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X-ray, lensing and Sunyaev Zel'dovich triaxial analysis of Abell 1835
out to R_{200} | Measuring the intrinsic shape and orientation of dark matter (DM) and
intracluster (IC) gas in galaxy clusters is crucial to constraining their
formation and evolution, and for enhancing the use of clusters as more precise
cosmological probes. Extending our previous works, we present for the first
time results from a triaxial joint analysis of the galaxy cluster Abell 1835,
by means of X-ray, strong lensing (SL) and Sunyaev Zel'dovich (SZ) data. We
parametrically reconstruct the full three-dimensional structure (triaxial shape
and principal axis orientation) of both the DM and the IC gas, and the level of
non-thermal pressure of the IC gas. We find that the intermediate-major and
minor-major axis ratios of the DM are 0.71+/-0.08 and 0.59+/-0.05,
respectively, and the major axis of the DM halo is inclined with respect to the
line of sight at 18.3+/-5.2 deg. We present the first observational measurement
of the non-thermal pressure out to R_{200}, which has been evaluated to be a
few percent of the total energy budget in the internal regions, while reaching
approximately 20% in the outer volumes. We discuss the implications of our
method for the viability of the CDM scenario, focusing on the concentration
parameter C and the inner slope of the DM gamma in order to test the cold dark
matter (CDM) paradigm for structure formation: we measure gamma=1.01+/-0.06 and
C=4.32+/-0.44, values which are close to the predictions of the CDM model. The
combination of X-ray/SL data at high spatial resolution, capable of resolving
the cluster core, with the SZ data, which are more sensitive to the cluster
outer volume, allows us to characterize the level and the gradient of the gas
entropy distribution and non-thermal pressure out to R_{200}, breaking the
degeneracy among the physical models describing the thermal history of the ICM.
|
IQ Collaboratory II: The Quiescent Fraction of Isolated, Low Mass
Galaxies Across Simulations and Observations | We compare three major large-scale hydrodynamical galaxy simulations (EAGLE,
Illustris-TNG, and SIMBA) by forward modeling simulated galaxies into
observational space and computing the fraction of isolated and quiescent low
mass galaxies as a function of stellar mass. Using SDSS as our observational
template, we create mock surveys and synthetic spectroscopic and photometric
observations of each simulation, adding realistic noise and observational
limits. All three simulations show a decrease in the number of quiescent,
isolated galaxies in the mass range $\mathrm{M}_* = 10^{9-10} \
\mathrm{M}_\odot$, in broad agreement with observations. However, even after
accounting for observational and selection biases, none of the simulations
reproduce the observed absence of quiescent field galaxies below
$\mathrm{M}_*=10^{9} \ \mathrm{M}_\odot$. We find that the low mass quiescent
populations selected via synthetic observations have consistent quenching
timescales, despite apparent variation in the late time star formation
histories. The effect of increased numerical resolution is not uniform across
simulations and cannot fully mitigate the differences between the simulations
and the observations. The framework presented here demonstrates a path towards
more robust and accurate comparisons between theoretical simulations and galaxy
survey observations, while the quenching threshold serves as a sensitive probe
of feedback implementations.
|
Canonical Analysis of Brans-Dicke Theory Addresses Hamiltonian
Inequivalence between Jordan and Einstein Frames | Jordan and Einstein frame are studied under the light of Hamiltonian
formalism. Dirac's constraint theory for Hamiltonian systems is applied to
Brans-Dicke theory in the Jordan Frame. In both Jordan and Einstein frame,
Brans-Dicke theory has four secondary first class constraints and their
constraint algebra is closed. We show, contrary to what is generally believed,
the Weyl (conformal) transformation, between the two frames, is not a canonical
transformation, in the sense of Hamiltonian formalism. This addresses quantum
mechanical inequivalence as well. A canonical transformation is shown.
|
Recovering the observed B/C ratio in a dynamic spiral-armed cosmic ray
model | We develop a fully three dimensional numerical code describing the diffusion
of cosmic rays in the Milky Way. It includes the nuclear spallation chain up to
Oxygen, and allows the study of various cosmic ray properties, such as the CR
age, grammage traversed, and the ratio between secondary and primary particles.
