Optical nuclei of radio-loud AGN and the Fanaroff-Riley divide

Abstract

We investigate the nature of the point-like optical nuclei that have been found in the centres of the host galaxies of a majority of radio galaxies by the Hubble Space Telescope. We examine the evidence that these optical nuclei are relativistically beamed, and look for differences in the behaviour of the nuclei found in radio galaxies of the two Fanaroff-Riley types. We also attempt to relate this behaviour to the properties of the optical nuclei in their highly beamed counterparts (the BL Lac objects and radio-loud quasars) as hypothesized by the simple Unified Scheme. Simple model-fitting of the data suggests that the emission may be coming from a non-thermal relativistic jet. It is also suggestive that the contribution from an accretion disk is not significant for the FRI objects and for the narrow-line radio galaxies of FRII type, while it may be significant for the Broad-line objects, and consistent with the idea that the FRII optical nuclei seem to suffer from extinction due to an obscuring torus while the FRI optical nuclei do not. These results are broadly in agreement with the Unified Scheme for radio-loud AGNs.