Molonglo Observatory Synthesis Telescope Research Articles

MULTIFREQUENCY RADIO MEASUREMENTS OF SUPERNOVA 1987A OVER 22 YEARS

We present extensive observations of the radio emission from the remnant of supernova (SN) 1987A made with the Australia Telescope Compact Array (ATCA), since the first detection of the remnant in 1990. The radio emission has evolved in time providing unique information on the interaction of the SN shock with the circumstellar medium. We particularly focus on the monitoring observations at 1.4, 2.4, 4.8, and 8.6 GHz, which have been made at intervals of 4–6 weeks. The flux density data show that the remnant brightness is now increasing exponentially, while the radio spectrum is flattening. The current spectral index value of −0.68 represents an 18 ± 3% increase over the last eight years. The exponential trend in the flux is also found in the ATCA imaging observations at 9 GHz, which have been made since 1992, approximately twice a year, as well as in the 843 MHz data set from the Molonglo Observatory Synthesis Telescope from 1987 to 2007 March. Comparisons with data at different wavelengths (X-ray, Hα) are made. The rich data set that has been assembled in the last 22 years forms a basis for a better understanding of the evolution of the supernova remnant.

The RASS–6dFGS catalogue: a sample of X-ray selected AGN from the 6dF Galaxy Survey

We present a catalogue of 3405 X-ray sources from the ROSAT All Sky Survey (RASS) Bright Source Catalogue which fall within the area covered by the 6dF Galaxy Survey (6dFGS). The catalogue is count-rate limited at 0.05 cts\s in the X-ray and covers the area of sky with delta < 0 deg and |b|>10 deg. The RASS--6dFGS sample was one of the additional target catalogues of the 6dFGS and as a result we obtained optical spectra for 2224 (65%) RASS sources. Of these, 1715 (77%) have reliable redshifts with a median redshift of z=0.16 (excluding the Galactic sources). For the optically bright sources (b_J < 17.5) in the observed sample, over 90% have reliable redshifts. The catalogue mainly comprises QSOs and active galaxies but also includes 238 Galactic sources. Of the sources with reliable redshifts the majority are Type 1 AGN (69%), while 12% are Type 2 AGN, 6% absorption-line galaxies and 13% are stars. We also identify a small number of optically-faint, very low redshift, compact objects which fall outside the general trend in the b_J-z plane. We detect 918 sources (27%) of the RASS--6dFGS sample in the radio using either the 1.4 GHz NRAO VLA Sky Survey (NVSS) or the 843 MHz Sydney University Molonglo Sky Survey (SUMSS) catalogues and find that the detection rate changes with redshift. At redshifts larger than 1 virtually all of these sources have radio counterparts and with a median flux density of 1.15 Jy, they are much stronger than the median flux density of 28.6 mJy for the full sample. We attribute this to the fact that the X-ray flux of these objects is being boosted by a jet component, possibly Doppler boosted, that is only present in radio-loud AGN. (abridged version)

Revisiting the relativistic ejection event in XTE J1550−564 during the 1998 outburst

We revisit the discovery outburst of the X-ray transient XTE J1550-564 during which relativistic jets were observed in 1998 September, and review the radio images obtained with the Australian Long Baseline Array, and lightcurves obtained with the Molonglo Observatory Synthesis Telescope and the Australia Telescope Compact Array. Based on HI spectra, we constrain the source distance to between 3.3 and 4.9 kpc. The radio images, taken some two days apart, show the evolution of an ejection event. The apparent separation velocity of the two outermost ejecta is at least 1.3c and may be as large as 1.9c; when relativistic effects are taken into account, the inferred true velocity is >0.8c. The flux densities appear to peak simultaneously during the outburst, with a rather flat (although still optically thin) spectral index of -0.2.