This code enables us to explore a model in which a large fraction of the cosmic
ray acceleration takes place in the vicinity of galactic spiral arms and that
these spiral arms are dynamic. We show that the effect of having dynamic spiral
arms is to limit the age of cosmic rays at low energies. This is because at low
energies the time since the last spiral arm passage governs the Cosmic Ray (CR)
age, and not diffusion. Using the model, the observed spectral dependence of
the secondary to primary ratio is recovered without requiring any further
assumptions such as a galactic wind, re-acceleration or various assumptions on
the diffusivity. In particular, we obtain a secondary to primary ratio which
increases with energy below about 1 GeV.
|
Observables in theories with a varying fine structure constant | We show how two seemingly different theories with a scalar multiplicative
coupling to electrodynamics are actually two equivalent parametrisations of the
same theory: despite some differences in the interpretation of some
phenemenological aspects of the parametrisations, they lead to the same
physical observables. This is illustrated on the interpretation of observations
of the Cosmic Microwave Background.
|
The current best estimate of the Galactocentric distance of the Sun
based on comparison of different statistical techniques | In this paper, the current best estimate of a fundamental Galactic parameter
Galactocentric distance of the Sun $R_0$ has been evaluated using all the
available estimates published during the last 20 years. Unlike some other
studies, our analysis of these results showed no statistically significant
trend in $R_0$ during this period. However we revealed a statistically valuable
improvement in the $R_0$ uncertainties with time of about 0.2 kpc for 20 years.
Several statistical techniques have been used and compared to obtain the most
reliable common mean of 52 determinations made in 1992--2011 and its realistic
uncertainty. The statistical methods used include unweighted mean, seven
variants of the weighted mean, and two variants of median technique. The $R_0$
estimates obtained with these methods range from 7.91 to 8.06 kpc with
uncertainties varying from 0.05 to 0.08 kpc. The final value derived in this
analysis is $R_0 = 7.98 \pm 0.15\,|_{stat} \pm 0.20\,|_{syst}$ kpc, which can
be recommended as the current best estimate of the Galactocentric distance of
the Sun. For most of the practical applications the value of $R_0 = 8.0 \pm
0.25$ kpc can be used.
|
Constraints on interacting dark energy models from Planck 2015 and
redshift-space distortion data | We investigate phenomenological interactions between dark matter and dark
energy and constrain these models by employing the most recent cosmological
data including the cosmic microwave background radiation anisotropies from
Planck 2015, Type Ia supernovae, baryon acoustic oscillations, the Hubble
constant and redshift-space distortions. We find that the interaction in the
dark sector parameterized as an energy transfer from dark matter to dark energy
is strongly suppressed by the whole updated cosmological data. On the other
hand, an interaction between dark sectors with the energy flow from dark energy
to dark matter is proved in better agreement with the available cosmological
observations. This coupling between dark sectors is needed to alleviate the
coincidence problem.
|
The Effects of Radiative Transfer on the PDFs of Molecular MHD
Turbulence | We study the effects of radiative transfer on the Probability Distribution
Functions (PDFs) of simulations of magnetohydrodynamic turbulence in the widely
studied $^{13}$CO 2-1 transition. We find that the integrated intensity maps
generally follow a log-normal distribution, with the cases that have $\tau
\approx 1$ best matching the PDF of the column density. We fit a 2D
variance-sonic Mach number relationship to our logarithmic PDFs of the form
$\sigma_{ln(\Sigma/\Sigma_0)}^2=A\times ln(1+b^2{\cal M}_s^2)$ and find that,
for parameter $b=1/3$, parameter $A$ depends on the radiative transfer
environment. We also explore the variance, skewness, and kurtosis of the linear
PDFs finding that higher moments reflect both higher sonic Mach number and
lower optical depth. Finally, we apply the Tsallis incremental PDF function and
find that the fit parameters depend on both Mach numbers, but also are
sensitive to the radiative transfer parameter space, with the $\tau \approx 1$
case best fitting the incremental PDF of the true column density. We conclude
that, for PDFs of low optical depth cases, part of the gas is always
sub-thermally excited so that the spread of the line intensities exceeds the
spread of the underlying column densities and hence the PDFs do not reflect the
true column density. Similarly, PDFs of optically thick cases are dominated by
the velocity dispersion and therefore do not represent the true column density
PDF. Thus, in the case of molecules like carbon monoxide, the dynamic range of
intensities, structures observed and consequently, the observable PDFs, are
less determined by turbulence and more-often determined by radiative transfer
effects.
