Centaurus A —
a long overdue synthesis imaging survey

"Surely it's been done" is the typical response I get when I mention that we are going to make an aperture synthesis mosaic of our closest radio galaxy, Centaurus A. But, of course, it really has not ever been done. We are now a quarter of the way through our imaging program and, I am pleased to report, it is by all accounts so-far-so-good. With only half of the required sensitivity, and (more importantly) only a quarter of the uv-coverage, we have a very long road ahead. But the results from the first of the four 750 metre array configurations look promising indeed.

Figure 1: Greyscale optical image (from the Digitised Sky Survey) of NGC 5128, the nearby elliptical galaxy that hosts the radio source Centaurus A. VLA 1.4-GHz radio continuum contours (Clarke, Burns & Norman, 1992, ApJ, 395, 444) are overlaid on the optical image. These radio jets represent less than 1% of the total extent of the radio structure of Centaurus A.

Centaurus A is by far the closest active supermassive black hole in the Universe, and has radio jet/lobe structures that span about 4 × 9 degrees on the sky. Figure 1 shows a greyscale image of NGC 5128, the elliptical galaxy that hosts the radio source Centaurus A, overlaid with Very Large Array (VLA) 1.4-GHz radio continuum contours of the inner jets. These inner jets amount to less than 1% of the entire radio structure. Figure 2 shows the full extent of the radio emission at 1.4 GHz, mapped with the Parkes telescope well over a decade ago (courtesy Norbert Junkes). The contours of the inner radio jets shown in Figure 1 are also displayed in Figure 2. So, why has Centaurus A never been fully imaged with an aperture synthesis telescope? Probably because the large angular size and the low surface brightness of the outer lobes make this project daunting, expensive in time and, up until recently, lacking in sophisticated software to deal with the high dynamic range and large field-of-view. The Compact Array really is the only array for the job, and finally it is being done!

Figure 2: Parkes 1.4 GHz greyscale image (courtesy of Norbert Junkes) overlaid with the same VLA contours as in Figure 1. The bottom right corner shows the angular resolution of this Parkes image (blue circle) with the Compact Array resolution from our current imaging program inset with a tiny red ellipse.

To date, the large-scale structure of Centaurus A is known only from the Parkes single dish images. But with such low resolution (~5 arcminutes at 4.8 GHz), it was not possible to study the lobe structure in much detail. The images we are making with the Compact Array will have a spatial resolution of about 600 parsecs, similar to the spatial resolution of the famous Virgo A (M87) and Cygnus A images made with the VLA. But the Centaurus A image will have at least four times the number of resolution elements, because the radio source is physically so much larger than both Virgo A (80 kpc) and Cygnus A (140 kpc). This essentially means that Centaurus A's radio lobe structure can be studied in more detail than is possible for any other radio galaxy!

The science goals of this imaging program encompass a wide range of astrophysics. Here I will mention the goals that our team are actively pursuing. Firstly, we will explore elements of feedback (i.e. the interaction, influence and impact) between the radio plasma and physical processes in the intergalactic medium (IGM), like shocks, ionised filaments, star formation and hot gas bubbles. The second science goal is to exploit the strong polarised emission of Centaurus A as a background screen to probe the magnetic fields of foreground sources (galaxies, high velocity clouds, our Galaxy) and as a foreground screen to investigate the Faraday rotation caused by the radio lobes and the IGM into which the lobes are expanding, using lines of sight to the hundreds of polarised background sources.

Figure 3: Parkes 1.4 GHz greyscale image (courtesy of Norbert Junkes) saturated to show the most diffuse large scale emission, overlaid with the Compact Array mosaic pointings used for this imaging survey. Each group corresponds to a single 12 hour observation.

Commencing on 20 December 2006, we began the first set of observations with the Compact Array at 1.4 GHz to mosaic the full area shown in Figure 2. We cover the field with about 500 pointings, in mosaic imaging mode, observed as blocks of 25 pointings per day; Figure 3 shows the Compact Array pointings overlaid on the Parkes 1.4 GHz continuum image. The complicated structure associated with this source means that uv-coverage is of paramount importance, and we therefore require all four 750 metre array configurations. The December 2006 observations were done in 750A and we will be observing in 750D configuration at the end of February 2007.

We are, at the time of writing, a quarter of the way through our observations and a first look at the 750A data shows promising results. The large-scale polar-ised emission seen in the Parkes images is already evident, although a much more densely sampled uv-plane is crucial to successfully image these diffuse, complicated lobe structures. In addition, there are several hundred background point sources in the field that are strong enough to probe the Faraday rotation in the radio lobes and the intergalactic medium into which the lobes are expanding. Our biggest hurdle is dealing with the strong sidelobes from the 200 Jy nucleus. But with Tim Cornwell on the case, implementing his multi-scale CLEAN and peeling algorithms, we are confident that this project will be a success!

References

Clarke, D. A., Burns, J. O., & Norman, M. L. 1992, ApJ, 395, 444

Ilana Klamer (on behalf of the whole team; in alphabetical order):
Joss Bland-Hawthorn (AAO), Tim Cornwell (ATNF), Ron Ekers (ATNF), Melanie Johnston-Hollitt (UTas), Ilana Klamer (ATNF), Enno Middelberg (ATNF), Ray Norris (ATNF), Jürgen Ott (NRAO)
(Ilana.Klamer@csiro.au)