18300265
Distant Extended Spiral Arm in the Fourth Quadrant of the Milky
Using data from the Southern Galactic Plane Survey we present a possible distant spiral arm in the fourth quadrant of the Milky Way. The very distinct and cohesive feature can be traced for over 70 • as the most extreme positive velocity feature in the longitude-velocity diagram. The feature is at a Galactic radius between 18 and 24 kpc and appears to be the last major structure before the end of the H i disk. We compare the feature with a Galactic spiral model and show that it is well reproduced by a spiral arm of pitch angle, i ∼ 9 •. The arm is quite well confined to the Galactic plane, dropping at most 1 kpc below the Galactic equator. Over most of its length the arm is 1-2 kpc thick.
7277352
A Neutral Hydrogen Self-Absorption Cloud in the SGPS
Using data from the Southern Galactic Plane Survey (SGPS) we analyze an HI self-absorption cloud centered on l = 318.0 • , b =-0.5 • , and velocity, v =-1.1 km s −1. The cloud was observed with the Australia Telescope Compact Array (ATCA) and the Parkes Radio Telescope, and is at a near kinematic distance of 400 pc with derived dimensions of 5 × 11 pc. We apply two different methods to find the optical depth and spin temperature. In both methods we find upper limit spin temperatures ranging from 20 K to 25 K and lower limit optical depths ∼ 1. We look into the nature of the HI emission and find that 60-70% originates behind the cloud. We analyze a second cloud at the same velocity centered on l = 319 • and b = 0.4 • with an upper limit spin temperature of 20 K and a lower limit optical depth of 1.6. The similarities in spin temperature, optical depth, velocity, and spatial location are evidence the clouds are associated, possibly as one large cloud consisting of smaller clumps of gas. We compare HI emission data with 12 CO emission and find a physical association of the HI self-absorption cloud with molecular gas. – 2 –
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.
17676935
ENHANCED SMALL-SCALE FARADAY ROTATION IN THE GALACTIC SPIRAL ARMS
We present an analysis of the rotation measures (RMs) of polarized extragalactic point sources in the Southern Galactic Plane Survey. This work demonstrates that the statistics of fluctuations in RM differ for the spiral arms and the interarm regions. Structure functions of RM are flat in the spiral arms, while they increase in the interarms. This indicates that there are no correlated RM fluctuations in the magnetoionized interstellar medium in the spiral arms on scales larger than ~05, corresponding to ~17 pc in the nearest spiral arm probed. The nonzero slopes in interarm regions imply a much larger scale of RM fluctuations. We conclude that fluctuations in the magnetoionic medium in the Milky Way spiral arms are not dominated by the mainly supernova-driven turbulent cascade in the global ISM but are probably due to a different source, most likely H II regions.
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.
123769958
OPTIMIZING WIMP DIRECTIONAL DETECTORS
117822403
The first WIMPy halos
The dark matter distribution on small scales may depend on the properties of the first generation of dark matter halos to form, which is in turn determined by the microphysics of the dark matter particles. We overview the microphysics of WIMPs and the calculation of the collisional damping and free streaming scales. We then present the resulting density perturbation power spectrum and the red‐shift at which typical halos form, taking into account the effect of uncertainties in the WIMP properties (mass and interaction channel) and the primordial power spectrum. Finally we review simulations of the formation of the first WIMPy halos and discuss their subsequent evolution, in particular disruption due to encounters with stars.
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.
119454410
Calculating exclusion limits for weakly interacting massive particle direct detection experiments without background subtraction
Competitive limits on the weakly interacting massive particle (WIMP) spin-independent scattering cross section are currently being produced by ${}^{76}\mathrm{Ge}$ detectors originally designed to search for neutrinoless double beta decay, such as the Heidelberg-Moscow and IGEX experiments. In the absence of background subtraction, limits on the WIMP interaction cross section are set by calculating the upper confidence limit on the theoretical event rate, given the observed event rate. The standard analysis technique involves calculating the $90%$ upper confidence limit on the number of events in each bin, and excluding any set of parameters (WIMP mass and cross section) which produces a theoretical event rate for any bin which exceeds the $90%$ upper confidence limit on the event rate for that bin. We show that, if there is more than one energy bin, this produces exclusion limits that are actually at a lower degree of confidence than $90%,$ and are hence erroneously tight. We formulate criteria which produce true $90%$ confidence exclusion limits in these circumstances, including calculating the individual bin confidence limit for which the overall probability that no bins exceed this confidence limit is $90%$ and calculating the $90%$ minimum confidence limit on the number of bins which exceed their individual bin $90%$ confidence limits. We then compare the limits on the WIMP cross section produced by these criteria with those found using the standard technique, using data from the Heidelberg-Moscow and IGEX experiments.
119401404
The WIMP annual modulation signal and non-standard halo models
Currently the best prospect for detecting Weakly Interacting Massive Particles (WIMPs) is via the annual modulation, which occurs due to the Earth's rotation around the Sun, of the direct detection signal. We investigate the effect of uncertainties in our knowledge of the structure of the galactic halo on the WIMP annual modulation signal. We compare the signal for three non-standard halo models: Evans' power-law halos, Michie models with an asymmetric velocity distribution and Maxwellian halos with bulk rotation. We then compare the theoretical predictions of these models with the experimental signal found by the DAMA experiment and investigate how the WIMP mass and interaction cross section determined depend on the halo model assumed. We find that the WIMP mass confidence limits are significantly extended to larger masses, with the shape of the allowed region in the mass-cross section plane depending on the model.