A new search for distant radio galaxies in the Southern hemisphere - II. 2.2 μm imaging*

We have compiled a sample of 234 ultra-steep-spectrum (USS) selected radio sources in order to find high-redshift radio galaxies. The sample covers the declination range −40 ◦ < δ< −30 ◦ in the overlap region between the 1400-MHz National Radio Astronomy Observatory (NRAO) Very Large Array (VLA) Sky Survey (NVSS), 408-MHz Revised Molonglo Reference Catalogue and the 843-MHz Sydney University Molonglo Sky Survey (the MRCR– SUMSS sample). This is the second in a series of papers on the MRCR–SUMSS sample, and here we present the K-band (2.2 μm) imaging of 173 of the sources primarily from the Magellan and the Anglo-Australian Telescopes. We detect a counterpart to the radio source in 93 per cent of the new K-band images which, along with previously published data, makes this the largest published sample of K-band counterparts to USS-selected radio galaxies. The location of the K-band identification has been compared to the features of the radio emission for the double sources. We find that the identification is most likely to lie near the mid-point of the radio lobes rather than closer to the brighter lobe, making the centroid a less likely place to find the optical counterpart. 79 per cent of the identifications are less than 1 arcsec from the radio lobe axis. These results differ from studies of low-redshift radio samples where the environments are typically not nearly so dense and disturbed as those at high redshift. In contrast to some literature samples, we find that the majority of our sample shows no alignment between the near-infrared and radio axes. Several different morphologies of aligned structures are found and those that are aligned within 10 ◦ are consistent with jet-induced star formation. The distribution and median value of the K-band magnitudes for the MRCR–SUMSS sample are found to be similar to several other USS-selected samples even though each sample has a very different median 1400 MHz flux density. USS selection from a lower radio-frequency sample has not netted fainter K-band magnitudes, which may imply that the k-correction is not responsible for the effectiveness of USS selection.

A radio survey of seven southern X-ray luminous clusters of galaxies

The Australia Telescope Compact Array (ATCA) has been used at 1.38 and 2.38 GHz to survey seven southern Abell clusters of galaxies with high X-ray luminosities: A2746, A2837, A3126, A3216, A3230, A3827 and A3836. The clusters have also been surveyed at 0.843 GHz with the Molonglo Observatory Synthesis Telescope (MOST). We have listed a complete 1.38-GHz sample of 149 radio sources within the Abell circles centred on their X-ray centroids. We compare their identification fractions, emitted 1.38-GHz and optical powers, radio spectral indices and radial variation in projected source density with those of the radio-selected samples of Slee et al. (1998). We compare our fractional radio luminosity function with that of the radio-selected samples of Ledlow and Owen (1996) and Slee et al. (1998). Three significant differences are noted between X-ray and radio-selected samples of clusters; (1) the X-ray sample has an excess of flat-spectrum radio sources; (2) the fractional radio luminosity function for the FR I sources in the X-ray selected sample is much steeper, implying that fewer of their cluster galaxies become hosts for the stronger FR I radio galaxies; (3) a complete absence of FR II radio galaxies in the X-ray selected sample. The average excess projected density of radio sources near our cluster centres is approx. 5 times the background source density.

A 20-yr radio light curve for the young supernova remnant G1.9+0.3

Abstract The radio source G1.9+0.3 has recently been identified as the youngest known Galactic supernova remnant, with a putative age of ∼100 yr. We present a radio light curve for G1.9+0.3 based on 25 epochs of observation with the Molonglo Observatory Synthesis Telescope, spanning 20 yr from 1988 to 2007. These observations are all at the same frequency (843 MHz) and comparable resolutions (43 × 91 or 43 × 95 arcsec2) and cover one fifth of the estimated lifetime of the supernova remnant. We find that the flux density has increased at a rate of 1.22 ± 0.240.16 per cent yr−1 over the last two decades, suggesting that G1.9+0.3 is undergoing a period of magnetic field amplification.

