The host galaxies of luminous quasars

We present the results of a deep Hubble Space Telescope (HST)/Wide Field and Planetary Camera 2 imaging study of 17 quasars at z~= 0.4, designed to determine the properties of their host galaxies. The sample consists of quasars with absolute magnitudes in the range -24 >=M_V>=- 28, allowing us to investigate host galaxy properties across a decade in quasar luminosity, but at a single redshift. Our previous imaging studies of active galactic nuclei hosts have focused primarily on quasars of moderate luminosity, but the most powerful objects in the current sample have powers comparable to the most luminous quasars found at high redshifts.

We find that the host galaxies of all the radio-loud quasars, and all the radio-quiet quasars in our sample with nuclear luminosities M_V < -24, are massive bulge-dominated galaxies, confirming and extending the trends deduced from our previous studies. From the best-fitting model host galaxies we have estimated spheroid and hence black hole (BH) masses, and the efficiency (with respect to the Eddington luminosity) with which each quasar emits radiation. The largest inferred black hole mass in our sample is M_BH~= 3 × 10⁹ M_solar, comparable to the mass of the black holes at the centres of M87 and Cygnus A. We find no evidence for super-Eddington accretion rates in even the most luminous objects (0.05 < L/L_Edd < 1.0).

We investigate the role of scatter in the black hole-spheroid mass relation in determining the ratio of quasar to host-galaxy luminosity, by generating simulated populations of quasars lying in hosts with a Schechter mass function. Within the subsample of the highest-luminosity quasars, the observed variation in nuclear-host luminosity ratio is consistent with being the result of the scatter in the black hole-spheroid relation. Quasars with high nuclear-host luminosity ratios can be explained in terms of sub-Eddington accretion rates on to black holes in the high-mass tail of the black hole-spheroid relation. Our results imply that, owing to the Schechter function cut-off, host mass should not continue to increase linearly with quasar luminosity, at the very highest luminosities. Any quasars more luminous than M_V=-27 should be found in massive elliptical hosts, which at the present day would have M_V~=-24.5.