A. J. Green

The 1000 Brightest HIPASS Galaxies: H I Properties

We present the HIPASS Bright Galaxy Catalog (BGC), which contains the 1000 H I brightest galaxies in the southern sky as obtained from the H i Parkes All-Sky Survey ( HIPASS). The selection of the brightest sources is based on their H I peak flux density (S-peak greater than or similar to116 mJy) as measured from the spatially integrated HIPASS spectrum. The derived H I masses range from similar to10(7) to 4 x 10(10) M-.. While the BGC ( z 10degrees. Therefore, the BGC yields no evidence for a population of free-floating intergalactic H I clouds without associated optical counterparts. HIPASS provides a clear view of the local large-scale structure. The dominant features in the sky distribution of the BGC are the Supergalactic Plane and the Local Void. In addition, one can clearly see the Centaurus Wall, which connects via the Hydra and Antlia Clusters to the Puppis Filament. Some previously hardly noticable galaxy groups stand out quite distinctly in the H I sky distribution. Several new structures, including some not behind the Milky Way, are seen for the first time.

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 Facilitys H I Surveys archive. This data set provides an unprecedented view of the neutral component of interstellar hydrogen in the inner Milky Way.

The 1000 brightest HIPASS galaxies: The H I mass function and ω

We present a new, accurate measurement of the H I mass function of galaxies from the HIPASS Bright Galaxy Catalog, a sample of 1000 galaxies with the highest H I peak flux densities in the southern (delta<0D) hemisphere. This sample spans nearly 4 orders of magnitude in H I mass [ log (M-H I/M-O) + 2 log h(75)=6.8-10.6] and is the largest sample of H I-selected galaxies to date. We develop a bivariate maximum likelihood technique to measure the space density of galaxies and show that this is a robust method, insensitive to the effects of large-scale structure. The resulting H I mass function can be fitted satisfactorily with a Schechter function with faint-end slope α=-1.30. This slope is found to be dependent on morphological type, with late-type galaxies giving steeper slopes. We extensively test various effects that potentially bias the determination of the H I mass function, including peculiar motions of galaxies, large-scale structure, selection bias, and inclination effects, and we quantify these biases. The large sample of galaxies enables an accurate measurement of the cosmological mass density of neutral gas: U(H) I=(3.8P0.6)x10(-4) h(75)(-1). Low surface brightness galaxies contribute only similar to15% to this value, consistent with previous findings.

Detection of 1720 MHz Hydroxyl Masers at the Galactic Center: Evidence for Shock-excited Gas and Milligauss Fields

Radio observations of the Sgr A region have been carried out at the 1720 MHz transition of the OH molecule, using the Very Large Array and the Australia Telescope Compact Array. OH(1720 MHz) maser emission is detected at seven different positions within a few arcminutes of the Galactic center. Most of the masers are located to the southeast of Sgr A*, at the boundary of the Sgr A East nonthermal source with the M-0.02-0.07 molecular cloud. One maser is located within the circumnuclear disk (CND), and another may arise as the result of an expansion of Sgr A East into molecular gas to the northwest of Sgr A*. It is likely that these maser features are excited by the interaction of shocks in the Galactic center with adjacent molecular gas. Significant circular polarization is observed toward 10 distinct spectral components in the seven maser spots; if the V signal is due to Zeeman splitting, preliminary measurements of strong fields [local line-of-sight components of B (BLOS) are estimated to be between 2 and 4 mG] are inferred toward all sources. The direction of these magnetic fields is positive for all masers except the CND maser, which is negative. The potential for these maser features as a diagnostic of shocked gas in the Galactic center is discussed.

A stellar wind bubble coincident with the anomalous X-ray pulsar 1E 1048.1-5937: Are magnetars formed from massive progenitors?

We present 21 cm H I observations from the Southern Galactic Plane Survey of the field around the anomalous X-ray pulsar 1E 1048.1-5937, a source whose X-ray properties imply that it is a highly magnetized neutron star (a magnetar). These data reveal an expanding hydrogen shell, GSH 288.3-0.5-28, centered on 1E 1048.1-5937, with a diameter of 35 × 23 pc (for a distance of 2.7 kpc) and an expansion velocity of ≈7.5 km s-1. We interpret GSH 288.3-0.5-28 as a wind bubble blown by a 30-40 M☉ star, but no such central star can be readily identified. We suggest that GSH 288.3-0.5-28 is the wind bubble blown by the massive progenitor of 1E 1048.1-5937 and consequently propose that magnetars originate from more massive progenitors than do radio pulsars. This may be evidence that the initial spin period of a neutron star is correlated with the mass of its progenitor and implies that the magnetar birthrate is only a small fraction of that for radio pulsars.