Evidence of a Curved Synchrotron Spectrum in the Supernova Remnant SN 1006

A joint spectral analysis of some Chandra ACIS X-ray data and Molonglo Observatory Synthesis Telescope radio data was performed for 13 small regions along the bright northeastern rim of the supernova remnant SN 1006. These data were fitted with a synchrotron radiation model. The nonthermal electron spectrum used to compute the photon emission spectra is the traditional exponentially cut off power law, with one notable difference: The power-law index is not a constant. It is a linear function of the logarithm of the momentum. This functional form enables us to show, for the first time, that the synchrotron spectrum of SN 1006 seems to flatten with increasing energy. The effective power-law index of the electron spectrum is 2.2 at 1 GeV (i.e., radio synchrotron-emitting momenta) and 2.0 at about 10 TeV (i.e., X-ray synchrotron-emitting momenta). This amount of change in the index is qualitatively consistent with theoretical models of the amount of curvature in the proton spectrum of the remnant. The evidence of spectral curvature implies that cosmic rays are dynamically important instead of being test particles. The spectral analysis also provides a means of determining the critical frequency of the synchrotron spectrum associated with the highest-energy electrons. The critical frequency seems to vary along the northeastern rim, with a maximum value of 1.1+1.0−0.5 × 1017 Hz. This value implies that the electron diffusion coefficient can be no larger than a factor of ~4.5-21 times the Bohm diffusion coefficient if the velocity of the forward shock is in the range 2300-5000 km s−1. Since the coefficient is close to the Bohm limit, electrons are accelerated nearly as fast as possible in the regions where the critical frequency is about 1017 Hz.

MOLONGLO OBSERVATORY: BUILDING THE CROSS AND MOST

When Bernard Mills left the CSIRO in 1960 to establish a radio astronomy group in the School of Physics, University of Sydney, he had not only invented the principle of cross-type radio telescopes but proved their great efficiency at surveying the positions, intensity and structure of radio sources. He had ambitious plans for a second generation Cross—a radio telescope with arms one mile long. This paper describes the circumstances of Mills' appointment as Professor of Astrophysics and the recruitment of an international Department that achieved his vision with the Molonglo Cross. The construction involved interaction with many colleagues—engineers in other university departments and government agencies, and with the contracting firms. Formal links were set up with the Electrical Engineering Department through The Radio Astronomy Centre in the University of Sydney and then with Arecibo Observatory through the Cornell-Sydney University Astronomy Center. When the Molonglo Cross completed its main survey in 1978 after eleven years, it was switched off and the EW arm was then converted to the Molonglo Observatory Synthesis Telescope. Many of the staff involved with the MOST are now challenged by SKAMP, testing systems for the Square Kilometre Array with cylindrical geometry in the Molonglo Prototype. These two later developments out of the original Cross telescope are described briefly.

SpitzerIRAC Observations of Newly Discovered Planetary Nebulae from the Macquarie‐AAO‐Strasbourg Hα Planetary Nebula Project

We compare H?, radio continuum, and Spitzer Space Telescope images of 58 planetary nebulae (PNe) recently discovered by the Macquarie-AAO-Strasbourg H? PN Project (MASH) of the SuperCOSMOS H? Survey. Using Infrared Array Camera (IRAC) data, we examine the relationships between optical and MIR morphologies from 3.6 to 8.0 ?m and explore the ratio of mid-infrared (MIR) to radio nebular fluxes, which is a valuable discriminant between thermal and nonthermal emission. MASH emphasizes late evolutionary stages of PNe compared with previous catalogs, enabling study of the changes in MIR and radio flux that attend the aging process. Spatially integrated fluxes are constructed for all MASH PNe observed by the GLIMPSE Legacy Project, using the H? morphologies to define the areas of MIR and radio continuum emission observed by the Midcourse Space Experiment (MSX), IRAC, the Molonglo Observatory Synthesis Telescope, and the Very Large Array (VLA). The ratio of IRAC 8.0 ?m to MSX 8.3 ?m fluxes provides an assessment of the absolute diffuse calibration of IRAC at 8.0 ?m. We independently confirm the aperture correction factor to be applied to IRAC at 8.0 ?m to align it with the diffuse calibration of MSX. The result is in accord with the recommendations of the Spitzer Science Center and with our results from a parallel study of H II regions in the MIR and radio. However, these PNe probe the diffuse calibration of IRAC on a spatial scale of 9''-77'', as opposed to the many-arcminute scale from the H II regions' study.

