On the host galaxies of quasars

I present the results of three distinct, but inter-connected studies of the nature and evolution of the host galaxies of quasars. These three are: i) An HST V-band study of a sample of 17 quasars at z ≈ 0.4, with absolute magnitudes in the range -24 ≥ Mv ≥ -28; ii) An HST rest-frame U-band study of matched subsamples of radio-loud and radio-quiet quasars at z ≈ 1 and z ≈ 2; and iii) A UKIRT K-band imaging study of a sample of ULIRG’s and IR-bright QSO’s A 2-dimensional modelling technique has been developed, and used throughout to disentangle the host galaxies from the nucleardominated UV, optical and infrared images.The first study allows an investigation of host galaxy properties across a decade in quasar luminosity, but at a single redshift, and forms a crucial low-z baseline against which to compare future host galaxy studies at higher z- Previous imaging studies of AGN hosts have focussed primarily on quasars of moderate luminosity, but the most powerful objects in this sample have powers comparable to the most luminous quasars found at high redshifts. All the radio-loud quasars, and all the radio-quiet quasars with nuclear luminosities Mv < -24, are found to have massive bulge-dominated galaxies, confirming and extending the trends deduced from previous studies. From the best-fitting model host galaxies I have estimated spheroid and hence black-hole masses, and the efficiency (with respect to the Eddington luminosity) with which each quasar is emitting radiation. The largest inferred black-hole mass in our sample is mBH ≈ 3 x 10⁹m⨀, comparable to the mass of the black holes at the centres of M87 and Cygnus A. No evidence is found for super-Eddington accretion rates in even the most luminous objects. The role of the scatter in the black-hole:spheroid mass relation in determining the ratio of quasar to host-galaxy luminosity is addressed 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-to-host luminosity ratios can be explained in terms of sub-Eddington accretion rates onto black holes in the high-mass tail of the black-hole:spheroid relation. The results imply that, owing to the Schechter function cutoff, host mass should not continue to increase linearly with quasar luminosity, at the very highest luminosities. Any quasars more luminous than Mv = -27 should be found in massive elliptical hosts which at the present day would have Mv ≈ -24.5.The second study is used in concert with existing rest-frame V-band data to calculate U - V colours for the host galaxies, and thus provide the first unbiased estimates of quasar host galaxy evolution out to cosmologically significant distances. The host galaxy colours are found to be broadly consistent with the assumption of passive evolution, but with a small amount (< 1%) of ongoing starformation. The hosts of the radio-quiet quasars are found to evolve slightly more rapidly than those of the radio-louds.Finally, the same modelling technique is applied to a sample of ULIRG’s and a control sample of IR-bright QSO’s, matched in terms of their 60μm luminosity. Using UKIRT K-band imaging to detect the presence of any well-evolved stellar population, the possibility of an evolutionary link between ULIRG’s and Quasars is explored. Large Tf-band bulges are found to be present in all of the quasars, with the majority of the ULIRG’s being best-fit by a disc. Despite the apparent difference, a significant overlap exists between the two populations in terms of their nuclear luminosity. A significant unresolved nuclear component is present in all of the objects, and this is found to correlate with the luminosity of the AT-band host across the sample. In general, the ULIRG’s do not appear to be on their way to becoming fully-fledged quasars, nor the first-ranked massive ellipticals that we have come to expect to find quasars situated in. It seems likely that such systems are the dusty equivalents of the Seyfert galaxies, and while some may end up as quite large (≤ L*) ellipticals, many have more in common with the disk galaxy populationTaken as a whole, this thesis pushes the study of Quasar Host Galaxies in three new directions, allowing a number of interesting questions on cosmology and galaxy evolution to be addressed