14950553
The cosmic ray distribution in sagittarius B
The H.E.S.S. instrument has observed a diffuse flux of ~TeV γ-rays from a large solid angle around the Galactic center (GC). This emission is correlated with the distribution of gas in the region, suggesting that the γ-rays originate in collisions between cosmic-ray hadrons (CRHs) and ambient matter. Of particular interest, H.E.S.S. has detected γ-rays from the Sagittarius (Sgr) B molecular cloud complex. Prompted by the suggestion of a hadronic origin for the γ-rays, we have examined archival 330 and 74 MHz Very Large Array radio data and 843 MHz Sydney University Molonglo Sky Survey data covering Sgr B, looking for synchrotron emission from secondary electrons and positrons (expected to be created in the same interactions that supply the observed γ-rays). Intriguingly, we have uncovered nonthermal emission, but at a level exceeding expectation. Adding to the overall picture, recent observations by the Atacama Pathfinder Experiment telescope show that the cosmic-ray ionization rate is 10 times greater in the Sgr B2 region of Sgr B than the local value. Lastly, Sgr B2 is also a very bright X-ray source. We examine scenarios for the spectra of CRHs and/or primary electrons that would reconcile all these different data. We determine that (1) a hard (~E-2.2), high-energy (TeV) population of CRHs is unavoidably required by the H.E.S.S. γ-ray data, and (2) the remaining broadband, nonthermal phenomenology is explained either by a rather steep (~E-2.9) spectrum of primary electrons or a (~E-2.7) population of CRHs. Perhaps unsurprisingly, no single power-law population of either leptons or hadrons can explain the totality of broadband, nonthermal Sgr B phenomenology.
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.
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.
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.
17007848
WIMP direct detection and halo structure
Weakly Interacting Massive Particle (WIMP) direct detection experiments are just reaching the sensitivity required to detect Galactic dark matter in the form of neutralinos (or indeed any stable weakly interacting particle). Detection strategies and data analyses are often based on the simplifying assumption of a standard spherical, isothermal halo model, but observations and numerical simulations indicate that galaxy halos are in fact triaxial and anisotropic, and contain substructure. The annual modulation and direction dependence of the event rate (due to the motion of the Earth) provide the best prospects of distinguishing WIMP scattering from background events, however these signals depend sensitively on the local WIMP velocity distribution. I briefly review the status of WIMP direct detection experiments before discussing the dependence of the annual modulation signal on astrophysical input, in particular the structure of the Milky Way halo, and the possibility that the local WIMP distribution is not smooth.
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.
14076551
On mini‐halo encounters with stars
We study, analytically and numerically, the energy input into dark matter mini-haloes by interactions with stars. We find that the fractional energy input in simulations of Plummer spheres agrees well with the impulse approximation for small and large impact parameters, with a rapid transition between these two regimes. Using the impulse approximation, the fractional energy input at large impact parameters is fairly independent of the mass and density profiles of the mini-halo; however, low-mass mini-haloes experience a greater fractional energy input in close encounters. We formulate a fitting function which encodes these results and use it to estimate the disruption time-scales of mini-haloes, taking into account the stellar velocity dispersion and mass distribution. For mini-haloes with mass formula on typical orbits which pass through the disc, we find that the estimated disruption time-scales are independent of mini-halo mass, and are of the order of the age of the Milky Way. For more massive mini-haloes, the estimated disruption time-scales increase rapidly with increasing mass.
13972481
Are there MACHOs in the Milky Way halo
Microlensing searches aim to detect compact halo dark matter via its gravitational lensing effect on stars within the Large Magellanic Cloud. These searches have led to the claim that roughly one fifth of the galactic halo dark matter may be in the form of compact, solar-mass objects. We analyze this hypothesis by considering the goodness-of-fit of the best-fit halo dark matter solutions to the observational data. We show that the distribution of the durations of the observed microlensing events is significantly narrower than that expected to result from a standard halo lens population at 90 to 95% confidence, casting doubt on the lenses constituting halo dark matter. This conclusion may possibly be avoided if (i) the Milky Way halo is sufficiently nonstandard or (ii) a large fraction of the events are due to non-halo populations with event durations coincidentally close to those of the putative halo population or (iii) individual event durations have been seriously underestimated due to blending.
118948395
Interpreting data from WIMP direct detection experiments
Weakly Interacting Massive Particle (WIMP) direct detection experiments are closing in on the region of parameter space where relic neutralinos may constitute the galactic halo dark matter. We discuss two issues in the interpretation of data, in particular the calculation of exclusion limits, from these experiments. Firstly we show that the technique that has been used for calculating exclusion limits from binned data without background subtraction produces erroneously tight limits, and discuss alternative methods which avoid this problem. We then argue that the standard maxwellian halo model is likely to be a poor approximation to the dark matter distribution and examine how halo models with triaxiality, velocity anisotropy and small scale clumping affect exclusion limits.