just constant, and nonlinear curvature perturbation
follows the simple master equation whose form is similar as one in linear
theory. So the calculation of bispectrum can be done in the next-leading order
in the expansion as similar as the case of deriving the power spectrum. We
discuss localized features of both primordial power and bispectrum generated by
the effect of varying sound speed with a finite duration time. We can see a
local feature like a bump in the equilateral bispectrum.
|
Ruling out $\sim 100-300$ GeV thermal relic annihilating dark matter by
radio observation of the Andromeda galaxy | In the past few years, some studies claimed that annihilating dark matter
with mass $\sim 10-100$ GeV can explain the GeV gamma-ray excess in our Galaxy.
However, recent analyses of the Fermi-LAT and radio observational data rule out
the possibility of the thermal relic annihilating dark matter with mass $m \le
100$ GeV for some popular annihilation channels. By using the new observed
radio data of the Andromeda galaxy, we rule out the existence of $\sim 100-300$
GeV thermal relic annihilating dark matter for ten annihilation channels. The
lower limits of annihilating dark matter mass are improved to larger than 330
GeV for the most conservative case, which is a few times larger than the
current best constraints. Moreover, these limits strongly disfavor the
benchmark model of weakly interacting massive particle (WIMP) produced through
the thermal freeze-out mechanism.
|
Continuous wavelet analysis of matter clustering using the
Gaussian-derived wavelet | Continuous wavelet analysis has been increasingly employed in various fields
of science and engineering due to its remarkable ability to maintain optimal
resolution in both space and scale. Here, we introduce wavelet-based
statistics, including the wavelet power spectrum, wavelet cross-correlation and
wavelet bicoherence, to analyze the large-scale clustering of matter. For this
purpose, we perform wavelet transforms on the density distribution obtained
from the one-dimensional Zel'dovich approximation and then measure the wavelet
power spectra and wavelet bicoherences of this density distribution. Our
results suggest that the wavelet power spectrum and wavelet bicoherence can
identify the effects of local environments on the clustering at different
scales. Moreover, we apply the statistics based on the three-dimensional
isotropic wavelet to the IllustrisTNG simulation at z = 0, and investigate the
environmental dependence of the matter clustering. We find that the clustering
strength of the total matter increases with increasing local density except on
the largest scales. Besides, we notice that the gas traces the dark matter
better than stars on large scales in all environments. On small scales, the
cross-correlation between the dark matter and gas first decreases and then
increases with increasing density. This is related to the impacts of the AGN
feedback on the matter distribution, which also varies with the density
environment in a similar trend to the cross-correlation between the dark matter
and gas. Our findings are qualitatively consistent with previous studies about
the matter clustering.
|
The ALFA ZOA Deep Survey: First Results | The Arecibo L-Band Feed Array Zone of Avoidance (ALFA ZOA) Deep Survey is the
deepest and most sensitive blind Hi survey undertaken in the ZOA. ALFA ZOA Deep
will cover about 300 square degrees of sky behind the Galactic plane in both
the inner (30 deg < l < 75 deg; b < |2 deg|) and outer (175 deg < l < 207 deg;
-2 deg < b < +1 deg) Galaxy, using the Arecibo Radio Telescope. First results
from the survey have found 61 galaxies within a 15 square degree area centered
on l = 192 deg and b = -2 deg. The survey reached its expected sensitivity of
rms = 1 mJy at 9 km/s channel resolution, and is shown to be complete above
integrated flux, F_HI = 0.5 Jy km/s. The positional accuracy of the survey is
28 arcsec and detections are found out to a recessional velocity of nearly
19,000 km/s. The survey confirms the extent of the Orion and Abell 539 clusters
behind the plane of the Milky Way and discovers expansive voids, at 10,000 km/s
and 18,000 km/s. 26 detections (43%) have a counterpart in the literature, but
only two of these have known redshift. Counterparts are 20% less common beyond
v_hel = 10,000 km/s and 33% less common at extinctions higher than AB = 3.5
mag. ALFA ZOA Deep survey is able to probe large scale structure beyond
redshifts that even the most modern wide-angle surveys have been able to detect
in the Zone of Avoidance at any wavelength.
