5. THE HOST GALAXIES OF FR-I AND FR-II RADIO GALAXIES

Elliptical galaxies have been the subject of numerous studies. Given that they form the hosts of radio galaxies it is only natural to examine some of the recent detailed observations of this family of galaxies for any bearing they may have on the formation of the two radio galaxy types. While we do not attempt to summarize the body of data gathered on elliptical galaxies we use some of the more recent detailed observational studies to highlight aspects that clarify the characteristics of radio galaxy hosts. It is now well established via a number of works that elliptical galaxies fall into two categories: the fast rotators and the slow rotators (Emsellem et al. 2007, Kormendy 2009 and references within). These two classes of ellipticals have distinct properties in several respects. Of relevance to the hosts of radio galaxies we note that the dividing optical luminosity for the fast and slow rotators is at M_b = -20.5 with slow rotators being more luminous. With the hosts of radio galaxies predominantly being as bright or brighter than M_b = -20.5 it appears that most of the radio galaxy hosts fall in the group of slow rotators. This is supported by two other properties of radio galaxy hosts: their ellipticities and masses. Slow rotators are found to have ellipticities flatter than 0.3 which is also the range estimated for a large fraction of 3CRR host ellipticals (Saripalli & Subrahmanyan 2009). Moreover the slow rotators are also the more massive with masses larger than 10¹¹ M which is also the mass regime for radio galaxy hosts.

There are several other distinctive characteristics of the two groups of ellipticals but we confine ourselves to the recognition that the hosts of radio galaxies resemble to a fair extent the slow rotating, massive elliptical galaxies. While this galaxy group is not as strongly oblate as the fast rotators the difference in the position angle of the photometric and kinematic axes is mostly within 20 for the sample studied by Emsellem et al. (2007) with as many as half having values within 10 (their Figure 4). It is reasonable to expect that a large fraction may be close to being oblate although there are clearly known triaxial galaxies.