Richard W. Hunstead

SUMSS: a wide-field radio imaging survey of the southern sky – II. The source catalogue

This paper is the second in a series describing the Sydney University Molonglo Sky Survey (SUMSS) being carried out at 843 MHz with the Molonglo Observatory Synthesis Telescope (MOST). The survey will consist of ∼590 4.3° x 4.3° mosaic images with 45 x 45 cosec‖δ‖ arcsec 2 resolution, and a source catalogue. In this paper we describe the initial release (version 1.0) of the source catalogue consisting of 107 765 radio sources made by fitting elliptical Gaussians in 271 SUMSS 4.3° × 4.3° mosaics to a limiting peak brightness of 6 mJy beam - 1 at δ ≤ -50° and 10 mJy beam - 1 at δ > -50°. The catalogue covers approximately 3500 deg 2 of the southern sky with δ ≤ -30°, about 43 per cent of the total survey area. Positions in the catalogue are accurate to within 1-2 arcsec for sources with peak brightness A 8 4 3 ≥ 20 mJy beam - 1 and are always better than 10 arcsec. The internal flux density scale is accurate to within 3 per cent. Image artefacts have been classified using a decision tree, which correctly identifies and rejects spurious sources in over 96 per cent of cases. Analysis of the catalogue shows that it is highly uniform and is complete to 8 mJy at δ ≤ -50° and 18 mJy at δ > -50°. In this release of the catalogue about 7000 sources are found in the overlap region with the National Radio Astronomy Observatories Very Large Array Sky Survey at 1.4 GHz. We calculate a median spectral index of a = -0.83 between 1.4 GHz and 843 MHz. This version of the catalogue will be released via the World Wide Web with future updates as new mosaics are released.

The Metallicity of high redshift galaxies: The Abundance of zinc in 34 damped Lyman alpha systems from Z = 0.7 to 3.4

We report new observations of Zn II and Cr II absorption lines in 10 damped Ly? systems (DLAs), mostly at redshift zabs 2.5. By combining these results with those from our earlier survey and other recent data, we construct a sample of 34 measurements (or upper limits) of the Zn abundance relative to hydrogen [Zn/H]; the sample includes more than one-third of the total number of DLAs known. The plot of the abundance of Zn as a function of redshift reinforces the two main findings of our previous study. (1) Damped Ly? systems are mostly metal poor, at all redshifts sampled; the column density-weighted mean for the whole data set is [Zn/H] = -1.13 ? 0.38 (on a logarithmic scale), or approximately 1/13 of solar. (2) There is a large spread, by up to 2 orders of magnitude, in the metallicities we measure at essentially the same redshifts. We propose that damped Ly? systems are drawn from a varied population of galaxies of different morphological types and at different stages of chemical evolution, supporting the idea of a protracted epoch of galaxy formation. At redshifts z 2, the typical metallicity of the damped Ly? systems is in agreement with expectations based on the consumption of H I gas implied by the recent measurements of ?DLA by Storrie-Lombardi et al., and with the metal ejection rates in the universe at these epochs deduced by P. Madau from the ultraviolet luminosities of high-redshift galaxies revealed by deep imaging surveys. There are indications in our data for an increase in the mean metallicity of the damped Ly? systems from z > 3 to ? 2, consistent with the rise in the comoving star formation rate indicated by the relative numbers of U and B drop-outs in the Hubble Deep Field. Although such comparisons are still tentative, it appears that these different avenues for exploring the early evolution of galaxies give a broadly consistent picture. At redshifts z < 1.5, DLAs evidently do not exhibit the higher abundances expected from a simple, closed-box model of global chemical evolution, although the number of measurements is still very small. We speculate that this may be due to an increasing contribution of low surface brightness galaxies to the cross section for damped Ly? absorption and to the increasing dust bias with decreasing redshift proposed by S. M. Fall and collaborators. However, more DLAs at intermediate redshifts need to be identified before the importance of these effects can be assessed quantitatively. The present sample is sufficiently large for a first attempt at constructing the metallicity distribution of damped Ly? systems and comparing it with those of different stellar populations of the Milky Way. The DLA abundance histogram is both broader and peaks at lower metallicities than those of either thin or thick disk stars. At the time when our Galaxys metal enrichment was at levels typical of DLAs, its kinematics were closer to those of the halo and bulge than a rotationally supported disk. This finding is at odds with the proposal that most DLAs are large disks with rotation velocities in excess of 200 km s-1, based on the asymmetric profiles of absorption lines recorded at high spectral resolution. Observations of the familiar optical emission lines from H II regions, which are within reach of near-infrared spectrographs on 8-10 m telescopes, may help resolve this discrepancy.

