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

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 similar to0.5 mag brighter than those of BLLs: <M-R >(RLQ) =-24.0, <M-R >(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 (M (BH) -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 similar to2 mag difference of intrinsic nuclear luminosity, BLLs and RLQs have black holes (BHs) of similar mass (<M (BH) /M-circle dot >(BLL) = 5.6 x 10(8) , <M (BH) /M-circle dot >(RLQ) = 1.0 x 10(9) ). This implies that the two types of objects are radiating at very different rates with respect to their Eddington luminosity.