|
R-symmetric Axion/Natural Inflation in Supergravity via Deformed Moduli
Dynamics | We construct a natural inflation model in supergravity where the inflaton is
identified with a modulus field possessing a shift symmetry. The superpotential
for the inflaton is generated by meson condensation due to strong dynamics with
deformed moduli constraints. In contrast to models based on gaugino
condensation, the inflaton potential is generated without $R$-symmetry breaking
and hence does not depend on the gravitino mass. Thus, our model is compatible
with low scale supersymmetry.
|
Super star cluster feedback driving ionization, shocks and outflows in
the halo of the nearby starburst ESO 338-IG04 | Stellar feedback strongly affects the interstellar medium (ISM) of galaxies.
Stellar feedback in the first galaxies likely plays a major role in enabling
the escape of LyC photons, which contribute to the re-ionization of the
Universe. Nearby starburst galaxies serve as local analogues allowing for a
spatially resolved assessment of the feedback processes in these galaxies. We
characterize the feedback effects from the star clusters in the local
high-redshift analogue ESO 338-IG04 on the ISM and compare the results with the
properties of the most massive clusters. We use high quality VLT/MUSE optical
integral field data to derive the physical properties of the ISM such as
ionization, density, shocks, and perform new fitting of the spectral energy
distributions of the brightest clusters in ESO 338-IG04 from HST imaging. ESO
338-IG04 has a large ionized halo which we detect to a distance of 9 kpc. We
identify 4 Wolf-Rayet (WR) clusters based on the blue and red WR bump. We
follow previously identified ionization cones and find that the ionization of
the halo increases with distance. Analysis of the galaxy kinematics shows two
complex outflows driven by the numerous young clusters in the galaxy. We find a
ring of shocked emission traced by an enhanced [OI]/H$\alpha$ ratio surrounding
the starburst and at the end of the outflow. Finally we detect nitrogen
enriched gas associated with the outflow, likely caused by the WR stars in the
massive star clusters. Photo-ionization dominates the central starburst and
sets the ionization structure of the entire halo, resulting in a density
bounded halo, facilitating the escape of LyC photons. Outside the central
starburst, shocks triggered by an expanding super bubble become important. The
shocks at the end of the outflow suggest interaction between the hot outflowing
material and the more quiescent halo gas.
|
A Cross-correlation Analysis of AGN and Galaxies using Virtual
Observatory: Dependence on Virial Mass of Super-Massive Black Hole | We present results of the cross-correlation analysis between active galactic
nuclei (AGNs) and galaxies at redshift 0.1-1. We obtain data of ~10,000 SDSS
AGNs in which their virial masses with a super-massive black hole (SMBH) were
estimated. The UKIDSS galaxy samples around the AGNs were obtained using the
virtual observatory. The scale length of AGN-galaxy cross-correlation for all
of the samples is measured to be r_0= 5.8+0.8-0.6 h^-1Mpc (for the fixed slope
parameter \gamma=1.8). We also derived a dependence of r_0 on the BH mass,
M_BH, and found an indication of an increasing trend of r_0 at M_BH > 10^8
Msun. It is suggested that the growth of SMBH is mainly driven by interactions
with the surrounding environment for M_BH > 10^8 Msun. On the other hand, at
M_BH <~10^8 Msun, we did not find the BH mass dependence. This would imply
that, for less massive BHs, the mass growth process can be different from that
for massive BHs.