Quenching of the Radio Jet during the X-Ray High State of GX 339−4

We have observed the black hole candidate X-ray binary GX 339-4 at radio wavelengths before, during, and after the 1998 high/soft X-ray state transition. We find that the radio emission from the system is strongly correlated with the hard X-ray emission and is reduced by a factor of ≥25 during the high/soft state compared with the more usual low/hard state. At the points of state transition, we note brief periods of unusually optically thin radio emission that may correspond to discrete ejection events. We propose that in the low/hard state, black hole X-ray binaries produce a quasi-continuous outflow, that in the high/soft state, this outflow is suppressed, and that state transitions often result in one or more discrete ejection events. Future models for low/hard states, such as advection-dominated solutions, need to take into account the strong outflow of relativistic electrons from the system. We propose that the inferred Comptonizing corona and the base of the jetlike outflow are the same thing, based on the strong correlation between radio and hard X-ray emission in GX 339-4 and other X-ray binaries and on the similarity in inferred location and composition of these two components.

A Long, Hard Look at the Low/Hard State in Accreting Black Holes

We present the first results of coordinated multiwavelength observations of the Galactic black hole GX 339-4 in a canonical low/hard state, obtained during its 2004 outburst. XMM-Newton observed the source for two revolutions, or approximately 280 ks; RXTE monitored the source throughout this long stare. The resulting data offer the best view yet obtained of the inner accretion flow geometry in the low/hard state, which is thought to be analogous to the geometry in low-luminosity active galactic nuclei. The XMM-Newton spectra clearly reveal the presence of a cool accretion disk component and a relativistic Fe K emission line. The results of fits made to both components strongly suggest that a standard thin disk remains at or near to the innermost stable circular orbit, at least in bright phases of the low/hard state. These findings indicate that potential links between the inner disk radius and the onset of a steady compact jet, and the paradigm of a radially recessed disk in the low/hard state, do not hold universally. The results of our observations can best be explained if a standard thin accretion disk fuels a corona that is closely related to, or consistent with, the base of a compact jet. In a brief examination of archival data, we show that Cygnus X-1 supports this picture of the low/hard state. We discuss our results within the context of disk-jet connections and prevailing models for accretion onto black holes.

A Powerful Radio Halo in the Hottest Known Cluster of Galaxies 1E 0657–56

We report the detection of a diffuse radio halo source in the hottest known cluster of galaxies 1E 0657-56 (RX J0658-5557). The radio halo has a morphology similar to the X-ray emission from the hot intracluster medium. The presence of a luminous radio halo in such a hot cluster is further evidence for a steep correlation between the radio halo power and the X-ray temperature. We favor models for the origin of radio halo sources involving a direct connection between the X-ray emitting thermal particles and the radio emitting relativistic particles.

Dust in High-Redshift Galaxies

Measurements of Zn and Cr abundances in 18 damped Ly? systems (DLAs) at absorption redshifts zabs = 0.692-3.390 (but mostly between zabs 2 and 3) show that metals and dust are much less abundant in high-redshift galaxies than in the Milky Way today. Typically, [Zn/H] -1.2; as Zn tracks Fe closely in Galactic stars of all metallicities and is only lightly depleted onto interstellar grains, we conclude that the overall degree of metal enrichment of damped Ly? galaxies ? 13.5 Gyr ago (H0 = 50 km s-1 Mpc-1, q0 = 0.05) was ~1/15 solar. Values of [Cr/Zn] span the range from 0 to - 0.65 which we interpret as evidence for selective depletion of Cr onto dust in some DLAs. On average Cr and other refractory elements are depleted by only a factor of ? 2, significantly less than in local interstellar clouds. We propose that this reflects an overall lower abundance of dust?which may be related to the lower metallicities, likely higher temperature of the ISM and higher supernova rates in these young galaxies?rather than an exotic composition of dust grains. Combining a metallicity ZDLA 1/15 Z? with a dust-to-metals ratio ? ? of that in local interstellar clouds, we deduce that the typical dust-to-gas ratio in damped Ly? galaxies is ? 1/30 of the Milky Way value. This amount of dust will introduce an extinction at 1500 ? of only A1500 ? 0.1 in the spectra of background QSOs. Similarly, we expect little reddening of the broad spectral energy distribution of the high-z field galaxies now being found routinely by deep imaging surveys. Even such trace amounts of dust, however, can explain the weakness of Ly? emission from star-forming regions. We stress the approximate nature of such general statements; in reality, the range of metallicities and dust depletions encountered indicates that some sight lines through high-redshift galaxies may be essentially dust-free, while others could suffer detectable extinction. Finally, we show that, despite claims to the contrary, these conclusions are not inconsistent with recent high-resolution observations of DLAs with the Keck telescope. We point out that the star formation histories of high-z galaxies are not necessarily the same as that of the Milky Way and that, if depletions of some elements onto dust are not taken into account correctly, it is possible to misinterpret the clues to early nucleosynthesis provided by nonsolar element ratios.

