Individual Objects

3C295 and its cluster

We investigated the properties of 3C295, a large FR2 radio galaxy located near the centre of a cluster. This makes use of packages developed by VOTech and its member VOs for:

• Data discovery (from Europe, USA and Japan)
• Interactive manipulation and exchange of data between different packages (PLASTIC)
• Extracting customised images and specialised selections from multiple catalogues using interfaces which do not require any specialised knowledge (Workbench, MERLINImager, ColourCutter)
• Automatic alignment and manipulation of images and manual astrometric correction (Aladin)
• Conversion of units including magnitude-to-flux (STILTS/TopCat)
• Spectral line identification, including a redshift correction (VOSpec).

Multi-wavelength data

• AstroScope was used to find images, spectra and catalogue entries for 3C295.
  • Selected images were displayed in Aladin, spectra in VOSpec and tables in TopCat.
  • This rapidly delivers a standard or static selection from each archive. These are in compatible formats for easy comparison.
• We used more specialised tools available via the VO (such as the AstroGrid ColourCutter and MERLINImager) to obtain customised images and catalogue selections.

Spectral Energy Distribution of 3C295

• We sent a selection of photometric catalogue entries from AstroScope to TopCat and used the built-in magnitude-to-flux converter to turn SDSS magnitudes into Jy. We also constructed functions for the simpler transformations between other flux density and spectral axis units. We did not apply any sophisticated corrections (e.g. for the photon index). We compared the resulting SED with the NED values. This shows that, in many cases, our very simple transformations are adequate for identifying the main features of an SED over a large frequency range. The highest-frequency NED point is for the Chandra (CXO) 0.2-2.0 keV band; our point is for the full band. The CXO measurements are significantly fainter than the Integral points, probably due to the larger beam of the latter detecting more extended cluster gas, as seen in the images in the next Section.

The Spectral Energy Distribution of 3C295

Improved methods for SED construction are described in the 'Hot Star' case.

Spatially resolved Radio Spectral Index and X-ray properties

• The MERLINImager was used to extract 21- and 6-cm wavelength images at a matching resolution of 0".1, from the MERLIN visibility data archive, to construct a spectral index image. The 21- and 6-cm data are shown in red and green respectively in the large image and an HST image is shown in blue, highlighting the core of 3C295. This core is also just visible in a high-resolution (0".05) 6-cm radio image (not shown), which was used to align the radio and optical images, whilst even more extended 21-cm lobe emission appears at 0".25 resolution. The insets show the use of Chandra high-resolution X-ray data (left, blue) and ASCA data at lower resolution (superimposed in yellow, right), allowing the X-ray emission associated with the AGN and its jets and with the cluster-centre IGM to be disentangled. In both insets the optical image is in magenta and the 21-cm contours are in red.

Radio, X-ray and optical images of 3C295

Identification of neighbouring galaxies by colour

• The AstroGrid Colour Cutter was used to select tabular data in particular colour ranges, cross-matching optical and IR catalogues.

Example workflow for selecting data by colour from more than one table (Right-click to open any image full-size in a new window)

• PLASTIC allows objects to be identified simultaneously in a colour-colour plot (left, i-z against J-K) and on an image, revealing the proximity of an interesting cluster galaxy to 3C295 itself (the red contours on the upper image). The lower SDSS image shows a larger region with the positions of other cluster members overlaid.

Selecting a point in the TopCat plot highlights the same data in the image display

Detailed spectral analysis

• We used VOSpec to display Migale spectra (from the Hyperleda archive) for objects in the direction of 3C295, corrected to the its redshift of 0.48. We used the SLAP line identification tool to see, for example, whether absorption lines belonged to the cluster or to foreground stars.

Spectral line identification at z=0.48

-- AnitaRichards - 23 Nov 2006