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

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-quie t quasars, radio-loud quasars and radio galaxies, We describe the design of this study and present the images that have been obtained for the first ha lf of our 33-source sample. We find that the hosts of all three classes of luminous AGN are massive elliptical galaxies, with scalelengths similar or equal to 10 kpc, and R-K colours consistent with mature stellar populations . Most importantly, this is first unambiguous evidence that, just like radi o-loud quasars, essentially all radio-quiet quasars brighter than M-R = -24 reside in massive ellipticals. This result removes the possibility that ra dio '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, We apply the relations given by Magorrian et al. to infer the expected Eddington luminosity of the putative black hol e at the centre of each of the spheroidal host galaxies we have uncovered. Comparison with the actual nuclear R-band luminosities suggests that the bl ack holes in most of these galaxies are radiating at a few per cent of the Eddington luminosity; the brightest host galaxies in our low-z sample are c apable of hosting quasars with M-R similar or equal to -28, comparable to t he most luminous quasars at z similar or equal to 3. Finally, we discuss ou r host derived black hole masses in the context of the radio luminosity:bla ck hole mass correlation recently uncovered for nearby galaxies by Francesc hini et al,, and consider the resulting implications for the physical origi n of radio loudness.