16012862
The Southern Galactic Plane Survey: H I Observations and Analysis
We describe the H I component of the Southern Galactic Plane Survey (SGPS). The SGPS is a large-scale project to image at arcminute resolution the H I spectral line and 21 cm continuum emission in parts of the plane of the Milky Way. The survey covers Galactic longitudes 253° ≤ l ≤ 358° and latitudes |b| ≤ 15 (SGPS I), plus a first quadrant extension covering 5° ≤ l ≤ 20° and |b| ≤ 15 (SGPS II). The survey combines data from the Australia Telescope Compact Array and the Parkes Radio Telescope for sensitivity to angular scales ranging from 2' to several degrees. The combined data cover 325 deg2 and have an rms sensitivity of 1.6 K. Here we describe the H I observations and data reduction in detail and present examples from the final data products. The complete data set is publicly available through the Australia Telescope National Facility's H I Surveys archive. This data set provides an unprecedented view of the neutral component of interstellar hydrogen in the inner Milky Way.
7431869
Magnetic fields and ionized gas in the inner galaxy: An outer scale for turbulence and the possible role of H II regions
We present an analysis of rotation measure (RM) fluctuations from the test region of the Southern Galactic Plane Survey (SGPS), along with emission measure (EM) fluctuations in the same field taken from the Southern H? Sky Survey Atlas. The structure function of RM fluctuations shows a relatively steep slope at small scales (1'-5'), a break in slope to a flatter structure function at intermediate scales (5'-60'), and a systematic variation of the strength of fluctuations as a function of position angle on the sky at the largest scales (60'-200'). The structure function of EM fluctuations shows similar behavior, although the lower resolution of the data prevents detection of a possible break in the spectrum. We interpret the anisotropy in RM/EM structure on large scales as resulting from a large-scale gradient in electron density (and possibly magnetic field) across the region. The break in the slope of the RM structure function at scales of ~5' can be explained by contributions from two spatially distinct magnetoionized screens, most likely in the Local and Carina spiral arms. The observed structure function then implies that the outer scale of RM fluctuations in these screens is ~2 pc. Such behavior is in striking contrast to the expectation that interstellar turbulence forms an unbroken spectrum from kiloparsec down to AU scales. We conclude that we have identified an additional source of enhanced turbulence, injected on scales of a few pc, possibly seen only in the Galactic plane. The most likely source of such turbulence is individual H II regions from relatively low mass stars, whose characteristic scale size is similar to the outer scale of turbulence inferred here. These sources may be the dominant source of density and velocity fluctuations in warm ionized gas in the Galactic plane.
117355245
Radio Observations of Masers in Post-AGB Stars
We present observations of 86 post-Asymptotic Giant Branch (post-AGB) stars of OH maser transitions, taken with the Parkes Telescope between September 2002 and August 2003. Post-AGB stars are the precursors of planetary nebulae, which have a wide range of morphologies that are not well explained. By studying the circumstellar envelopes of post-AGB stars through the masers produced in them, we hope to shed light on the origin of planetary nebula morphologies.
16776986
The Southern Galactic Plane Survey: the test region
The Southern Galactic Plane Survey (SGPS) is a project to image the H I line emission and 1.4 GHz continuum in the fourth quadrant of the Milky Way at high resolution using the Australia Telescope Compact Array and the Parkes Radio Telescope. This paper describes the survey details and goals, presents 21 cm continuum data, and discusses H I absorption and emission characteristics of the SGPS test region (325.5d<=l<=333.5d -0.5d<=b<=+3.5d). We explore the effects of massive stars on the interstellar medium through a study of H I shells and the H I environments of H II regions and supernova remnants (SNRs). We find an H I shell surrounding the H II region RCW 94, which indicates that the region is embedded in a molecular cloud. We give lower limits for the kinematic distances to SNRs G327.4+0.4 and G330.2+1.0 of 4.3 and 4.9 kpc, respectively. We find evidence of interaction with the surrounding H I for both of these remnants. We also present images of a possible new SNR G328.6-0.0. Additionally, we have discovered two small H I shells with no counterparts in continuum emission.
