In the initial days of radio astronomy, several point sources of radio emission were located. These objects came to be called radio stars. As the angular resolution of the radio telescopes improved, it was seen that some of the point sources were in fact extended in nature, and could be identified either with gas clouds in our Galaxy or with other galaxies. Cygnus A was the first radio source to be associated with another galaxy. Since then we have learnt a great deal about radio as well as optical morphology of radio galaxies.
In this chapter, we describe the radio properties of elliptical galaxies. We describe the classification scheme of radio galaxies with examples from the radio sample. We study the relationship of the radio properties of the radio galaxies with their optical properties. In particular, we show the strong dependence of properties like dust content, diskyness, excess blue and morphological peculiarities on the radio power.
Operationally, a galaxy is said to be
a radio galaxy if its radio luminosity exceeds the optical luminosity
( 
). However, the criteria for separating radio
loud objects from radio quiet objects tends to be subjective and gives rise
to a boundary that is not very sharp.
Powerful radio sources
( 
) have radio emission extending to 
scales. The emission is non-thermal in nature and arises from relativistic
electrons accelerated in a magnetic field.
Objects that are not ``radio loud'' are not totally radio quiet. All
elliptical and spiral galaxies have some amount of radio emission
at a level of 
.
Radio emission
from normal ellipticals almost always originates from a small, unresolved
region at the center of the galaxy.
In spiral galaxies the radio emission primarily traces star formation in
the disk. It can either be non-thermal in nature (supernovae) or
thermal in nature (H II regions).
Seyferts and star burst galaxies
typically emit 
. For the powerful radio sources,
the typical structure consists of two giant radio lobes extending to
several 100 
straddling a central galaxy.
Fanaroff and Riley (1974), based on a sample of 57 radio galaxies
and quasars from
the 3CR catalogue, showed that the radio objects can be
divided into two distinct classes, based on the
relative positions of regions of high and low surface brightness in
the radio lobes. They used the ratio 
, of the distance between
the regions of highest surface brightness on opposite sides of the
central galaxy or quasar, to the total extent of the source upto the
lowest brightness contour in the map.
Sources with 
were called FR I sources
(also called edge-darkened sources) and sources with 
were
called FR II sources (also called edge-brightened sources).
An example of FR I type, from the 3C catalogue, 3C 449, is shown in
Figure 
. An example from our sample, 1346-252,
is shown in Figure .
An example of FR II type, again from the 3C catalogue, 3C 47, is shown
in Figure and an example from our sample, 1222-252,
is shown in Figure .
It turns out that this is a very basic distinction and is closely
related to several other properties, indicating a direct link
between luminosity and the energy transport and conversion mechanism.
An important correlation is that most FR II sources have radio luminosity
, while most FR I sources have
.
This dividing line is not very sharp and the luminosity
overlap at higher frequencies can be almost two orders
of magnitude.
FR I galaxies have a steeper radio spectral index than
FR II galaxies.
The mean spectral index for FR II galaxies is 0.89 while that
for FR I is 1.07.
The incidence of jets in FR I type of sources is common,
while 
of FR II sources have jets.
Owen and Laing (1989)
introduced the fat double class as being intermediate between the
FR I and FR II classes since several galaxies could not be
unambiguously classified in the original scheme. Fat double sources
have characteristics of both FR classes.
Owen and White (1991) showed that the FR I/II division
depended not only on radio power but also on the optical luminosity.
Theoretical arguments have now been provided (Bicknell, 1995)
for the slope of the line in the radio/optical plane which
separates the two classes.
FR I sources are found to be located in denser environments as compared to FR II sources, though the immediate environment for both type of sources is seen to be similar (Fanti, 1984; Ledlow and Owen, 1996).
Radio sources are found 3-5 times more frequently in distant clusters (z;SPMgt;0.2) than in nearby clusters (z;SPMlt;0.1) (Owen et al., 1996). Cluster merging is seen at higher redshifts and there may be a relation between the increased activity and the cluster merging and is likely to be related to blue galaxies.
Heckman et al. (1986) have shown using a sample of 43 radio
galaxies that 
of the powerful radio galaxies in the
sample exhibit disturbed
optical morphologies. They found a lower incidence ( ) of such
features in less luminous radio galaxies
( 
). Later work,
e.g. Gonzalez-Serrano et al. (1993), on low-luminosity radio
galaxies however showed that as many as 
of the galaxies show peculiar optical features.
In the following sections we present our findings related to the incidence of peculiar features in the two FR classes as well as other correlations. We find in general that our results are intermediate between the two mentioned above.
