We present the final results from our deep Hubble Space Telescope (HST) imaging study of the host galaxies of radio‐quiet quasars (RQQs), radio‐loud quasars (RLQs) and radio galaxies (RGs). We describe and analyse new Wide Field & Planetary Camera 2 (WFPC2) R‐band observations for 14 objects, which when combined with the first tranche of HST imaging reported in McLure et al., provide a complete and consistent set of deep, red, line‐free images for statistically matched samples of 13 RQQs, 10 RLQs and 10 RGs in the redshift band 0.1 < z < 0.25. We also report the results of new deep VLA imaging that has yielded a 5‐GHz detection of all but one of the 33 active galactic nuclei (AGN) in our sample. Careful modelling of our images, aided by a high dynamic‐range point spread function, has allowed us to determine accurately the morphology, luminosity, scalelength and axial ratio of every host galaxy in our sample. Armed with this information we have undertaken a detailed comparison of the properties of the hosts of these three types of powerful AGN, both internally and with the galaxy population in general. We find that spheroidal hosts become more prevalent with increasing nuclear luminosity such that, for nuclear luminosities MV < −23.5, the hosts of both radio‐loud and radio‐quiet AGN are virtually all massive ellipticals. Moreover, we demonstrate that the basic properties of these hosts are indistinguishable from those of quiescent, evolved, low‐redshift ellipticals of comparable mass. This result rules out the possibility that radio‐loudness is determined by host‐galaxy morphology, and also sets severe constraints on evolutionary schemes that attempt to link low‐z ultraluminous infrared galaxies with RQQs. Instead, we show that our results are as expected given the relationship between black hole and spheroid mass established for nearby galaxies, and apply this relation to estimate the mass of the black hole in each object. The results agree remarkably well with completely independent estimates based on nuclear emission‐line widths; all the quasars in our sample have Mbh > 5 × 108 M⊙, while the radio‐loud objects are confined to Mbh > 109 M⊙. This apparent mass‐threshold difference, which provides a natural explanation for why RQQs outnumber RLQs by a factor of 10, appears to reflect the existence of a minimum and a maximum level of black hole radio output, which is a strong function of black hole mass (∝M2−2.5bh). Finally, we use our results to estimate the fraction of massive spheroids/black holes that produce quasar‐level activity. This fraction is ≃0.1 per cent at the present day, rising to >10 per cent at z≃ 2–3.

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 that have been obtained for the first half of our 33‐source sample. We find that the hosts of all three classes of luminous AGN are massive elliptical galaxies, with scalelengths ≃10 kpc, and R−K colours consistent with mature stellar populations. Most importantly, this is 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. We apply the relations given by Magorrian et al. to infer the expected Eddington luminosity of the putative black hole at the centre of each of the spheroidal host galaxies we have uncovered. Comparison with the actual nuclear R‐band luminosities suggests that the black 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 capable of hosting quasars with MR≃− 28, comparable to the most luminous quasars at z≃3. Finally, we discuss our host‐derived black hole masses in the context of the radio luminosity:black hole mass correlation recently uncovered for nearby galaxies by Franceschini et al., and consider the resulting implications for the physical origin of radio loudness.