Host galaxies and black hole masses of low- And high-luminosity radio-loud active nuclei

Abstract

We investigate the host galaxy luminosities of BL Lac objects (BLLs) and radio-loud quasars (RLQs) at z < 0.5, imaged with the Hubble Space Telescope (HST). From a homogeneous treatment of the data, we construct the host-galaxy luminosity functions (HGLFs) and find that RLQ hosts are ∼0.5 mag brighter than those of BLLs: (MR)RLQ = -24.0, (MR) BLL = -23.5. For both classes, the HGLFs exhibit a remarkably different distribution with respect to that of normal (inactive) ellipticals, with clear preference for more luminous galaxies to show nuclear activity. We make use of the black hole mass-bulge luminosity (MBH-L bulge) relation, derived for nearby inactive ellipticals, to estimate the central black hole mass in our sample of radio-loud active galaxies. In spite of a ∼2 mag difference of intrinsic nuclear luminosity, BLLs and RLQs have blackholes(BHs) of similar mass ((MBH/M ⊙BLL = 5.6 × 108, (M BH/M⊙)RLQ = 1.0 × 109). This implies that the two types of objects are radiating at very different rates with respect to their Eddington luminosity.