|
Gamma-ray Flux Distribution and Non-linear behavior of Four LAT Bright
AGNs | We present a statistical characterization of the $\gamma$-ray emission from
the four \emph{Fermi}-LAT sources: FR I radio galaxy NGC 1275, BL Lac Mrk 421,
FSRQs B2 1520+31 and PKS 1510-089 detected almost continuously over a time
integration of 3-days between August 2008 - October 2015. The observed flux
variation is large, spanning $\gtrsim 2$ orders of magnitude between the
extremes except for Mrk~421. We compute the flux distributions and compare with
Gaussian and lognormal ones. We find that the 3 blazars have distribution
consistent with a lognormal, suggesting that the variability is of a
non-linear, multiplicative nature. This is further supported by the computation
of the flux-rms relation, which is observed to be linear for the 3 blazars.
However, for NGC 1275, the distribution does not seem to be represented either
by a lognormal or a Gaussian, while its flux-rms relation is still found to be
linear. We also compute the power spectra, which suggest the presence of a
break, but are consistent with typical scale-free power-law shot noise. The
results are broadly consistent with the statistical properties of the magnetic
reconnection powered minijets-in-a-jet model. We discuss other possible
scenarios and implications of these observations on jet processes and
connections with the central engine.
|
Natural cliff inflation | We propose a novel scenario of inflation, in which the inflaton is identified
as the lightest mode of an angular field in a compactified fifth dimension. The
periodic effective potential exhibits exponentially flat plateaus, so that a
sub-Planckian field excursion without hilltop initial conditions is naturally
realized. We can obtain consistent predictions with observations on the
spectral index and the tensor-to-scalar ratio.
|
Quintessential and phantom power-law solutions in scalar tensor model of
dark energy | We consider a scalar-tensor model of dark energy with kinetic and Gauss
Bonnet couplings. We study the conditions for the existence of quintessential
and phantom power-law expansion, and also analyze these conditions in absence
of potential (closely related to string theory). A mechanism to avoid the Big
Rip singularity in various asymptotic limits of the model has been studied. It
was found that the kinetic and Gauss-Bonnet couplings might prevent the Big Rip
singularity in a phantom scenario. The autonomous system for the model has been
used to study the stability properties of the power-law solution, and the
centre manifold analysis was used to treat zero eigenvalues.
|
Models for SIMP dark matter and dark photon | We give a review on the SIMP paradigm and discuss a consistent model for SIMP
dark mesons in the context of a dark QCD with flavor symmetry. The $Z'$-portal
interaction is introduced being compatible with stable dark mesons and is
responsible for making the SIMP dark mesons remain in kinetic equilibrium with
the SM during the freeze-out process. The SIMP parameter space of the $Z'$
gauge boson can be probed by future collider and direct detection experiments.
|
The variance of the CMB temperature gradient: a new signature of a
multiply connected Universe | In this work we investigate the standard deviation of the Cosmic Microwave
Background (CMB) temperature gradient field as a signature for a multiply
connected nature of the Universe. CMB simulations of a spatially infinite
universe model within the paradigm of the standard cosmological model present
non-zero two-point correlations at any angular scale. This is in contradiction
with the extreme suppression of correlations at scales above $60^{\circ}$ in
the observed CMB maps. Universe models with spatially multiply connected
topology contain typically a discrete spectrum of the Laplacian with a specific
wave-length cut-off and thus lead to a suppression of the correlations at large
angular scales, as observed in the CMB (in general there can be also an
additional continuous spectrum). Among the simplest examples are 3-dimensional
tori which possess only a discrete spectrum. To date, the universe models with
non-trivial topology such as the toroidal space are the only models that
possess a two-point correlation function showing a similar behaviour as the one
derived from the observed Planck CMB maps. In this work it is shown that the
normalized standard deviation of the CMB temperature gradient field does
hierarchically detect the change in size of the cubic 3-torus, if the volume of
the Universe is smaller than $\simeq 2.5 \cdot 10^3$ Gpc$^3$. It is also shown
that the variance of the temperature gradient of the Planck maps is consistent
with the median value of simulations within the standard cosmological model.
