A Pc-scale study of radio-loud AGN : The Fanaroff–Riley Divide and Unification

Abstract

In this thesis an attempt is made to address the Fanaroff-Riley (FR) dichotomy observed in radio-powerful active galactic nuclei (AGNs) within the framework of the “Unification Scheme” for radio-loud AGNs, using high angular resolution total intensity and polarization observations in the radio and optical wavebands. We describe the technique of Very Long Baseline Interferometry (VLBI) and VLBI polarimetry (VLBP) which were made use of in order to study the magnetic (B) field structure and orientation in AGN jets on parsec-scales. In this thesis we present the first VLBP images of four relatively low-luminosity FRI radio galaxies with detected polarized emission on parsecscales. We further present the results from our multi-frequency, multi-epoch VLBP observations of the broad-line FRII radio galaxy – 3C111. We compare and contrast our VLBP results in the light of the Fanaroff-Riley dichotomy and the Unified Scheme that relates the FRI and FRII radio galaxies to BL Lac objects and quasars, respectively. We find that the B-field geometry in FRIs and FRII(s) is similar to radio-powerful BL Lacs and quasars respectively, in accordance with the Unified Scheme. We present VLBP images of the relatively poorly studied subclass of BL Lacertae objects, the high-energy peaked BL Lacs (HBLs). We find that they tend to exhibit a predominantly longitudinal B-field geometry in their VLBI jets, similar to the radio-loud quasars but different from the low-energy peaked BL Lacs (LBLs) which show a predominantly transverse B-field geometry in their jets. We examine the nature of the unresolved optical nuclei discovered by the Hubble Space Telescope in the centres of a majority of FR radio galaxies by fitting simple models to the optical and radio data. We conclude that the nature of the optical nuclei in the FRI population (FRI radio galaxies and BL Lac objects) and the FRII population (FRII radio galaxies and radio-loud quasars) differs in a significant manner. FRI sources probably lack the optically thick obscuring tori around their central engines while FRII sources do not. Further, the contribution to the nuclear optical luminosity from the accretion disk is much more significant in the FRII population. xx