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Control Sample

Powerful radio sources have been believed to be hosted by early-type galaxies. One of the main aims of this thesis is to test this claim in the face of numerous examples of morphological distortions found in radio galaxy hosts following better observations. The aim is to see if there exist any systematic differences between radio galaxy hosts and normal galaxies.

We have compared our results on the MRC radio galaxies with other radio samples and with normal ellipticals mentioned in the literature. In addition, we have carried out exhaustive comparison with a sample of early-type galaxies extracted from the radio galaxy fields and processed in a fashion identical to the radio galaxies. We detail below the procedure used to extract this sample and the advantages of having such a comparison sample.

Selecting the Comparison Sample

The radio galaxies have angular sizes which range from several arcsec to a few 10s of arcsec. However, the CCD used in 1995 was tex2html_wrap_inline4451 and that used in 1996 was tex2html_wrap_inline4453 . As a result several other galaxies, besides the object of primary interest, are present in each frame. It is from these images that the control sample has been chosen. The advantages of such a selection are:

  1. The control objects are observed using the same telescope, filters and CCD as the radio sample. As a result, during flat fielding etc. the instrument signature removal is automatically identical to that for the radio galaxies.
  2. The objects are observed on the same nights as the program objects, thereby allowing one to use exactly the same extinction corrections.
  3. The accuracy of the various reduction techniques used is same both for the comparison sample and the actual sample.
  4. Since the raw data is available just as in the case of the actual sample, everything that is done for the actual sample can be done for the comparison sample.

We selected from these galaxies, for the comparison sample, those galaxies which have a semi-major axis length of at least tex2html_wrap_inline4455 in both the tex2html_wrap_inline3807 bands. To decide this objectively we went through the following steps: (1) determine the background level and its tex2html_wrap_inline3823 , (2) subtract the background, (3) set points having a count below tex2html_wrap_inline4461 to zero and (4) display the frame.

The resulting set contained 82 galaxies including several program objects. Classification by eye was done to group the objects into five classes viz. E/S0, spiral, multiple (including dumbbell), distorted, and marginal. The marginal included galaxies that seemed to be bulge dominant but had at least one feature that looked either like a spiral arm or a tidal interaction. The bulge disk decomposition program was run on all these galaxies to obtain a quantitative measure of the various parameters. The resulting numbers were used to test the classification by eye. It was found that in eight cases the two differed. These included two ellipticals, four edge-on spirals and two marginal cases. The program is known not to be able to handle edge-on cases where the disk dominates and the bulge and disk ellipticities tend to be very different. In the two marginal cases the program detected a dominant disk ( tex2html_wrap_inline4463 ). One of the galaxies put into the class of ellipticals was reclassified as a marginal case as it did have some distorting features in the form of faint spiral arms. The other elliptical galaxy where the eye and the program differed in judgment had a kink at the point where ellipse fitting was begun. Changing the starting point made the program give proper elliptical parameters. Finally, only galaxies that were classified as E/S0 galaxies by the program as well as the eye were chosen as member of the control sample. These numbered 30, coincidentally the same as the number in the radio sample.

The comparison sample thus consists of bulge-dominant galaxies having semi-major axis length tex2html_wrap_inline4465 .


next up previous contents
Next: Brightness distribution Up: Observations: the raw material Previous: Calibration