|
New Perspective on Galaxy Clustering as a Cosmological Probe: General
Relativistic Effects | We present a general relativistic description of galaxy clustering in a FLRW
universe. The observed redshift and position of galaxies are affected by the
matter fluctuations and the gravity waves between the source galaxies and the
observer, and the volume element constructed by using the observables differs
from the physical volume occupied by the observed galaxies. Therefore, the
observed galaxy fluctuation field contains additional contributions arising
from the distortion in observable quantities and these include tensor
contributions as well as numerous scalar contributions. We generalize the
linear bias approximation to relate the observed galaxy fluctuation field to
the underlying matter distribution in a gauge-invariant way. Our full formalism
is essential for the consistency of theoretical predictions. As our first
application, we compute the angular auto correlation of large-scale structure
and its cross correlation with CMB temperature anisotropies. We comment on the
possibility of detecting primordial gravity waves using galaxy clustering and
discuss further applications of our formalism.
|
Quasars as a tracer of large-scale structures in the distant universe | We study the dependence of the number density and properties of quasars on
the background galaxy density using the currently largest spectroscopic
datasets of quasars and galaxies. We construct a galaxy number density field
smoothed over the variable smoothing scale of between approximately 10 and
$20\,h^{-1}$Mpc over the redshift range of $0.46<z<0.59$ using the Sloan
Digital Sky Survey (SDSS) Data Release 12 (DR12) Constant MASS (CMASS)
galaxies. The quasar sample is prepared from the SDSS I/II DR7. We examine the
correlation of incidence of quasars with the large-scale background density and
dependence of quasar properties such as bolometric luminosity, black hole mass,
and Eddington ratio on the large-scale density. We find a monotonic correlation
between the quasar number density and large-scale galaxy number density, which
is fitted well with a power law relation, $n_Q\propto\rho_G^{0.618}$. We detect
weak dependences of quasar properties on the large-scale density such as a
positive correlation between black hole mass and density, and a negative
correlation between luminosity and density. We discuss the possibility of using
quasars as a tracer of large-scale structures at high redshifts, which may be
useful for studies of growth of structures in the high redshift universe.
|
The Quasar Feedback Survey: Revealing the Interplay of Jets, Winds \&
Emission Line Gas in Type 2 Quasars with Radio Polarization | We present results from a combined radio polarization and emission line study
of five type 2 quasars at $z<0.2$ with the Karl G. Jansky Very Large Array
(VLA) B-array at 5 GHz and Hubble Space Telescope (HST) [O~{\sc{iii}}]
observations. These five sources are known to exhibit close association between
radio structures and ionized gas morphology and kinematics. Four sources
(J0945+1737, J1000+1242, J1356+1026 and J1430+1339) show polarization in the
current data. J1010+1413 is the unpolarized source in our sample. We detect
$0.5-1\%$ fractional polarization in the radio cores and a high fractional
polarization ($10-30\%$) in the lobes of these sources. The morphological,
spectral and polarization properties suggest a jet origin for radio emission in
J0945+1737, J1000+1242, J1010+1413 and J1430+1339 whereas the current data
cannot fully discern the origin of radio emission (jet or wind) in J1356+1026.
An anti-correlation between various polarized knots in the radio and
[O~{\sc{iii}}] emission is observed in our sources, similar to that observed in
some radio-loud AGN in the literature. This suggests that the radio emission is
likely to be depolarized by the emission-line gas. By modeling the
depolarization effects, we estimate the size of the emission-line gas clouds to
be $\sim(2.8\pm1.7)\times10^{-5}$ parsec and the amount of thermal material
mixed with the synchrotron plasma to be $\sim(9.2\pm0.8)\times10^{5}$~M$_\odot$
in the lobe of J0945+1737 (which exhibits the most prominent polarization
signature in its lobe). The current work demonstrates that the interplay of
jets/winds and emission-line gas is most likely responsible for the nature of
radio outflows in radio-quiet AGN.