The second epoch Molonglo Galactic Plane Survey: compact source catalogue

We present the first data release from the second epoch Molonglo Galactic Plane Survey (MGPS-2). MGPS-2 was carried out with the Molonglo Observatory Synthesis Telescope at a frequency of 843 MHz and with a restoring beam of 45 × 45 arcsec2, making it the highest resolution large-scale radio survey of the southern Galactic plane. It covers the range |b| 10°). In this paper, we present the MGPS-2 compact source catalogue. The catalogue has 48 850 sources above a limiting peak brightness of 10 mJy beam−1. Positions in the catalogue are accurate to 1–2 arcsec. A full catalogue including extended sources is in preparation. We have carried out an analysis of the compact source density across the Galactic plane and find that the source density is not statistically higher than the density expected from the extragalactic source density alone. We also present version 2.0 of the SUMSS image data and catalogue which is now available online. The data consist of 629 4.3° × 4.3° mosaic images covering the 8100 deg2 of sky with δ ≤ −30° and |b| > 10°. The catalogue contains 210 412 radio sources to a limiting peak brightness of 6 mJy beam−1 at δ ≤ −50° and 10 mJy beam−1 at δ > −50°. We describe the updates and improvements made to the SUMSS cataloguing process.

An Expanding Radio Nebula Produced by a Giant Flare from the Magnetar SGR 1806-20

Soft γ-ray repeaters (SGRs) are ‘magnetars’, a small class of slowly spinning neutron stars with extreme surface magnetic fields, B ≈ 1015 gauss (refs 1 , 2 –3). On 27 December 2004, a giant flare was detected from the magnetar SGR 1806 - 20 (ref. 2), only the third such event recorded. This burst of energy was detected by a variety of instruments and even caused an ionospheric disturbance in the Earths upper atmosphere that was recorded around the globe. Here we report the detection of a fading radio afterglow produced by this outburst, with a luminosity 500 times larger than the only other detection of a similar source. From day 6 to day 19 after the flare from SGR 1806 - 20, a resolved, linearly polarized, radio nebula was seen, expanding at approximately a quarter of the speed of light. To create this nebula, at least 4 × 1043 ergs of energy must have been emitted by the giant flare in the form of magnetic fields and relativistic particles.

Element Abundances at High Redshifts: The N/O Ratio in a Primeval Galaxy

The damped Lyman alpha systems seen in the spectra of high redshift QSOs offer the means to determine element abundances in galaxies observed while still at an early stage of evolution. Such measurements, which have only recently come within reach, complement and extend the data provided by studies of different stellar populations in our Galaxy and of extragalactic H~II regions which have up to now formed the basis of galactic chemical evolution models. In this paper we demonstrate the potential of this new approach with high-resolution echelle observations of several elements in the z(abs) = 2.27936 absorption system in the bright z(em) = 2.940 QSO 2348-147. The absorbing galaxy appears to be chemically unevolved, with heavy element abundances only 1/100 of solar; if it is the progenitor of a spiral galaxy like our own, it is unlikely to have collapsed to form a thin disk by z = 2.3 (corresponding to a look-back time of approximately 13 Gyr for H_0 = 50 km/s/Mpc and q_0 = 0.01). Our data allow us to measure the nitrogen-to-oxygen ratio at a metallicity lower than those of the most metal-poor dwarf galaxies known. We find that, relative to the solar scale, N is more underabundant than O by at least a factor of 15. This result is broadly in line with current ideas on the relative importance of primary and secondary production of N; future measurements in several damped Lyman alpha systems will permit more stringent tests of models for the evolution of the N/O ratio with time. Oxygen and other alpha-elements are overabundant relative to Fe by no more than the factor of about 3 typical of metal-poor stars in the disk and halo of the Milky Way.

Light Curves and Radio Structure of the 1999 September Transient Event in V4641 Sagittarii (=XTE J1819-254=SAX J1819.3-2525)

We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (=XTE J1819-254=SAX J1819.3-2525). This event was extremely rapid in its rise and decay across radio, optical, and X-ray wavelengths; the X-rays rose to 12 crab within 8 hr and faded to below 0.1 crab in less than 2 hr. Radio observations were made with seven telescopes during the first day following the onset of the strong X-ray event, revealing a strong radio source that was detected for 3 further weeks by the more sensitive telescopes. The radio source was resolved even in the first Very Large Array (VLA) images (September 16.027 UT), being ~025 long with an axis ratio of at least 10 : 1. The total flux density decayed by a factor of ~4 over the first day, and by September 17.94 UT the radio emission was confined to a slowly decaying, marginally resolved remnant located at one side of the early elongated emission. The H I absorption spectrum gives a minimum kinematic distance of about 400 pc; various other arguments suggest that the true distance is not much greater than this. The inferred proper motions for the early extended emission (04-11 day-1) correspond to v/c ~ 1.0-3.2 (d/0.5 kpc), and this together with the radio morphology argues that this is a relativistic jet source like GRS 1915+105 and GRO J1655-40. The proper motion of the late-time remnant is at least 100 times smaller. One simple interpretation posits the ejection of a single short-lived jet segment, followed by a more slowly decaying, optically thin jet segment ejection. These two components can explain both the multifrequency radio light curves and the radio images. The most likely parameters for the fast-jet system with net-averaged proper motion of ~04 day-1, assuming d = 0.5 kpc, are v ~ 0.85c and i ~ 63°, where i is the inclination to the line of sight. The corresponding apparent velocities are 1.4c and 0.6c for the approaching and receding jets, making V4641 Sgr the closest superluminal jet source known.