Goal

To investigate the large FR2 radio galaxy 3C295 and its cluster. We compare radio, optical and X-ray images and also show how a catalogue can be overlaid and sources selected. Start by launching the AstroGrid Workbench (you will also need to have an AstroGrid account) and check the links at each stage for more details. You will also need to have the Helper applications Aladin and TopCat running.

Images

Flexible resolution radio images

It can be hard to make meaningful comparisons between images at different resolutions.  Although there are limitations inherant in telescope properties, the resolution can often be tweaked usefully by a factor of 2 or more. The MERLIN Imager is used to create interferometry images to your exact requirements (within the constrains of the baselines available).

These settings request images of 3C295 between 1000 and 2000 MHz at 0".2 resolution. We also requested images between 4500 and 5500 MHz at 0".07 resolution.  These are close to optimum resolutions at each frequency band.  However, in order to  make spectral index maps, we remade a set of images at 0".1 resolution (after the previous images had finished). These may  be missing flux at the higher frequencies on spatial scales only present at the lower frequencies, but it is a useful way to obtain data otherwise inaccessible without specialised  software and the brighter emission is reliable.

Once the MERLINImager has finished, you can use either the Lookout or MySpace (shown) to send the list of images to Aladin.   Right-click on your output table and select To Aladin as VOTable.  The list of images is dispayed as a tree and you can select which ones you want to view; only then are any data moved over the internet.

Use the RGB button in Aladin to create a spectral index image of the matched-resolution (0".1) MERLIN data at 5 GHz (green) and 1.6 GHz (red).  The blue shows the aligned optical image (see below).  The jet hot-spots clearly show up as yellow.  At this resolution the core is only seen in the optical.

Chandra and HST images

You can load Chandra images via the Mission button in Aladin, or use AstroScope to find HST images, and send those to Aladin. We selected the image most closely overlapping 3C295 and edited the header to align it with the radio core (see later).

The 1.4 GHz radio image at top left is overlaid with its contours in reds and with the 5-GHz contours at 0.07-arcsec resolution in yellow/oranges; the yellow contour near the cross marks the radio core which is only visible in the high-resolution 5-GHz data.  The radio contours are also overlaid on the Chandra image (top right) and the aligned HST image (lower left).  The false-colour image at lower right shows radio in red, optical in green and X-ray in blue. The core, NW and SE lobes are the brightest X-ray sources (in decreasing order) but the Chandra-only image shows a lot of diffuse emission.  The data points are taken from Stanford et al. 2002, photometry of distant cluster galaxies.

Using catalogues

Plastic allows catalogue points selected in Aladin to be highlighted in TopCat or visa versa. The properties of optically-selected galaxies in the cluster can be investigated with respect to their position in the X-ray gas.

Astrometry

The HST image is at a different orientation but the tools take care of this; however there is also a small astrometric error which can be corrected approximately by adjusting the FITS header to align the optical and radio core centres.  This can be done using a contour or tag (as in this example); for larger fields an astrometric catalogue is more accurate.