|
Searches for topological defect dark matter via non-gravitational
signatures | We propose schemes for the detection of topological defect dark matter using
pulsars and other luminous extraterrestrial systems via non-gravitational
signatures. The dark matter field, which makes up a defect, may interact with
standard model particles, including quarks and the photon, resulting in the
alteration of their masses. When a topological defect passes through a pulsar,
its mass, radius and internal structure may be altered, resulting in a pulsar
`quake'. A topological defect may also function as a cosmic dielectric material
with a distinctive frequency-dependent index of refraction, which would give
rise to the time delay of a periodic extraterrestrial light or radio signal,
and the dispersion of a light or radio source in a manner distinct to a
gravitational lens. A topological defect passing through Earth may give rise to
temporary non-zero electric dipole moments for an electron, proton, neutron,
nuclei and atoms. The biggest advantage of such astrophysical observations over
recently proposed terrestrial detection methods is the much higher probability
of a defect been found in the vast volumes of outer space compared with one
passing through Earth itself.
|
The structure and evolution of cold dark matter halos | In the standard cosmological model a mysterious cold dark matter (CDM)
component dominates the formation of structures. Numerical studies of the
formation of CDM halos have produced several robust results that allow unique
tests of the hierarchical clustering paradigm. Universal properties of halos,
including their mass profiles and substructure properties are roughly
consistent with observational data from the scales of dwarf galaxies to galaxy
clusters. Resolving the fine grained structure of halos has enabled us to make
predictions for ongoing and planned direct and indirect dark matter detection
experiments.
While simulations of pure CDM halos are now very accurate and in good
agreement (recently claimed discrepancies are addressed in detail in this
review), we are still unable to make robust, quantitative predictions about
galaxy formation and about how the dark matter distribution changes in the
process. Whilst discrepancies between observations and simulations have been
the subject of much debate in the literature, galaxy formation and evolution
needs to be understood in more detail in order to fully test the CDM paradigm.
Whatever the true nature of the dark matter particle is, its clustering
properties must not be too different from a cold neutralino like particle to
maintain all the successes of the model in matching large scale structure data
and the global properties of halos which are mostly in good agreement with
observations.
|
Overlapping abundance gradients and azimuthal gradients related to the
spiral structure of the Galaxy | The connection between some features of the metallicity gradient in the
Galactic disk, best revealed by Open Clusters and Cepheids, and the spiral
structure, is explored. The step-like abrupt decrease in metallicity at 8.5 kpc
(with R_0= 7.5 kpc, or at 9.5 kpc if R_0 = 8.5 kpc is adopted) is well
explained by the corotation ring-shaped gap in the density of gas, which
isolates the internal and external regions of the disk one from the other. This
solves a long standing problem of understanding the different chemical
characteristics of the inner and outer parts of the disk. The time required to
build up the metallicity difference between the two sides of the step is a
measure of the minimal life-time of the present grand-design spiral pattern
structure, of the order of 3 Gyr. The plateaux observed on each side of the
step are interpreted in terms of the large scale radial motion of the stars and
of the gas flow induced by the spiral structure. The star-formation rate
revealed by the density of open clusters is maximum in the Galactic radial
range from 6 to 12 kpc (with an exception of a narrow gap at corotation),
coinciding with the region where the 4-arms mode is allowed to exist. We argue
that most of the old open clusters situated at large galactocentric radii were
born in this inner region where conditions more favorable to star-formation are
found. The ratio of $\alpha$-elements to Fe of the sample of Cepheids does not
vary appreciably with the Galactic radius, which reveals an homogeneous history
of star formation. Different arguments are given showing that usual
approximations of chemical evolution models, which assume fast mixing of
metallicity in the azimuthal direction and ignore the existence of the spiral
arms, are a poor ones.
|
The effect of the environment on the Faber Jackson relation | We investigate the effect of the environment on the Faber Jackson (FJ)
relation, using a sample of 384 nearby elliptical galaxies and estimating
objectively their environment on the typical scale of galaxy clusters. We show
that the intrinsic scatter of the FJ is significantly reduced when ellipticals
in high density environments are compared to ellipticals in low density ones.