All flat tori are globally homogeneous, but are globally anisotropic. However,
this study also presents a test showing a level of homogeneity and isotropy of
all the CMB map ensembles for the different torus sizes considered that are
nearly at the same weak level of anisotropy revealed by the CMB in the standard
cosmological model.
|
Andromeda and its satellites - a kinematic perspective | Using spectroscopic data taken with Keck II DEIMOS by the Z-PAndAS team in
the Andromeda-Triangulum region, I present a comparison of the disc and
satellite systems of Andromeda with those of our own Galaxy. I discuss the
observed discrepancies between the masses and scale radii of Andromeda dwarf
spheroidal galaxies of a given luminosity with those of the Milky Way. I also
also present an analysis of the newly discovered M31 thick disc, which is
measured to be hotter, more extended and thicker than that seen in the Milky
Way.
|
Stellar Populations of over one thousand $z\sim0.8$ Galaxies from
LEGA-C: Ages and Star Formation Histories from D$_n$4000 and H$\delta$ | Drawing from the LEGA-C dataset, we present the spectroscopic view of the
stellar population across a large volume- and mass-selected sample of galaxies
at large lookback time. We measure the 4000\AA\ break (D$_n$4000) and Balmer
absorption line strengths (probed by H$\delta$) from 1019 high-quality spectra
of $z=0.6 - 1.0$ galaxies with $M_\ast = 2 \times 10^{10} M_\odot - 3 \times
10^{11} M_\odot$. Our analysis serves as a first illustration of the power of
high-resolution, high-S/N continuum spectroscopy at intermediate redshifts as a
qualitatively new tool to constrain galaxy formation models. The observed
D$_n$4000-EW(H$\delta$) distribution of our sample overlaps with the
distribution traced by present-day galaxies, but $z\sim 0.8$ galaxies populate
that locus in a fundamentally different manner. While old galaxies dominate the
present-day population at all stellar masses $> 2\times10^{10} M_\odot$, we see
a bimodal D$_n$4000-EW(H$\delta$) distribution at $z\sim0.8$, implying a
bimodal light-weighted age distribution. The light-weighted age depends
strongly on stellar mass, with the most massive galaxies
$>1\times10^{11}M_\odot$ being almost all older than 2 Gyr. At the same time we
estimate that galaxies in this high mass range are only $\sim3$ Gyr younger
than their $z\sim0.1$ counterparts, at odd with pure passive evolution given a
difference in lookback time of $>5$ Gyr; younger galaxies must grow to
$>10^{11}M_\odot$ in the meantime, and/or small amounts of young stars must
keep the light-weighted ages young. Star-forming galaxies at $z\sim0.8$ have
stronger H$\delta$ absorption than present-day galaxies with the same
D$_n$4000, implying larger short-term variations in star-formation activity.
|
Constraints on the presence of water megamaser emission in z~2.5
ultraluminous infrared starburst galaxies | We present Expanded Very Large Array and Arecibo observations of two lensed
submm galaxies at z~2.5, in order to search for redshifted 22.235 GHz water
megamaser emission. Both SMM J14011+0252 and SMM J16359+6612 have
multi-wavelength characteristics consistent with ongoing starburst activity, as
well as CO line emission indicating the presence of warm molecular gas. Our
observations do not reveal any evidence for H2O megamaser emission in either
target, while the lensing allows us to obtain deep limits to the H_2O line
luminosities, L(H2O) < 7470 Lsun (3-sigma) in the case of SMM J14011+0252, and
L(H2O) < 1893 Lsun for SMM J16359+6612, assuming linewidths of 80 km/s. Our
search for, and subsequent non-detection of H2O megamaser emission in two
strongly lensed starburst galaxies, rich in gas and dust, suggests that such
megamaser emission is not likely to be common within the unlensed population of
high-redshift starburst galaxies. We use the recent detection of strong H2O
megamaser emission in the lensed quasar, MG J0414+0534 at z = 2.64 to make
predictions for future EVLA C-band surveys of H2O megamaser emission in submm
galaxies hosting AGN.