Shock-excited Maser Emission from Supernova Remnants: G32.8−0.1, G337.8−0.1, G346.6−0.2, and the HB 3/W3 Complex

?????We present the results of VLA observations in the ground-state hydroxyl (OH) transition at 1720 MHz toward 20 supernova remnants (SNRs). We detect compact emission from four objects. For three of these objects (G32.8-0.1, G337.8-0.1, and G346.6-0.2), we argue that the emission results from masers that are shock-excited due to the interaction of the SNR and an adjacent molecular cloud. We observe a characteristic Zeeman profile in the Stokes V spectrum, which allows us to derive a magnetic field of 1.5 and 1.7 mG for G32.8-0.1 and G346.6-0.2, respectively. The velocity of the masers also allows us to determine a kinematic distance to the supernova remnants (SNRs). Our criteria for a maser to be associated with an SNR along the line of sight are that the position and velocity of the maser and SNR must agree and the OH (1720) emission must be unaccompanied by other OH lines.

Low-Mach-number turbulence in interstellar gas revealed by radio polarization gradients

The interstellar medium of the Milky Way is multiphase, magnetized and turbulent. Turbulence in the interstellar medium produces a global cascade of random gas motions, spanning scales ranging from 100 parsecs to 1,000 kilometres (ref. 4). Fundamental parameters of interstellar turbulence such as the sonic Mach number (the speed of sound) have been difficult to determine, because observations have lacked the sensitivity and resolution to image the small-scale structure associated with turbulent motion. Observations of linear polarization and Faraday rotation in radio emission from the Milky Way have identified unusual polarized structures that often have no counterparts in the total radiation intensity or at other wavelengths, and whose physical significance has been unclear. Here we report that the gradient of the Stokes vector (Q, U), where Q and U are parameters describing the polarization state of radiation, provides an image of magnetized turbulence in diffuse, ionized gas, manifested as a complex filamentary web of discontinuities in gas density and magnetic field. Through comparison with simulations, we demonstrate that turbulence in the warm, ionized medium has a relatively low sonic Mach number, Msu2009≲u20092. The development of statistical tools for the analysis of polarization gradients will allow accurate determinations of the Mach number, Reynolds number and magnetic field strength in interstellar turbulence over a wide range of conditions.

The Southern Galactic Plane Survey: Polarized Radio Continuum Observations and Analysis

The Southern Galactic Plane Survey (SGPS) is a radio survey in the 21 cm H I line and in 1.4 GHz full-polarization continuum, observed with the Australia Telescope Compact Array and the Parkes 64 m single-dish telescope. The survey spans a Galactic longitude of 253° < l < 358° and a latitude of |b| < 15 at a resolution of 100 and a sensitivity below 1 mJy beam-1. This paper presents interferometer only polarized continuum survey data and describes the data taking, analysis processes, and data products. The primary data products are the four Stokes parameters I, Q, U, and V in 25 overlapping fields of 55 by 3°, from which polarized intensity, polarization angle, and rotation measure are calculated. We describe the effects of missing short spacings, and discuss the importance of the polarized continuum data in the SGPS for studies of fluctuations and turbulence in the ionized interstellar medium and for studying the strength and structure of the Galactic magnetic field.

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.

H2O Maser Observations of Candidate Post-AGB Stars and Discovery of Three High-Velocity Water Sources

We present the results of 22 GHz H2O maser observations of a sample of 85 post-asymptotic giant branch (post-AGB) candidate stars, selected on the basis of their OH 1612 MHz maser and far-infrared properties. All sources were observed with the Tidbinbilla 70 m radio telescope, and 21 detections were made; 86 GHz SiO Mopra observations of a subset of the sample are also presented. Of the 21 H2O detections, 15 are from sources that are likely to be massive AGB stars and most of these show typical, regular H2O maser profiles. In contrast, nearly all the detections of more evolved stars exhibited high-velocity H2O maser emission. Of the five sources seen, v223 (W43A, IRAS 18450-0148) is a well-known water-fountain source that belongs to a small group of post-AGB stars with highly collimated, high-velocity H2O maser emission. A second source in our sample, v270 (IRAS 18596+0315), is also known to have high-velocity emission. We report the discovery of similar emission from a further three sources, d46 (IRAS 15445-5449), d62 (IRAS 15544-5332), and b292 (IRAS 18043-2116). The source d46 is an evolved post-AGB star with highly unusual maser properties. The H2O maser emission from d62 is probably associated with a massive star. The source b292 is a young post-AGB star that is highly likely to be a water-fountain source, with masers detected over a velocity range of 210 km s-1.