This result, which holds on a limited range of overdensities, is likely to
provide an important observational link between scaling relations and formation
mechanisms in galaxies.
|
Existence of the Metal-Rich Stellar Halo and High-velocity Thick Disk in
the Galaxy | Based on the second Gaia data release (DR2), combined with the LAMOST and
APOGEE spectroscopic surveys, we study the kinematics and metallicity
distribution of the high-velocity stars that have a relative speed of at least
220 ${\rm km\ s^{-1}}$ with respect to the local standard of rest in the
Galaxy. The rotational velocity distribution of the high-velocity stars with
[Fe/H] $>-1.0$ dex can be well described by a two-Gaussian model, with peaks at
$V_{\phi}\sim +164.2\pm0.7$ and $V_{\phi}\sim +3.0\pm0.3$ ${\rm km\ s^{-1}}$,
associated with the thick disk and halo, respectively. This implies that there
should exist a high-velocity thick disk (HVTD) and a metal-rich stellar halo
(MRSH) in the Galaxy. The HVTD stars have the same position as the halo in the
Toomre diagram and but show the same rotational velocity and metallicity as the
canonical thick disk. The MRSH stars have basically the same rotational
velocity, orbital eccentricity, and position in the Lindblad and Toomre diagram
as the canonical halo stars, but they are more metal-rich. Furthermore, the
metallicity distribution function (MDF) of our sample stars are well fitted by
a four-Gaussian model, associated with the outer-halo, inner-halo, MRSH, and
HVTD, respectively. Chemical and kinematic properties and age imply that the
MRSH and HVTD stars may form in situ.
|
The Ejecta Distributions of the Heavy Elements in the Cygnus Loop | We analyzed the metal distribution of the Cygnus Loop using 14 and 7
pointings observation data obtained by the \textit{Suzaku} and the
\textit{XMM-Newton} observatories. The spectral analysis shows that all the
spectra are well fitted by the two-$kT_e$ non-equilibrium ionization plasma
model as shown by the earlier observations. From the best-fit parameters of the
high-$kT_e$ component, we calculated the emission measures about various
elements and showed the metal distribution of the ejecta component. We found
that the distributions of Si and Fe are centered at the southwest of the
geometric center toward the blow-out region. From the best-fit parameters, we
also estimated the progenitor mass of the Cygnus Loop from our field of view
and the metal rich region with a radius of 25 arcmin from the metal center. The
result from the metal circle is similar to that from our entire FOV, which
suggests the mixing of the metal. From the results, we estimated the mass of
the progenitor star at 12-15\MO.
|
Planets Across Space and Time (PAST). III. Morphology of the Planetary
Radius Valley as a Function of Stellar Age and Metallicity in the Galactic
Context Revealed by the LAMOST-Gaia-Kepler Sample | The radius valley, a dip in the radius distribution of exoplanets at ~1.9
Earth radii separates compact rocky Super-Earths and Sub-Neptunes with lower
density. Various hypotheses have been put forward to explain the radius valley.
Characterizing the radius valley morphology and its correlation to stellar
properties will provide crucial observation constraints on its origin mechanism
and deepen the understanding of planet formation and evolution. In this paper,
the third part of the Planets Across the Space and Time (PAST) series, using
the LAMOST-Gaia-Kepler catalog, we perform a systematical investigation into
how the radius valley morphology varies in the Galactic context, i.e.,
thin/thick galactic disks, stellar age and metallicity abundance ([Fe/H] and
[alpha/Fe]). We find that (1) The valley becomes more prominent with the
increase of both age and [Fe/H]. (2) The number ratio of super-Earths to
sub-Neptunes monotonically increases with age but decreases with [Fe/H] and
[alpha/Fe]. (3) The average radius of planets above the valley (2.1-6 Earth
radii) decreases with age but increases with [Fe/H]. (4) In contrast, the
average radius of planets below the valley (R < 1.7 Earth radii) is broadly
independent on age and metallicity. Our results demonstrate that the valley
morphology as well as the whole planetary radius distribution evolves on a long
timescale of giga-years, and metallicities (not only Fe but also other metal
elements, e.g., Mg, Si, Ca, Ti) play important roles in planet formation and in
the long term planetary evolution.