|
A Sample of OB Stars That Formed in the Field | We present a sample of 14 OB stars in the Small Magellanic Cloud that meet
strong criteria for having formed under extremely sparse star-forming
conditions in the field. These stars are a minimum of 28 pc in projection from
other OB stars, and they are centered within symmetric, round HII regions. They
show no evidence of bow shocks, implying that the targets are not transverse
runaway stars. Their radial velocities relative to local HI also indicate that
they are not line-of-sight runaway stars. A friends-of-friends analysis shows
that 9 of the objects present a few low-mass companion stars, with typical mass
ratios for the two highest-mass stars of around 0.1. This further substantiates
that these OB stars formed in place, and that they can and do form in extremely
sparse conditions. This poses strong constraints on theories of star formation
and challenges proposed relations between cluster mass and maximum stellar
mass.
|
The difference in metallicity distribution functions of halo stars and
globular clusters as a function of galaxy type: A tracer of globular cluster
formation and evolution | Observations of globular clusters (GCs) and field stars in the halos of the
giant elliptical galaxy Cen A and the spiral galaxy M31 show a large range of
cluster-to-star number ratios ('specific frequencies'). The cluster-to-star
ratio decreases with increasing metallicity by a factor of 100-1000, at all
galactocentric radii and with a slope that does not seem to depend on radius.
In dwarf galaxies, the GCs are also more metal-poor than the field stars on
average. These observations indicate a strong dependence of either the cluster
formation efficiency or the cluster destruction rate on metallicity and
environment. We aim to explain these trends by considering various effects that
may influence the observed cluster-to-star ratio as a function of metallicity,
environment and cosmological history. We show that both the cluster formation
efficiency and the maximum cluster mass increase with metallicity, so they
cannot explain the observed trend. Destruction of GCs by tidal stripping and
dynamical friction destroy clusters over too small a range of galactocentric
radii. We show that cluster destruction by tidal shocks from giant molecular
clouds in the high-density formation environments of GCs becomes increasingly
efficient towards high galaxy masses and, hence, towards high metallicities.
The predicted cluster-to-star ratio decreases by a factor 100-1000 towards high
metallicities and should only weakly depend on galactocentric radius due to
orbital mixing during hierarchical galaxy merging, consistent with the
observations. The observed, strong dependence of the cluster-to-star ratio on
metallicity and the independence of its slope on galactocentric radius can be
explained by cluster destruction and hierarchical galaxy growth. As a result,
we find that the metallicity-dependence of the cluster-to-star ratio does not
reflect a GC formation efficiency, but a survival fraction. (Abridged)
|
Fully resolved array of simulations investigating the influence of the
magnetic Prandtl number on magnetohydrodynamic turbulence | We explore the effect of the magnetic Prandtl number Pm on energy and
dissipation in fully resolved direct numerical simulations of steady-state,
mechanically forced homogeneous magnetohydrodynamic turbulence in the range Pm
= 1/32 to 32. We compare the spectra and show that if the simulations are not
fully resolved, the steepness of the scaling of the kinetic-to-magnetic
dissipation ratio with Pm is overestimated. We also present results of decaying
turbulence with helical and nonhelical magnetic fields, where we find
nonhelical reverse spectral transfer for Pm < 1. The results of this systematic
analysis have applications including stars, planetary dynamos, and accretion
disks.
|
Revealing the dust grain size in the inner envelope of the Class I
protostar Per-emb-50 | A good constraint of when the growth of dust grains from sub-micrometer to
millimeter sizes occurs, is crucial for planet formation models. This provides
the first step towards the production of pebbles and planetesimals in
protoplanetary disks. Currently, it is well established that Class II objects
have large dust grains. However, it is not clear when in the star formation
process this grain growth occurs. We use multi-wavelength millimeter
observations of a Class I protostar to obtain the spectral index of the
observed flux densities $\alpha_\mathrm{mm}$ of the unresolved disk and the
surrounding envelope. Our goal is to compare our observational results with
visibility modeling at both wavelengths simultaneously. We present data from
NOEMA at 2.7 mm and SMA at 1.3 mm of the Class I protostar, Per-emb-50. We
model the dust emission with a variety of parametric and radiative transfer
models to deduce the grain size from the observed emission spectral index. We
find a spectral index in the envelope of Per-emb-50 of $\alpha_{\rm
env}$=$3.3\pm0.3$, similar to the typical ISM values. The radiative transfer
modeling of the source confirms this value of $\alpha_{\rm env}$ with the
presence of dust with a $a_\mathrm{max}$$\leq$100 $\mu$m. Additionally, we
explore the backwarming effect, where we find that the envelope structure
affects the millimeter emission of the disk. Our results reveal grains with a
maximum size no larger than $100$ $\mu$m in the inner envelope of the Class I
protostar Per-emb-50, providing an interesting case to test the universality of
millimeter grain growth expected in these sources.
