An Infrared Study of Centaurus A

We present J, H and K-band images and 0.9-2.5 micron spectra of the nuclear regions of Centaurus A obtained with IRIS on the AAT. While K band has a point source coincident with the nucleus, at H and J we identify diffuse structure extending to the NE as a possible ionisation cone. By considering the NIR colours we show that the point-like K-band emission originates not from the nucleus itself, but from dust which has been shock heated by nuclear outflows. A J-K image reveals a band of high extinction across the nucleus lying perpendicular to the radio jet axis, as suggested by previous authors (Israel et al. 1990, Turner et al. 1992). We model the detailed structure of this extinction image with a circumnuclear torus of diameter 240+/-20 pc, thickness 75+/-4 pc, tilt 80+/-2 degrees to the line of sight and with the torus and radio jet axes aligned.Comment: Submitted to MNRAS on 6th Aug 1998. 12 latex pages including 4 postscript figures. 6 jpg colour figures are attached and can also be found at http://www.physics.usyd.edu.au/~jbryant/cenapub.htm

Long-term monitoring of Molonglo calibrators

Before and after every 12 hour synthesis observation, the Molonglo Observatory Synthesis Telescope (MOST) measures the flux densities of ~5 compact extragalactic radio sources, chosen from a list of 55 calibrators. From 1984 to 1996, the MOST made some 58 000 such measurements. We have developed an algorithm to process this dataset to produce a light curve for each source spanning this thirteen year period. We find that 18 of the 55 calibrators are variable, on time scales between one and ten years. There is the tendency for sources closer to the Galactic Plane to be more likely to vary, which suggests that the variability is a result of refractive scintillation in the Galactic interstellar medium. The sources with the flattest radio spectra show the highest levels of variability, an effect possibly resulting from differing orientations of the radio axes to the line of sight.Comment: 18 pages, 9 embedded EPS files. To appear in Publications of the Astronomical Society of Australia. Data available electronically at http://www.physics.usyd.edu.au/astrop/scan

An X-ray and radio study of the cluster A 2717

We present an X-ray, radio and optical study of the cluster A 2717. The central D galaxy is associated with aWide- Angled-Tailed (WAT) radio source. A Rosat PSPC observation of the cluster shows that the cluster has a well constrained temperature of 1.9+0.3 −0.2 × 107 K. The pressure of the intracluster medium was found to be comparable to the mininum pressure of the radio source suggesting that the tails may in fact be in equipartition with the surrounding hot gas

An X-ray and Radio study of the Cluster A2717

We present an X-ray, radio and optical study of the cluster A2717. The central D galaxy is associated with a Wide-Angled-Tailed (WAT) radio source. A Rosat PSPC observation of the cluster shows that the cluster has a well constrained temperature of 2x10^7 K. The pressure of the intracluster medium was found to be comparable to the mininum pressure of the radio source suggesting that the tails may in fact be in equipartition with the surrounding hot gas.Comment: 7 pages, 6 Postscript figures, to appear in Astronomy and Astrophysics 199

Radio Properties of the Shapley Concentration. III. Merging Clusters in the A3558 Complex

We present the results of a 22 cm radio survey carried out with the A3558 complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and thetwo groups SC1327-312 and SC1323-313, located in the central region of the complex, a chain formed by the merging ACO clusters A3556-A3558-A3562 and the two groups SC1327-312 and SC1323-313, located in the central region of the Shapley Concentration. The purpose of our survey is to study the effects of cluster mergers on the statistical properties of radio galaxies and to investigate the connection between mergers and the presence of radio halos and relic sources. We found that the radio source counts in the A3558 complex are consistent with the background source counts. Furthermore, we found that no correlation exists between the local density and the radio source power, and that steep spectrum radio galaxies are not segregated in denser optical regions. The radio luminosity function for elliptical and S0 galaxies is significantly lower than that for cluster type galaxies and for those not selected to be in clusters at radio powers logP(1.4) > 22.5, implying that the probability of a galaxy becoming a radio source above this power limit is lower in the Shapley Concentration compared with any other environment. The detection of a head-tail source in the centre of A3562, coupled with careful inspection of the 20 cm NRAO VLA Sky Survey (NVSS) and of 36 cm MOST observations, allowed us to spot two extended sources in the region between A3562 and SC1329-313, i.e. a candidate radio halo at the centre of A3562, and low brightness extended emission around a 14.96 magnitude Shapley galaxy.Comment: 18 pages, 14 figures. Accepted for publication on MNRA