|
Constraining ultra large-scale cosmology with multiple tracers in
optical and radio surveys | Multiple tracers of the cosmic density field, with different bias, number and
luminosity evolution, can be used to measure the large-scale properties of the
Universe. We show how an optimal combination of tracers can be used to detect
general-relativistic effects in the observed density of sources. We forecast
for the detectability of these effects, as well as measurements of primordial
non-Gaussianity and large-scale lensing magnification with current and upcoming
large-scale structure experiments. In particular we quantify the significance
of these detections in the short term with experiments such as the Dark Energy
Survey (DES), and in the long term with the Large Synoptic Survey Telescope
(LSST) and the Square Kilometre Array (SKA). We review the main observational
challenges that must be overcome to carry out these measurements.
|
Sterile Neutrino Status | We review the results of global analyses of short-baseline neutrino
oscillation data in 3+1, 3+2 and 3+1+1 neutrino mixing schemes.
|
Photoionization models for extreme Ly$\alpha$ $\lambda$1216 and HeII
$\lambda$1640 ratios in quasar halos, and PopIII vs AGN diagnostics | We explore mechanisms to produce extremely high Ly-alpha/HeII flux ratios, or
to enhance the observed number of Ly-alpha photons per incident ionizing
photon, in extended AGN-photoionized nebulae at high-redshift. Using
photoionization models, we explore the impact of ionization parameter, gas
metallicity, ionizing spectrum, electron energy distribution, and cloud viewing
angle on the relative fluxes of Ly-alpha, HeII and other lines, and on the
observed number of Ly-alpha photons per incident ionizing photon. We find that
low ionization parameter, a relatively soft or filtered ionizing spectrum, low
gas metallicity, kappa-distributed electron energies, or reflection of Ly-alpha
photons by HI can all result in significantly enhanced Ly-alpha relative to
other lines (>10%), with log Ly-alpha/HeII reaching values up to 4.6. In the
cases of low gas metallicity, reflection by HI, or a hard or filtered ionizing
spectrum, the observed number of Ly-alpha photons per incident ionizing photon
is itself significantly enhanced above the nominal Case B value of 0.66 due to
collisional excitation, reaching values up to 5.3 in our 'extreme case' model.
At low gas metallicity (e.g. 0.1 x Solar), the production of Ly-alpha is
predominantly via collisional excitation rather than recombination. In
addition, we find that collisional excitation of Ly-alpha becomes more
efficient if the ionizing continuum is pre-filtered through an optically thin
screen of gas closer to the AGN. We also show that Ly-alpha / HeII ratios of
the z~3.5 quasars studied by Borisova et al. (2016) are consistent with
AGN-photoionization of gas with moderate to low metallicity and/or low
ionization parameter, without requiring exotic ionization/excitation mechanisms
such as strong line-transfer effects. We also present UV-optical diagnostic
diagrams to distinguish between photoionization by Pop III stars and AGN
photoionization.
|
A search for steep spectrum radio relics and halos with the GMRT | Context: Diffuse radio emission, in the form of radio halos and relics,
traces regions in clusters with shocks or turbulence, probably produced by
cluster mergers. Some models of diffuse radio emission in clusters indicate
that virtually all clusters should contain diffuse radio sources with a steep
spectrum. External accretion shocks associated with filamentary structures of
galaxies could also accelerate electrons to relativistic energies and hence
produce diffuse synchrotron emitting regions. Here we report on Giant Metrewave
Radio Telescope (GMRT) observations of a sample of steep spectrum sources from
the 74 MHz VLSS survey. These sources are diffuse and not associated with
nearby galaxies.
