236 research outputs found
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
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
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
A cluster pair : A3532 and A3530
We present a detailed study of a close pair of clusters of galaxies, A3532
and A3530, and their environments. The \textit{Chandra} X-ray image of A3532
reveals presence of substructures on scales of 20 in its
core. XMM-Newton maps of the clusters show excess X-ray emission from an
overlapping region between them. Spectrally determined projected temperature
and entropy maps do not show any signs of cluster scale mergers either in the
overlapping region or in any of the clusters. In A3532, however, some signs of
the presence of galaxy scale mergers are visible e.g., anisotropic temperature
variations in the projected thermodynamic maps, a wide angled tailed (WAT)
radio source in the brighter nucleus of its dumbbell Brightest Cluster Galaxy
(BCG), and a candidate X-ray cavity coincident with the northwestern extension
of the WAT source in the low-frequency radio observations. The northwestern
extension in A3532 seems either a part of the WAT or an unrelated diffuse
source in A3532 or in the background. There is an indication that the cool core
in A3532 has been disrupted by the central AGN activity. A reanalysis of the
redshift data reinforces the close proximity of the clusters. The excess
emission in the overlapping region appears to be a result of tidal interactions
as the two clusters approach each other for the first time. However, we can not
rule out the possibility of the excess being due to the chance superposition of
their X-ray halos.Comment: Accepted for publication in the Astrophysical Journa
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