A comparative HST imaging study of the host galaxies of radio-quiet quasars, radio-loud quasars and radio galaxies: I

Abstract

We present the first results from a major HST WFPC2 imaging study aimed at providing the first statistically meaningful comparison of the morphologies, luminosities, scalelengths and colours of the host galaxies of radio-quiet quasars, radio-loud quasars, and radio galaxies. We describe the design of this study and present the images which have now been obtained for approximately the first half of our 33-source sample. We also report and discuss the results of combining high-dynamic range PSF determination with 2-dimensional modelling to extract the key parameters of the host galaxies of these AGN; this is the first substantial study of quasar hosts in which it is proving possible to determine unambiguously the morphological type of the hosts of all quasars in the sample. A full statistical analysis is deferred until the complete sample has been observed, but within the sub-sample presented here we have already obtained some remarkably clean results. We find that the underlying hosts of all three classes of luminous AGN are massive elliptical galaxies, with scalelengths ~=10 kpc, and R−K colours consistent with old stellar populations. Most importantly this is the the first unambiguous evidence that, just like radio-loud quasars, essentially all radio-quiet quasars brighter than MR=−24 reside in massive ellipticals. This result removes the possibility that radio ‘loudness’ is directly linked to host galaxy morphology, but is however in excellent accord with the black-hole/spheroid mass correlation recently highlighted by Magorrian et al. (1998). To demonstrate this we apply the spheroid luminosity/spheroid mass/black-hole mass relations given by Magorrian et al. to infer the expected Eddington luminosity (and hence maximum expected nuclear R-band luminosity) of the putative black hole at the centre of each of the spheroidal host galaxies we have uncovered. Comparison of the predicted Eddington luminosities with the actual nuclear R-band luminosities produces a clear relationship, and suggests that the black holes in most of these galaxies are radiating at a few percent of the Eddington luminosity (although a few appear to be radiating close to the Eddington limit); the brightest host galaxies in our low-redshift sample are capable of hosting quasars with MR~=−28, comparable to the most luminous quasars at z~=3, if fueled at the Eddington rate. Finally we discuss our host-galaxy-derived black-hole masses in the context of the radio-luminosity:black-hole mass correlation recently uncovered for nearby galaxies by Franceschini et al. (1998). The radio loud AGN (both RGs and RLQs) lie on the Ptotal 5GHz : mbh relation, while the RQQs lie on the Pcore 5GHz : mbh relation. This finding may hold the key to identifying the physical origin of the radio-loud radio-quiet dichotomy (Refer to PDF file for exact formulas).