The masses of AGN host galaxies & the origin of radio loudness

Abstract

We highlight some of the principal results from our recent Hubble Space Telescope studies of quasars and radio galaxies. The hosts of these powerful AGN are normal massive ellipticals which lie on the region of the fundamental plane populated predominantly by massive ellipticals with boxy isophotes and distinct cores. The hosts of the radio-loud sources are on average 1.5 times brighter than their radio-quiet counterparts and appear to lie above a mass threshold of 4 x 10^11 solar masses. This suggests that black holes more massive than 5 x 10^8 solar masses are required to produce a powerful radio source. However we show that this apparent threshold appears to be a consequence of an upper bound on radio output which is a strong function of black-hole mass, L_5GHz proportional to M_bh^2.5. This steep mass dependence can explain why the hosts of the most powerful radio sources are good standard candles. Such objects were certainly fully assembled by z = 1, and appear to have formed the bulk of their stars prior to z = 3.