We have fit the surface brightness profiles of radio galaxies and the
control sample galaxies with de Vaucouleurs' law and an exponential disk as described
in Chapter 
. We present in this section our findings
with regards to the goodness of fit, the values obtained for various
parameters and their dependence on the wavelength.
We have obtained the
scale lengths using a 
minimization between the observed
radial brightness profile of a galaxy and a model profile.
We say that a fit is very good for
and acceptable for 
.
The limit of 2 is chosen as a fiducial boundary, and has no particular
significance.
The distribution of minimum values are shown in Table .
We note the following points with respect to this distribution:
of the cases
in the radio as well as the control sample. Strong radio sources
therefore do not prefer highly distorted galaxies. values for the radio and control
samples is similar (See Table and Figure ). This
indicates that the radio galaxies are not very highly disturbed. does not increase as a function of redshift, i.e.,\
we get good fits right upto the redshift of 0.3 that we have
considered. (See Figure ).
is in general smaller than
the corresponding 
(Figure ).
This is partly due to
the fact that both dust and star formation are likely to affect the
B light more thus increasing the 
which in turn will
lead to smaller values provided all other factors remain the same.
[Distribution of for the two samples]
Distribution of for the radio and control samples. The number of cases
for which we obtain a good is roughly the same for both the samples.
| Radio | Control | ||||
|
| B | R | B | R | |
 | 18 | 18 | 18 | 17 | |
 | 8 | 7 | 7 | 8 | |
 | 4 | 5 | 5 | 5 | |
Here we present the parameters obtained following the exercise of fitting the surface brightness profiles with B+D models. So far as the disks are concerned, we note the following:
).
The fraction of such D/B
values is just a bit higher for the control sample.
Values of D/B;SPMlt;0.1 are not taken to be significant in our data.
smaller than the FWHM of the PSF, for the radio as
well as the control sample.
[D/B values for the two samples]
D/B ratio for the radio and control samples. is
the disk scale length.
| Radio | Control | ||||
|
| B | R | B | R | |
 | 15 | 15 | 20 | 16 | |
| and | |||||
 | 8 | 10 | 10 | 7 | |
 | 7 | 5 | 10 | 9 | |
As shown above, in a majority of cases, we find that D/B is negligible.
In cases where D/B is appreciable (;SPMgt;0.1) we find that
(1) 
and (2) 
.
As a result
we will, in general, not refer to the disk component, while considering
the dependence of fitted parameters on the observing band, even though we have
fit the profile with a bulge-disk combination. There are cases where we
do find large D/B (;SPMgt;0.3). We will discuss these separately in
Chapter .
We now turn to the bulge scale lengths in the B and R bands for the radio galaxies and the control sample. We note the following points about bulge scale lengths:
errors on 
are typically 
.
. The distribution amongst these classes is shown in Table .
, the surface brightness in R increases more
rapidly towards the center than the surface brightness in B
(See Figure ).
In other words, half the red light from the galaxy is contained in a
smaller region than half the blue light. therefore implies that
the galaxy on the average becomes redder inwards.
, the galaxy becomes
bluer as one moves towards the center of the galaxy.
[Comparative 
and 
values for the two samples]
and values for the samples.
Scale lengths in the two filters are taken to be equal if they
differ by less than 
.
| Radio | Control | |
 | 5 | 4 |
 | 2 | 10 |
 | 4 | 8 |
 | 3 | 6 |
 | 13 | 2 |
| Indeterminate | 3 | 0 |
Figure shows the scale lengths for the control and
radio samples in the B and R filters. Figure and Table
indicate that for the control sample galaxies typically the points
are on or below the line of equality ( ), whereas, for the
radio galaxy sample a large number of points lie above the line
( ). This
suggests that the scale lengths for the two samples could be worth
comparing in detail from the point of view of differentiating between the host
galaxies.
With this in mind we obtained the ratio 
for the
radio as well as the control sample.
We show the distribution of the ratio in the form of a
histogram in Figure .
It is clear that there is a qualitative difference in the distribution
for the two samples. The mean values of the ratio for the two samples
are
and
Equation indicates that the control
galaxies become redder inward which is consistent with previous
studies (e.g. Sandage and Visvanathan, 1978).
However, the interesting new fact is that contrary to the behavior of
control
galaxies, a majority of the radio galaxies become bluer
as one approaches the center, i.e., the color variation in
radio galaxies is opposite of that in the control galaxies.
In the next section we obtain
color gradients for the radio and control samples using conventional
techniques and compare the results obtained with inferences drawn from
the scale length ratios.
An important point to remember is that points within 1.5 times the FWHM of the PSF from the center have been excluded from the fit. The corresponding physical distance is upto 4 kpc from the center of the galaxy depending on its redshift. As a result, it cannot be the light from the AGN that is responsible for the bluer colors of the radio galaxies. However, the AGN may contribute indirectly, by triggering star formation which makes the light blue. Later on in this chapter we quantify the blue excess and discuss the possible mechanism giving rise to it.