118984034
The Molonglo Galactic Plane Survey. I. Overview and Images
The first-epoch Molonglo Galactic Plane Survey is a radio continuum survey made using the Molonglo Observatory Synthesis Telescope at 843 MHz with a resolution of 43''×43'' csc|δ|. The region surveyed is 245°≤l≤355°, |b|≤1.5°. The 13 9°×3° mosaic images presented here are the superposition of over 450 complete synthesis observations, each taking 12 h and covering a field of 70'×70' csc|δ|. The root-mean-square sensitivity over much of the mosaicked survey is 1-2 mJy beam-1 (1 σ), and the positional accuracy is ≈ 1''×1'' csc|δ| for sources brighter than 20 mJy. The dynamic range is no better than 250:1, and this also constrains the sensitivity in some parts of the images. The survey area of 330 deg2 contains well over 1.2×104 unresolved or barely resolved objects, almost all of which are extragalactic sources lying in the zone of avoidance. In addition, a significant fraction of this area is covered by extended, diffuse emission associated with thermal complexes, discrete H II regions, supernova remnants, and other structures in the Galactic interstellar medium.
16815360
H I-bright Galaxies in the Southern Zone of Avoidance
A blind survey for H I–bright galaxies in the southern zone of avoidance (212° ≤ l ≤ 36°, |b| ≤ 5°) has been made with the 21 cm multibeam receiver on the Parkes 64 m radio telescope. The survey, sensitive to normal spiral galaxies to a distance of ~40 Mpc and more nearby dwarfs, detected 110 galaxies. Of these, 67 have no counterparts cataloged in the NASA/IPAC Extragalactic Database. In general, the uncataloged galaxies lie behind thicker obscuration than do the cataloged objects. All the newly discovered galaxies have H I flux integrals that are more than an order of magnitude lower than those of the Circinus galaxy. The survey recovers the Puppis Cluster and foreground group, and the Local Void remains empty. The H I mass function derived for the sample is satisfactorily fitted by a Schechter function with parameters α = 1.51 ± 0.12, Φ* = 0.006 ± 0.003, and log M* = 9.7 ± 0.10.
119187222
Ultrafine dark matter structure in the solar neighbourhood
Dark matter plays a fundamental role in theories of the formation and evolution of galaxies. Thus every attempt to model galaxy formation and evolution has to take into consideration the presence of dark halos. Moreover, mergers and accretion appear to be fundamental driving mechanisms in determining the present day properties of galaxies.
The aim of this thesis is to study the ultra-fine distribution of dark matter in the Solar neighbourhood, and to investigate the implications for the current and next generation of dark matter directional detectors. For this purpose we develop a model for halo mergers in a Milky Way-like galaxy. The signals expected in lab-based dark matter detection experiments depend on the phase-space distribution on sub-milliparsec scales. With our numerical technique it is possible to resolve structures produced by minor mergers of subhalos with a larger parent halo. This type of substructure is unaccessible to conventional N-body simulations. When applied in a cosmological context,this method becomes a powerful instrument to reproduce and analyse the complete multiple merger history of a Milky Way-like system.
The results obtained simulating the Galactic halo suggest that the velocity distribution in the solar neighbourhood after an evolution time corresponding to the lifetime of our galaxy (≃ 14Gyr) is smooth. This result suggests the presence of a huge number of dark matter streams that overlap to form a smooth distribution. Nevertheless, the final velocity distribution has overdensities for all the cases that has been analysed. They are generated by a very large number of merger events, but the current generation of detectors have not the angular resolution required to observe these features. A future generation of detectors with a resolution of ~ 1◦ would start to resolve them, allowing the merger history of the Galaxy to begin to be unravelled using this diagnostic.