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° and 245° < l < 365°, and is the Galactic counterpart to the Sydney University Molonglo Sky Survey (SUMSS) which covers the whole southern sky with δ≤−30° (|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.

Multiwavelength study of a new Galactic SNR G332.5−5.6

We present compelling evidence for confirmation of a Galactic supernova remnant (SNR) candidate, G332.5−5.6, based initially on identification of new, filamentary, optical emission-line nebulosity seen in the arcsecond resolution images from the Anglo Australian Observatory (AAO)/United Kingdom Schmidt Telescope (UKST) Hα survey. The extant radio observations and X-ray data which we have independently re-reduced, together with new optical spectroscopy of the large-scale fragmented nebulosity, confirm the identification. Optical spectra, taken across five different, widely separated nebula regions of the remnant as seen in the Hα images, show average ratios of [N II]/Hα = 2.42, [S II]/Hα = 2.10 and [S II] 6717/6731 = 1.23, as well as strong [O I] 6300, 6364 A and [O II] 3727 A emission. These ratios are firmly 3within those typical of SNRs. Here, we also present the radio-continuum detection of the SNR at 20/13 cm from observations with the Australia Telescope Compact Array (ATCA). Radio emission is also seen at 4850 MHz, in the Parkes‐MIT‐NRAO (PMN) survey and at 843 MHz from the Sydney University Molonglo Sky Survey (SUMSS) survey. We estimate an angular diameter of ∼30 arcmin and obtain an average radio spectral index of α =− 0.6 ± 0.1 which indicates the non-thermal nature of G332.5−5.6. Fresh analysis of existing ROSAT X-ray data in the vicinity also confirms the existence of the SNR. The distance to G332.5−5.6 has been independently estimated by Reynoso and Green as 3.4 kpc based on measurements of the H I λ21-cm line seen in absorption against the continuum emission. Our cruder estimates via assumptions on the height of the dust layer (3.1 kpc) and using the � ‐D relation (4 kpc) are in good agreement.

A new search for distant radio galaxies in the Southern hemisphere – I. Sample definition and radio properties

This paper introduces a new program to find high-redshift radio galaxies in the Southern hemisphere through ultrasteep spectrum (USS) selection. We define a sample of 234 USS radio sources with spectral indices α843408≤−1.0 (Sν∝να) and flux densities S408≥ 200 mJy in a region of 0.35 sr, chosen by cross-correlating the revised 408 MHz Molonglo Reference Catalogue, the 843 MHz Sydney University Molonglo Sky Survey and the 1400 MHz NRAO VLA Sky Survey in the overlap region −40° < δ < −30°. We present Australia Telescope Compact Array (ATCA) high-resolution 1384 and 2368 MHz radio data for each source, which we use to analyse the morphological, spectral index and polarization properties of our sample. We find that 85 per cent of the sources have observed-frame spectral energy distributions that are straight over the frequency range 408–2368 MHz, and that, on average, sources with smaller angular sizes have slightly steeper spectral indices and lower fractional linear polarization. Fractional polarization is anticorrelated with flux density at both 1400 and 2368 MHz. We also use the ATCA data to determine observed-frame Faraday rotation measures for half of the sample.

The Cosmic Ray Distribution in Sagittarius B

The HESS instrument has observed a diffuse flux of ~ TeV gamma 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 gamma rays originate in collisions between cosmic ray hadrons (CRHs) and ambient matter. Of particular interest, HESS has detected gamma rays from the Sagittarius (Sgr) B Molecular Cloud Complex. Prompted by the suggestion of a hadronic origin for the gamma 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 gamma rays). Intriguingly, we have uncovered non-thermal 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 ten 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 (i) a hard (~ E^-2.2), high-energy (> TeV) population CRHs is unavoidably required by the HESS gamma ray data and (ii) the remaining broad-band, non-thermal phenomenology is explained either by a rather steep (~ E^-2.9) spectrum of primary electrons or a (~ E^-2.7) population of CRHs. No single, power-law population of either leptons or hadrons can explain the totality of broadband, non-thermal Sgr B phenomenology.