|
The Runaway Binary LP 400-22 is Leaving the Galaxy | We present optical spectroscopy, astrometry, radio, and X-ray observations of
the runaway binary LP 400-22. We refine the orbital parameters of the system
based on our new radial velocity observations. Our parallax data indicate that
LP 400-22 is significantly more distant (3 sigma lower limit of 840 pc) than
initially predicted. LP 400-22 has a tangential velocity in excess of 830 km/s;
it is unbound to the Galaxy. Our radio and X-ray observations fail to detect a
recycled millisecond pulsar companion, indicating that LP 400-22 is a double
white dwarf system. This essentially rules out a supernova runaway ejection
mechanism. Based on its orbit, a Galactic center origin is also unlikely.
However, its orbit intersects the locations of several globular clusters;
dynamical interactions between LP 400-22 and other binary stars or a central
black hole in a dense cluster could explain the origin of this unusual binary.
|
Inflection-point Higgs Inflation | Inflection-point inflation is an interesting possibility to realize a
successful slow-roll inflation when inflation is driven by a single scalar
field with its initial value below the Planck mass ($\phi_I \lesssim M_{Pl}$).
In order for a renormalization group (RG) improved effective $\lambda \phi^4$
potential to develop an inflection-point, the quartic coupling $\lambda(\phi)$
must exhibit a minimum with an almost vanishing value in its RG evolution,
namely $\lambda(\phi_I) \simeq 0$ and $\beta_{\lambda}(\phi_I) \simeq 0$, where
$\beta_{\lambda}$ is the beta-function of the quartic coupling. As an example,
we consider the minimal gauged $B-L$ extended Standard Model at the TeV scale,
where we identify the $B-L$ Higgs field as the inflaton field. For a successful
inflection-point inflation, which is consistent with the current cosmological
observations, the mass ratios among the $Z^{\prime}$ gauge boson, the
right-handed neutrinos and the $B-L$ Higgs boson are fixed. Our scenario can be
tested in the future collider experiments such as the High-Luminosity LHC and
the SHiP experiments. In addition, the inflection-point inflation provides a
unique prediction for the running of the spectral index $\alpha \simeq - 2.7
\times 10^{-3}\left(\frac{60}{N}\right)^2$ ($N$ is the $e$-folding number),
which can be tested in the near future.
|
Improved CMB Map from WMAP Data | The cosmic microwave background (CMB) temperature maps published by the
Wilkinson Microwave Anisotropy Probe (WMAP) team are found to be inconsistent
with the differential time-ordered data (TOD), from which the maps are
reconstructed. The inconsistency indicates that there is a serious problem in
the map making routine of the WMAP team, and it is necessary to reprocess the
WMAP data. We develop a self-consistent software package of map-making and
power spectrum estimation independently of the WMAP team. Our software passes a
variety of tests. New CMB maps are then reconstructed, which are significantly
different from the official WMAP maps. In the new maps, the inconsistency
disappeared, along with the hitherto unexplained high level alignment between
the CMB quadrupole and octopole components detected in released WMAP maps. An
improved CMB cross-power spectrum is then derived from the new maps which
better agrees with that of BOOMRANG. Two important results are hence obtained:
the CMB quadrupole drops to nearly zero, and the power in multiple moment range
between 200 and 675 decreases on average by about 13%, causing the best-fit
cosmological parameters to change considerably, e.g., the total matter density
increases from 0.26 up to 0.32 and the dark energy density decreases from 0.74
down to 0.68. These new parameters match with improved accuracy those of other
independent experiments. Our results indicate that there is still room for
significant revision in the cosmological model parameters.
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