Aims: The main aim of the observations is to search for diffuse radio
emission associated with galaxy clusters or the cosmic web.
Methods: We carried out GMRT 610 MHz continuum observations of unidentified
diffuse steep spectrum sources.
Results: We have constructed a sample of diffuse steep spectrum sources,
selected from the 74 MHz VLSS survey. We identified eight diffuse radio sources
probably all located in clusters. We found five radio relics, one cluster with
a giant radio halo and a radio relic, and one radio mini-halo. By complementing
our observations with measurements from the literature we find correlations
between the physical size of relics and the spectral index, in the sense that
smaller relics have steeper spectra. Furthermore, larger relics are mostly
located in the outskirts of clusters while smaller relics are located closer to
the cluster center.
|
A Herschel view of the far-infrared properties of submillimetre galaxies | We study a sample of 61 submillimetre galaxies (SMGs) selected from
ground-based surveys, with known spectroscopic redshifts and observed with
Herschel as part of the PACS Evolutionary Probe (PEP) and the Herschel
Multi-tiered Extragalactic Survey (HerMES) key programmes. We use the broad
far-infrared wavelength coverage (100-600um) provided by the combination of
PACS and SPIRE observations. Using a power-law temperature distribution model
to derive infrared luminosities and dust temperatures, we measure a dust
emissivity spectral index for SMGs of beta=2.0+/-0.2. Our results unveil the
diversity of the SMG population. Some SMGs exhibit extreme infrared
luminosities of ~10^13 Lsun and relatively warm dust components, while others
are fainter (~10^12 Lsun) and are biased towards cold dust temperatures. The
extreme infrared luminosities of some SMGs (LIR>10^12.7 Lsun, 26/61 systems)
imply SFRs of >500Msun yr^-1. Such high SFRs are difficult to reconcile with a
secular mode of star formation, and may instead correspond to a merger-driven
stage in the evolution of these galaxies. Another observational argument in
favour of this scenario is the presence of dust temperatures warmer than that
of SMGs of lower luminosities (~40K as opposed to ~25K), consistent with
observations of local ULIRGs triggered by major mergers and with results from
hydrodynamic simulations of major mergers combined with radiative transfer
calculations. Luminous SMGs are also offset from normal star-forming galaxies
in the stellar mass-SFR plane, suggesting that they are undergoing starburst
events with short duty cycles, compatible with the major merger scenario. On
the other hand, a significant fraction of the low infrared luminosity SMGs have
cold dust temperatures, are located close to the main sequence of star
formation, and thus might be evolving through a secular mode of star formation.
[abridged]
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The Evolution of Luminous Red Nova AT 2017jfs in NGC 4470 | We present the results of our photometric and spectroscopic follow-up of the
intermediate-luminosity optical transient AT 2017jfs. At peak, the object
reaches an absolute magnitude of Mg=-15.46+-0.15 mag and a bolometric
luminosity of 5.5x10^41 erg/s. Its light curve has the double-peak shape
typical of Luminous Red Novae (LRNe), with a narrow first peak bright in the
blue bands, while the second peak is longer lasting and more luminous in the
red and near-infrared (NIR) bands. During the first peak, the spectrum shows a
blue continuum with narrow emission lines of H and Fe II. During the second
peak, the spectrum becomes cooler, resembling that of a K-type star, and the
emission lines are replaced by a forest of narrow lines in absorption. About 5
months later, while the optical light curves are characterized by a fast linear
decline, the NIR ones show a moderate rebrightening, observed until the
transient disappeared in solar conjunction. At these late epochs, the spectrum
becomes reminiscent of that of M-type stars, with prominent molecular
absorption bands. The late-time properties suggest the formation of some dust
in the expanding common envelope or an IR echo from foreground pre-existing
dust. We propose that the object is a common-envelope transient, possibly the
outcome of a merging event in a massive binary, similar to NGC4490-2011OT1.
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