G315.1+2.7: a new Galactic supernova remnant from the AAO/UKST H survey

New narrow-band Hα imaging and subsequent optical spectra confirm G315.1+2.7, a previously identified candidate supernova remnant (SNR), as a bona fide Galactic SNR. Present observations are based on independent discovery of filamentary optical emission nebulosity on images of the Anglo-Australian Observatory/United Kingdom Schmidt Telescope Hα survey of the southern Galactic plane which were found to coincide with existing multifrequency radio detections. Separate medium- and high-dispersion spectra were taken across two locations of this 11-arcmin north–south (NS) aligned optical filament. The resulting spectral signatures were found to strongly confirm the SNR identification based on standard emission-line ratio discriminators which characterize emission from shock-heated gas. The average observed ratios of S ii/Hα= 1.13, N ii/Hα= 1.43 and S ii 6717/6731 = 1.46, together with the simultaneous detection of [O ii] 3727, [O iii] 5007 and [O i] 6300 A, all point to a SNR origin of the observed optical emission. There is also an excellent positional coincidence between the new Hα filament and the north-east radio arc of G315.1+2.7 seen at several frequencies. Careful scrutiny of the low-resolution but high-sensitivity Southern Hα Sky Survey Atlas also revealed a low-level but distinct optical emission arc. This arc precisely correlates with the large, 2.5°, NS angular extent of the proposed new SNR also seen as a fractured structure in the extant radio data. G315.1+2.7 was detected previously at 2400 and 4800 MHz and at 408 and 1420 MHz. We also identified associated radio emission at 843 MHz from the now publicly available Sydney University Molonglo Sky Survey. On the basis of optical imaging and spectra and radio observations at five frequencies, we identify G315.1+2.7 as a new Galactic SNR. The large projected angular extent of the new remnant, together with the distance estimate of ∼1.7 kpc and diameter of ∼80 pc, make G315.1+2.7 one of the largest remnants known.

Spatial clustering of Ultra Steep Spectrum sources and galaxies

We present measurements of the clustering properties of galaxies in the field of redshift range 0.5 ≤ z ≤ 1.5 Ultra Steep Spectrum radio sources selected from the Sydney University Molonglo Sky Survey and the National Radio Astronomy Observatories Very Large Array Sky Survey. Galaxies in these USS fields were identified in deep near-infrared observations, complete down to K s = 20, using the IRIS2 instrument at the Anglo-Australian Telescope. We used the redshift distribution of K s < 20 galaxies taken from Cimatti et al. (2002) to constrain the correlation length r 0 . We find a strong correlation signal of galaxies with K s < 20 around our USS sample. A comoving correlation length r 0 = 14.0 ± 2.8 h -1 Mpc and y = 1.98 ± 0.15 are derived in a flat cosmological model universe. We compare our findings with those obtained in a cosmological N-body simulation populated with GALFORM semi-analytic galaxies. We find that clusters of galaxies with masses in the range M = 10 13.4-14.2 2 h -1 M ⊙ have a cluster-galaxy cross-correlation amplitude comparable to those found between the USS hosts and galaxies. These results suggest that distant radio galaxies are excellent tracers of galaxy overdensities and pinpoint the progenitors of present day rich clusters of galaxies.

