The Host Galaxies of Radio-Loud AGN: The Black Hole--Galaxy Connection

We have studied the host galaxies of a sample of radio-loud AGN spanning more than four decades in the energy output of the nucleus. The core sample includes 40 low-power sources (BL Lac objects) and 22 high-power sources (radio-loud quasars) spanning the redshift range z~0.15 to z~0.5, all imaged with the high spatial resolution of HST. All of the sources are found to lie in luminous elliptical galaxies, which follow the Kormendy relation for normal ellipticals. A very shallow trend is detected between nuclear brightness (corrected for beaming) and host galaxy luminosity. Black hole masses are estimated for the entire sample, using both the bulge luminosity--black hole mass and the velocity dispersion--black hole mass relations for local galaxies. The latter involves a new method, using the host galaxy morphological parameters, mu_e and r_e, to infer the velocity dispersion, sigma, via the fundamental plane correlation. Both methods indicate that the entire sample of radio-loud AGN are powered by very massive central black holes, with M_{black hole} ~ 10^8 to 10^10 M_{sun}$. Eddington ratios range from L/L_{Eddington} ~ 2 x 10^-4 to ~1, with the high- power sources having higher Eddington ratios than the low-power sources. Overall, radio-loud AGN appear to span a very large range in accretion efficiency, which is all but independent of the mass of the host galaxy.