7156638
New H I-detected Galaxies in the Zone of Avoidance
We present the first results of a blind H I survey for galaxies in the southern zone of avoidance with a multibeam receiver on the Parkes telescope. This survey is expected to eventually catalog several thousand galaxies within Galactic latitude |b| < 5°, mostly unrecognized before because of Galactic extinction and confusion. We present here the results of the first three detections to have been imaged with the Australia Telescope Compact Array. All the galaxies lie near Galactic longitude 325° and were selected because of their large angular sizes, up to 13. Linear sizes range from 53 to 108 kpc. The first galaxy is a massive 5.7 × 1011 M⊙ disk galaxy with a faint optical counterpart, SGC 1511.1-5249. The second is probably an interacting group of galaxies straddling the Galactic equator. No optical identification is possible. The third object appears to be an interacting pair of low column density galaxies, possibly belonging to an extended Circinus or Centaurus A galaxy group. No optical counterpart has been seen despite the predicted extinction (AB = 2.7–4.4 mag) not being excessive. We discuss the implications of the results, in particular, the low H I column densities (~1019 atoms cm-2) found for two of the three galaxies.
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.
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.
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.
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.
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.
118549443
Coupled quintessence and the halo mass function
A sufficiently light scalar field slowly evolving in a potential can account for the dark energy that presently dominates the Universe. This quintessence field is expected to couple directly to matter components, unless some symmetry of a more fundamental theory protects or suppresses it. Such a coupling would leave distinctive signatures in the background expansion history of the Universe and on cosmic structure formation, particularly at galaxy cluster scales. Using semianalytic expressions for the cold dark matter (CDM) halo mass function, we make predictions for halo abundance in models where the quintessence scalar field is coupled to cold dark matter, for a variety of quintessence potentials. We evaluate the linearly extrapolated density contrast at the redshift of collapse using the spherical collapse model and we compare this result to the corresponding prediction obtained from the nonlinear perturbation equations in the Newtonian limit. For all the models considered in this work, if there is a continuous flow of energy from the quintessence scalar field to the CDM component, then the predicted number of CDM haloes can only lie below that of $\ensuremath{\Lambda}\mathrm{CDM}$, when each model shares the same cosmological parameters today. In the last stage of our analysis we perform a global MCMC fit to data to find the best fit values for the cosmological model parameters. We find that for some forms of the quintessence potential, coupled dark energy models can offer a viable alternative to $\ensuremath{\Lambda}\mathrm{CDM}$ in light of the recent detections of massive high-$z$ galaxy clusters, while other models of coupled quintessence predict a smaller number of massive clusters at high redshift compared to $\ensuremath{\Lambda}\mathrm{CDM}$.
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.
1364874
The First wimpy halos
Dark matter direct and indirect detection signals depend crucially on the dark matter distribution. While the formation of large scale structure is independent of the nature of the cold dark matter (CDM), the fate of inhomogeneities on subgalactic scales, and hence the present day CDM distribution on these scales, depends on the microphysics of the CDM particles. We study the density contrast of weakly interacting massive particles (WIMPs) on subgalactic scales. We calculate the damping of the primordial power spectrum due to collisional damping and free streaming of WIMPy CDM and show that free streaming leads to a CDM power spectrum with a sharp cut-off at about 10(-6) M-circle dot. We also calculate the transfer function for the growth of the inhomogeneities in the linear regime, taking into account the suppression in the growth of the CDM density contrast after matter-radiation equality due to baryons and show that our analytic results are in good agreement with numerical calculations. Combining the transfer function with the damping of the primordial fluctuations we produce a WMAP normalized primordial CDM power spectrum, which can serve as an input for high resolution CDM simulations. We find that the smallest inhomogeneities typically have comoving radius of about 1 pc and enter the nonlinear regime at a red-shift of 60 +/- 20. We study the effect of scale dependence of the primordial power spectrum on these numbers and also use the spherical collapse model to make simple estimates of the properties of the first generation of WIMP halos to form. We find that the very first WIMPy halos may have a significant impact on indirect dark matter searches.
118388583
Probing light WIMPs with directional detection experiments
The CoGeNT and CRESST WIMP direct detection experiments have recently observed excesses of nuclear recoil events, while the DAMA/LIBRA experiment has a long-standing annual modulation signal. It has been suggested that these excesses may be due to light masses of mχ∼5–10  GeV, weakly interacting massive particles (WIMPs). The Earth’s motion with respect to the Galactic rest frame leads to a directional dependence in the WIMP scattering rate, providing a powerful signal of the Galactic origin of any recoil excess. We investigate whether direct detection experiments with directional sensitivity have the potential to observe this anisotropic scattering rate with the elastically scattering light WIMPs proposed to explain the observed excesses. We find that the number of recoils required to detect an anisotropic signal from light WIMPs at 5σ significance varies from 7 to more than 190 over the set of target nuclei and energy thresholds expected for directional detectors. Smaller numbers arise from configurations where the detector is only sensitive to recoils from the highest-speed, and hence most anisotropic, WIMPs. However, the event rate above the threshold is very small in these cases, leading to the need for large experimental exposures to accumulate even a small number of events. To account for this sensitivity to the tail of the WIMP velocity distribution, whose shape is not well known, we consider two exemplar halo models spanning the range of possibilities. We also note that for an accurate calculation, the Earth’s orbital speed must be averaged over. We find that the exposures required to detect 10 GeV WIMPs at a WIMP-proton cross section of 10-4  pb are of order 103  kg day for a 20 keV energy threshold, within reach of planned directional detectors. Lower WIMP masses require higher exposures and/or lower energy thresholds for detection.