Hst Images of B2 Radio Galaxies: the Link between Circum-nuclear Dust and Radio Properties

A long standing question in radio astronomy has been why some elliptical galaxies host strong radio sources and others not. The reason may have something to do with the physical conditions near the (active) nucleus from which the radio source emanates. The resolution of HST images is such that dust features close to the nucleus can be detected, and these may often be in the form of circumnuclear disks. A study of dust features in 3C radio galaxies and their relation to the radio source properties was presented by de Koff et al. (2000). The large majority of the 3C radio galaxies are of the Fanaroff-Riley type II, i.e. they have high radio luminosities. A number of lower luminosity (FR I) radio sources from the B2 sample (see e.g. Fanti et al. 1987) have been imaged recently in two colors (V and I) with the WFPC2 of the Hubble Space Telescope (Capetti et al. 2000). Although only about 60 % of the B2 sample has been observed, this should not have introduced any selection bias (the selection for observation was in practice done in random fashion). In order to study the influence of dust on the radio sources we produced images which show only regions with dust (if present). These absorption maps were obtained by first constructing a model of the brightness profile of the galaxy (in I), which was then, properly scaled, divided by the V image of the galaxy.