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: <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 ~2 mag difference of intrinsic nuclear luminosity, BLLs and RLQs have black holes (BHs) of similar mass (<M_BH/M_solar>_BLL=5.6 × 10⁸, <M_BH/M_solar>_RLQ= 1.0 × 10⁹). This implies that the two types of objects are radiating at very different rates with respect to their Eddington luminosity.