A Complete Sample of Megaparsec-sized Double Radio Sources from the Sydney University Molonglo Sky Survey

We present a complete sample of megaparsec-sized double radio sources compiled from the Sydney University Molonglo Sky Survey. Almost complete redshift information has been obtained for the sample. The sample has the following defining criteria: Galactic latitude > 125, declination δ 5'. All the sources have a projected linear size larger than 0.7 Mpc (assuming H0 = 71 km s-1 Mpc-1). The sample is chosen from a region of the sky covering 2100 deg2. In this paper we present 843 MHz radio images of the extended radio morphologies made using the Molonglo Observatory Synthesis Telescope, higher resolution radio observations of any compact radio structures using the Australia Telescope Compact Array, and low-resolution optical spectra of the host galaxies from the 2.3 m Australian National University telescope at Siding Spring Observatory. The sample presented here is the first in the southern hemisphere and significantly enhances the database of known giant radio sources. The giant radio sources with linear size exceeding 0.7 Mpc have an abundance of (215 Mpc)-3 at the sensitivity of the survey. In the low-redshift universe, the survey may be suggesting the possibility that giant radio sources with relict lobes are more numerous than giant sources in which beams from the center currently energize the lobes.

A new Wolf-Rayet star and its ring nebula: PCG 11

In a search for new Galactic planetary nebulae from our systematic scans of the Anglo-Australian Observatory/United Kingdom Schmidt Telescope (AAO/UKST) Halpha survey of the Southern Galactic Plane, we have identified a Pop I Wolf-Rayet star of type WN7h associated with an unusual ring nebula that has a fractured rim. We present imagery in Halpha, the 843-MHz continuum from the Molonglo Observatory Synthesis Telescope (MOST), the mid-infrared from the Midcourse Space Experiment (MSX), and confirmatory optical spectroscopy of the character of the nebula and of its central star. The inner edge of the Halpha shell shows gravitational instabilities with a well-defined wavelength around its complete circumference.

Mapping the Local and Distant Universe with SUMSS

The Sydney University Molonglo Sky Survey (SUMSS) is a radio continuum imaging survey of the southern sky at 843 MHz, with similar sensitivity and resolution to the northern NRAO VLA Sky Survey (NVSS). We have combined radio data from SUMSS and NVSS with optical spectra from the 2dF Galaxy Redshift Survey and 6dF Galaxy Survey to study the space distribution and properties of both AGN and star-forming galaxies in the local universe (redshift range 0 &lt; z &lt; 0.3). We also discuss new results on radio-source clustering in the more distant universe (z ∼ 1) and present the first results from a search for the most distant (z &gt; 3) southern radio galaxies.

Angular clustering in the Sydney University Molonglo Sky Survey

We measure the angular correlation function of radio galaxies selected by the 843-MHz Sydney University Molonglo Sky Survey (SUMSS). We find that the characteristic imprint of large-scale structure is clearly detectable and that the survey is very uniform. Through comparison with similar analyses for other wide-area radio surveys ‐ the 1400MHz National Radio Astronomy Observatories Very Large Array Sky Survey (NVSS) and the 325-MHz Westerbork Northern Sky Survey ‐ we are able to derive consistent angular clustering parameters, including a steep slope for the clustering function, w(θ ) ∝ θ −1.1 . We revise upwards previous estimates of the NVSS clustering amplitude, and find no evidence for dependence of clustering properties on radio frequency. It is important to incorporate the full covariance matrix when fitting parameters to the measured correlation function. Once the redshift distribution for mJy radio galaxies has been determined, these projected clustering measurements will permit a robust description of large-scale structure at z ∼ 0.8, the median redshift of the sources.

Radio observations of the supernova remnant candidate G312.5-3.0

The radio images from the Parkes-MIT-NRAO (PMN) Southern Sky Survey at 4850 MHz have revealed a number of previously unknown radio sources. One such source, G312.5-3.0 (PMN J1421-6415), has been observed using the multi-frequency capabilities of the Australia Telescope Compact Array (ATCA) at frequencies of 1380 MHz and 2378 MHz. Further observations of the source were made using the Molonglo Observatory Synthesis Telescope (MOST) at a frequency of 843 MHz. The source has an angular size of 18 arcmin and has a distinct shell structure. We present the reduced multi-frequency observations of this source and provide a brief argument for its possible identification as a supernova remnant.