11640854
Critical collapse and the primordial black hole initial mass function
It has normally been assumed that primordial black holes (PBHs) always form with a mass approximately equal to the mass contained within the horizon at that time. Recent work studying the application of critical phenomena in gravitational collapse to PBH formation has shown that in fact, at a fixed time, PBHs with a range of masses are formed. When calculating the PBH initial mass function it is usually assumed that all PBHs form at the same horizon mass. It is not clear, however, that it is consistent to consider the spread in the mass of PBHs formed at a single horizon mass, whilst neglecting the range of horizon masses at which PBHs can form. We use the excursion set formalism to compute the PBH initial mass function, allowing for PBH formation at a range of horizon masses, for two forms of the density perturbation spectrum. First we examine power-law spectra with $ng1,$ where PBHs form on small scales. We find that, in the limit where the number of PBHs formed is small enough to satisfy the observational constraints on their initial abundance, the mass function approaches that found by Niemeyer and Jedamzik under the assumption that all PBHs form at a single horizon mass. Second, we consider a flat perturbation spectrum with a spike at a scale corresponding to horizon mass $\ensuremath{\sim}{0.5M}_{\ensuremath{\bigodot}},$ and compare the resulting PBH mass function with that of the MACHOs (massive compact halo objects) detected by microlensing observations. The predicted mass spectrum appears significantly wider than the steeply falling spectrum found observationally.
119363323
Effect of halo modelling on WIMP exclusion limits
WIMP direct detection experiments are just reaching the sensitivity required to detect galactic dark matter in the form of neutralinos. Data from these experiments are usually analysed under the simplifying assumption that the Milky Way halo is an isothermal sphere with maxwellian velocity distribution. Observations and numerical simulations indicate that galaxy halos are in fact triaxial and anisotropic. Furthermore, in the cold dark matter paradigm galactic halos form via the merger of smaller subhalos, and at least some residual substructure survives. We examine the effect of halo modelling on WIMP exclusion limits, taking into account the detector response. Triaxial and anisotropic halo models, with parameters motivated by observations and numerical simulations, lead to significant changes which are different for different experiments, while if the local WIMP distribution is dominated by small scale clumps then the exclusion limits are changed dramatically.
53576835
Primordial black holes as a tool for constraining non-Gaussianity
Primordial Black Holes (PBH's) can form in the early Universe from thecollapse of large density fluctuations. Tight observational limits on theirabundance constrain the amplitude of the primordial fluctuations on very smallscales which can not otherwise be constrained, with PBH's only forming from theextremely rare large fluctuations. The number of PBH's formed is thereforesensitive to small changes in the shape of the tail of the fluctuationdistribution, which itself depends on the amount of non-Gaussianity present. Westudy, for the first time, how quadratic and cubic local non-Gaussianity ofarbitrary size (parameterised by f_nl and g_nl respectively) affects the PBHabundance and the resulting constraints on the amplitude of the fluctuations onvery small scales. Intriguingly we find that even non-linearity parameters oforder unity have a significant impact on the PBH abundance. The sign of thenon-Gaussianity is particularly important, with the constraint on the allowedfluctuation amplitude tightening by an order of magnitude as f_nl changes fromjust -0.5 to 0.5. We find that if PBH's are observed in the future, thenregardless of the amplitude of the fluctuations, non-negligible negative f_nlwould be ruled out. Finally we show that g_nl can have an even larger effect onthe number of PBH's formed than f_nl.
17400054
Modelling ultrafine structure in dark matter haloes
Various laboratory-based experiments are underway attempting to detect dark matter directly. The event rates and detailed signals expected in these experiments depend on the dark matter phase-space distribution on submilliparsec scales. These scales are many orders of magnitude smaller than those that can be resolved by conventional N-body simulations, so one cannot hope to use such tools to investigate the effect of mergers in the history of the Milky Way on the detailed phase-space structure probed by the current experiments. In this paper, we present an alternative approach to investigate the results of such mergers, by studying a simplified model for a merger of a subhalo with a larger parent halo. With an appropriate choice of parent halo potential, the evolution of material from the subhalo can be expressed analytically in action-angle variables, so it is possible to obtain its entire orbit history very rapidly without numerical integration. Furthermore by evolving backwards in time, we can obtain arbitrarily high spatial resolution for the current velocity distribution at a fixed point. Although this model cannot provide a detailed quantitative comparison with the Milky Way, its properties are sufficiently generic that it offers qualitative insight into the expected structure arising from a merger at a resolution that cannot be approached with full numerical simulations. Preliminary results indicate that the velocity-space distribution of dark matter particles remains characterized by discrete and well-defined peaks over an extended period of time, both for single and multimerging systems, in contrast to the simple smooth velocity distributions sometimes assumed in predicting laboratory experiment detection rates. In principle, this structure contains a wealth of information about the formation history of the Milky Way's dark halo.