Introduction

Powerful radio sources are known to be hosted almost universally by early type galaxies. Recent observations with high dynamic range detectors have shown that elliptical galaxies hosting radio sources show the presence of faint structure in various forms. In this thesis we study the optical and near infrared morphology of low redshift (z;SPMlt;0.3) radio galaxies with particular emphasis on morphologically peculiar features and the distribution of dust. In the following sections of this chapter, we briefly summarize known properties of normal elliptical and radio galaxies, mention the most important results we have obtained, and indicate what we have left for the future.

Elliptical galaxies

In the 1930's Hubble introduced the concept of elliptical galaxies as a separate class of objects that did not contain any substructure. This ``traditional'' view persisted till the 1970s. As a result, most research on elliptical galaxies concentrated on studying the properties of their stellar component. Initially, it was thought that the flattening of elliptical galaxies is due to a rotation about the polar axis (Osterbrock, 1960). More detailed observations indicated that elliptical galaxies rotated much more slowly than required to support the flattening (Bertola and Cappacioli, 1975; Illingworth, 1977). It was also shown that the elliptical galaxies were supported by anisotropic random velocities of stars (Binney, 1976) and that these galaxies are likely to be triaxial bodies rather than axisymmetric oblate spheroids. These results led to dynamical modeling of elliptical galaxies. Modeling related to accretions and mergers produced features like arcs, shells and ripples which had been seen in elliptical galaxies (e.g. Malin and Carter, 1983; Quinn, 1984). We have extensively used existing techniques and developed a few of our own to carefully look for and quantify the occurrence of small scale structure in radio galaxies and in a control sample. However, we have not carried out any modeling as part of this thesis work.

As per the definition of elliptical galaxies, they are diskless. Galaxies whose morphological characteristics are intermediate between disk-dominated spirals and spheroidal ellipticals are classified as S0 galaxies, also called lenticulars. Recently, it has been suggested that elliptical galaxies with disky isophotes are the bridge between the normal, diskless ellipticals and S0 galaxies (Bender et al., 1993). The disky ellipticals are likely to be rapidly rotating, low-luminosity, coreless, nearly isotropic and oblate spheroidal galaxies (Kormendy and Bender, 1996). We find that of the radio galaxies in our sample contain an appreciable disk component (D/B;SPMgt;0.3). These galaxies are described in detail in Chapter .

The radio galaxy Cen A (NGC 5128) is a peculiar elliptical galaxy with a prominent dust lane. Bertola and Galletta (1978) showed that it is a prototype of a new kind of objects, the dust lane galaxies. Subsequent searches showed that several galaxies contained dust lanes (e.g. Hawarden et al., 1981) and astronomers began to ponder on the origin of dust and the possible correlations of the presence of dust with other properties. Studies have shown that ellipticals with boxy isophotes are associated with radio-loud galaxies (Bender et al., 1989) and with essentially nonrotating giant ellipticals (Kormendy and Bender, 1996). A dust lane along the major-axis leads to boxy isophotes and it may be possible that radio galaxies, in general, are dust lanes galaxies. One of the triggers of this thesis has been to examine the incidence and distribution of dust in radio galaxies. These issues have been covered in Chapter .

Radio galaxies

If the radio luminosity of a galaxy is comparable to or exceeds its optical luminosity ( ), then the galaxy can be called a radio galaxy. The dividing line, however, is not sharp since the criteria for separating radio loud and radio quiet objects tends to be subjective. Cygnus A was the first radio source to be associated with a galaxy. Jennison and Das Gupta (1953) showed that the radio source had a double lobed structure. The list of radio galaxies grew rapidly thereafter. About a tenth of giant elliptical galaxies are radio loud ( ).

Early observations indicated that the hosts of all powerful radio sources are elliptical galaxies. Seyfert nuclei, on the other hand, are almost always associated with spiral hosts. Seyferts are towards the low luminosity end of the radio galaxy luminosity function and may form the bridge between powerful radio galaxies and spirals. Recent work using detectors with higher dynamic range has indicated that though the radio galaxies appear to be of the early type, there are morphological peculiarities associated with them. Heckman et al. (1985, 1986), showed that the enhanced local galaxy density and galaxy mergers can act as producers of powerful radio sources in early-type galaxies. It is now believed that strong radio sources are associated with elliptical galaxies or recent merger remnants (see e.g. Urry and Padovani, 1995).

Fanaroff and Riley (1974) showed that, from their radio structures, one can divide radio galaxies into two distinct classes viz. FR I and FR II. This dichotomy is seen to extend to several properties associated with the radio galaxies. More recently, Owen and White (1991) showed that the FR I/II division depends not only on radio power but also on the optical luminosity. In Chapter  we discuss the findings for our radio sample in this context.