This workflow deals with 3C295, a powerful lobe-dominated radio-galaxy embedded in the X-ray emission of a galaxy cluster. It shows you how to: At the end of the workflow a red (i.e. elliptical) galaxy is identified, in agreement with expectations that ellipticals reside at the center of massive cluster.

The science case is based on the VOTECH Science Framework Document. The reader is referred to this document for more details on the underlying scientific drivers.

In order to effectively follow this workflow, some knowledge of basic functionalities of VO tools such as Aladin, topcat, and VOSpec may be helpful. Please, refer to the EURO-VO recipes user manual for more information.


  1. launch AstroScope from the AstroGrid Workbench

    The AstroGrid Workbench main panel.

  2. launch topcat
  3. register topcat in the PLASTIC hub, through: Interop/Register with PLASTIC

Data Selection

  1. Search on Position or Object Name=3C295, Search Radius=0.01, Catalogues=Yes, Images=No, Spectra=No

    The results of a query in AstroScope on 3C295.

  2. For each of the following catalogues: do the following: The search radius can be the default [0.01 deg], except for SDSS and 2MASS (see values in square brackets above), to avoid that too many entries appear.

Data manipulation

  1. Photometric measurements in the selected catalogue come in different formats and units. Some manipulation is required before the SED can be displayed.

SED display

  1. The photometric SED can be displayed in VOSpec by reading each of the files produced in the previous step. Before the photometric point is actually displayed, a panel asks for the column where the X- and Y-axis are defined, and their units.

    3C295 photometric SED displayed by VOSpec.

Images display

  1. launch Aladin [automatically PLASTICized]
  2. launch the MERLINImager from the AstroGrid Workbench Task Launcher

    AstroGrid Workbench Task Launcher panel.

    MERLINImager selection panel.

  3. generate three images:

    MERLINImager input parameters selection panel.

  4. send them to Aladin via MERLINImages -> to Aladin

    Radio images produced by the MERLINImager broadcasted to Aladin.

  5. search in the Aladin All VO the available images for 3C295, and select the following ones:
  6. perform an astrometric correction of the HST image, by:
  7. generate an RGB image of the small field-of-view radio images and the F702W/HST image through the rgb Aladin function

    RGB composite image of MERLIN (red, green) and HST (blue) images of 3C295, without (left) and with (right) astrometric correction. The pink cross indicates the nominal position of the radio active nucleus.

  8. generate contours of the Chandra image (~20, more thickly spaced towards the X-ray centroid) and superpose them to the large field-of-view radio image

    Merlin (greyscale) and Chandra/ACIS (contours) images of the 3C295 nucleus.

Identification of extreme red sources

  1. load the following catalogues in Aladin (from the Vizier menu):

    List of VIZIER catalogues including an entry for 3C295 (from the all VO panel in Aladin

  2. Cross-match the SDSS and the 2MASS catalogue (Tools/Catalogue cross ...)

    Catalogue cross-match panel in Aladin

  3. Broadcast the correlation catalogue to topcat [tip: use the menu activated by the small antenna icon at the right bottom of the Aladin window]
  4. Create the following additional columns in topcat:
  5. Plot J_K vs. r-g
  6. Configure the plot axis (Plot/Configure axis) to show the quadrant where both colors are >0.5 (see, e.g., Davoodi et al., 2006, MNRAS, 371, 1113)

    topcat J-K versus r-g plot, where only the sources for which both colors are >0.5 are shown.

  7. Select (Subsets/Draw subset region) the source with the highest but one IR color
  8. Broadcast 1-source sub-table back to Aladin (Interop/Send table to ...)
  9. load r and g SDSS images from the all VO Aladin panel. The red source nicely coincide with a galaxy in the SDSS [tip: in order to see the faint diffuse emission, one need to modify the contrast through the "pixel" button in Aladin. A logarithmic scale is advisable].
  10. Enjoy the results

    Multipanel Aladin view, showing all the images used in this workflow.

Color selection via a pre-canned workflow: Colour Cutter

The steps in the previous section can be automatized through the usage of the Colour Cutter workflow:
  1. Select Data Analysis/Science Workflows in the AstroGrid Workbench

    Science Workflows launcher in the AstroGrid Workbench.

  2. After log-in, select Colour Cutter in the workflow selection panel
  3. Define the proper parameters in the Task Editor user interface:

    Colour Cutter input parameter set-up.

  4. send the output catalogue to Aladin via [right mouse button] merged_output -> to Aladin
This workflow step takes ~1 hour to run.