This workflow deals with 3C295, a powerful lobe-dominated
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.
- Generate (and display) a photometric Spectral Energy Distribution (SED)
- Study the multi-wavelength (radio, optical, X-ray)
emission from the lobes and the cluster
- Identify red objects in the field
- Study the properties of individual galaxies in the cluster
The science case is based on the VOTECH
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
VOSpec may be helpful.
Please, refer to the
EURO-VO recipes user
manual for more information.
- launch AstroScope from the AstroGrid Workbench
The AstroGrid Workbench main panel.
- launch topcat
- register topcat in the PLASTIC hub, through: Interop/Register with PLASTIC
- 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.
- For each of the following catalogues:
do the following:
- Green Bank 6-cm (GB6) Radio
- Two Micron All Sky Survey (2MASS) [0.001 deg]
- SDSS Data Release 5 (DR5) [0.001 deg]
- RASS ROSAT All Sky Survey
- Chandra Xassist Source List
- Integral Reference Catalogue
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.
- select the closest catalogue entry to the (resolved) nominal position of
3C295, as indicated by the distance estimator in
The SDSS catalogue entries associated to 3C295 in
- send the catalogue entry to PLASTIC, by clicking with the right
mouse button on the asterisk (*) box and choosing to Topcat
The main topcat window. The SDSS catalogue entries corresponding to 3C295 have been
sent ("broadcasted") to topcat through the PLASTIC hub.
- save each catalogue as VOTable (tabledata)
- Photometric measurements in the selected catalogue come in different
formats and units. Some manipulation is required before the SED can
- magnitudes need to be converted in fluxes. Generate a new flux
density column it in the following way:
- by applying the formula: Flux = F_0 * pow(10,-2.5/m) for 2MASS
magnitudes, where m is the magnitude in band H, J, K, and
F_O is the
flux density for 0 magnitude
- by applying the intrinsic AB magnitude to flux conversion function in
TOPCAT for SDSS
The topcat panel, allowing to define a new synthetic column in a table. The intrinsic
function to convert from magnitude to fluxes is being used.
- by dividing the flux by a median bandpass wavelength for the X-ray
- one needs to generate explicitly a new column with the
- GB6 -> 5E+09 (Hz)
- 2MASS (J) -> 1.235E-06 (m)
- 2MASS (H) -> 1.662E-06 (m)
- 2MASS (K) -> 2.159E-06 (m)
- SDSS (u) -> 3551 (Å)
- SDSS (g) -> 4686 (Å)
- SDSS (r) -> 6165 (Å)
- SDSS (i) -> 7481 (Å)
- SDSS (z) -> 8931 (Å)
- RASS -> 2.42E17 (Hz)
- Chandra -> 3.63E17 (Hz)
- one needs to generate one VOTable for each photometric point one wants
to display in VOSpec.
This can be conveniently done by highlighting only
the "frequency/wavelength" column through the
panel, and saving the file.
The topcat window listing all the columns in a catalogue (SDSS in the example). By
selecting a sub-set of columns in this panel (using the
tick boxes under the Visible column) one can create a smaller
version of the original file.
- The photometric SED can be displayed in
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.
- launch Aladin
- launch the MERLINImager from the AstroGrid Workbench Task Launcher
AstroGrid Workbench Task Launcher panel.
MERLINImager selection panel.
- generate three images:
- small field-of-view 5GHz image (Freq=4850, Ftol=350, Image Size=5,
Resolution = 0.1)
- small field-of-view 1.4GHz image (Freq=1400, Ftol=30, Image Size=5,
Resolution = 0.1)
- large field-of-view 1.4GHz image (Freq=1400, Ftol=30, Image Size=200,
Resolution = 0.25)
MERLINImager input parameters selection panel.
- send them to Aladin
via MERLINImages -> to Aladin
Radio images produced by the MERLINImager broadcasted to Aladin.
- search in the Aladin
All VO the available images for 3C295, and select
the following ones:
- Chandra (1.2'x1.2')
- HST (F555W)
- HST (F702W)
- perform an astrometric correction of the HST image, by:
- run the option Tools .../Image astrometric re-calibration with the following parameters:
- Coordinates (J2000): 14 11 20.6 +52 12 09
nominal coordinates of the radio galaxy in NED)
- Corresp. pos. (x,y): the pixel coordinates of the closest source
in the HST image
- 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.
- 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
Merlin (greyscale) and Chandra/ACIS (contours) images of
the 3C295 nucleus.
Identification of extreme red sources
- load the following catalogues in Aladin (from the Vizier menu):
- II/276 (SDSS photometric catalogue)
- II/246 (2MASS)
- 2MASS All-Sky Extended Source Catalogue
List of VIZIER catalogues including an entry for 3C295 (from the all VO panel in Aladin
- Cross-match the SDSS and the 2MASS catalogue (Tools/Catalogue cross ...)
Catalogue cross-match panel in
- Broadcast the correlation catalogue to
[tip: use the menu activated
by the small antenna icon at the right bottom of the Aladin window]
- Create the following additional columns in
- Plot J_K vs. r-g
- Configure the plot axis (Plot/Configure axis) to show the
where both colors are >0.5 (see, e.g.,
Davoodi et al., 2006, MNRAS, 371,
J-K versus r-g plot, where only the sources for which both colors
are >0.5 are shown.
- Select (Subsets/Draw subset region) the source with the highest but one
sub-table back to Aladin (Interop/Send table to ...)
- 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
A logarithmic scale is advisable].
- 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
This workflow step takes ~1 hour to run.
- Select Data Analysis/Science Workflows in the
Science Workflows launcher in the
- After log-in, select Colour Cutter in the workflow selection
- Define the proper parameters in the Task Editor user interface:
- Sky Region -> 212.83, 52.20, 1.0
- IR coluor limit -> j_m, k_m, 1.0
- Optical catalogue -> SDSS DR-5
- Optical colour limit -> g,r,0.5
Cutter input parameter set-up.
- send the output catalogue to
via [right mouse
button] merged_output -> to Aladin