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 BLL: <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 order of magnitude difference of intrinsic nuclear luminosity BLL and RLQ have BH of similar mass <M[BH]/M[sun]>_BLL= 5.6x10^{8}, <M[BH]/M[sun]_RLQ = 1.0 10^{9}). This implies that the two types of objects are radiating at very different rates with respect to their Eddington luminosity.