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.

I review the radio to X-ray properties of gigahertz peaked-spectrum (GPS) and compact steepspectrum (CSS) sources, the current hypotheses for their origin, and their use to constrain the evolution of powerful radio galaxies. The GPS and CSS sources are compact, powerful radio sources with well-defined peaks in their radio spectra (near 1 GHz in the GPS and near 100 MHz in the CSS). The GPS sources are entirely contained within the extent of the narrow-line region (Շ1 kpc), while the CSS sources are contained entirely within the host galaxy (Շ15 kpc). The peaks in the spectra are probably due to synchrotron self-absorption, though free-free absorption through an inhomogeneous screen may also play a role. The turnover frequency varies with linear size l as , suggesting a simple physical relationship between these parameters. The radion ∝ l m morphologies are strikingly like those of the large-scale classical doubles, though some sources can have very distorted morphologies suggestive of interactions. Radio polarization tends to be low, and in some cases the Faraday rotation measures can be extremely large. The IR properties are consistent with stellar populations and active galactic nucleus (AGN) bolometric luminosity similar to that of the 3CR classical doubles. The optical host galaxy properties (absolute magnitude, Hubble diagram, evidence for interaction) are consistent with those of the 3CR classical doubles. CSS sources at all redshifts exhibit high surface brightness optical light (most likely emission-line gas) that is aligned with the radio axis. The optical emission-line properties suggest (1) interaction of the radio source with the emission-line gas and (2) the presence of dust toward the emission-line regions. Xray observations of high-redshift GPS quasars and a couple of GPS galaxies suggest the presence of significant columns of gas toward the nuclei. Searches for cold gas in the host galaxies have revealed large amounts of molecular gas and smaller amounts of atomic gas in several sources, though probably not enough to confine the radio sources. The main competing models for the GPS and CSS sources are that (1) they are frustrated by interaction with dense gas in their environments and (2) they are young and evolving radio sources that will become large-scale sources. Combining the bright GPS and CSS samples with the 3CR results in a sample spanning a range in source size of 10 5 that can be used to study source evolution. The number density versus linear size relation is consistent with a picture in which the sources expand with constant velocity and the radio power drops with linear size l according to . This strong evolution suggests that at least some of the Ϫ0.5 P ∝ l GPS and CSS sources evolve to become lower luminosity FR 1 radio sources. The GPS and CSS sources are important probes of their host galaxies and will provide critical clues to the origin and evolution of powerful radio sources. 1 ergs s Ϫ1 cm Ϫ2 Hz Ϫ1 . Ϫ23 1 Jy ϭ 10 2 We define spectral index a such that .

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Quillen et al. presented an imaging survey with the Spitzer Space Telescope of 62 brightest cluster galaxies with optical line emission located in the cores of X-ray-luminous clusters. They found that at least half of these sources have signs of excess IR emission. Here we discuss the nature of the IR emission and its implications for cool core clusters. The strength of the mid-IR excess emission correlates with the luminosity of the optical emission lines. Excluding the four systems dominated by an AGN, the excess mid-IR emission in the remaining brightest cluster galaxies is likely related to star formation. The mass of molecular gas (estimated from CO observations) is correlated with the IR luminosity as found for normal star-forming galaxies. The gas depletion timescale is about 1 Gyr. The physical extent of the IR excess is consistent with that of the optical emission-line nebulae. This supports the hypothesis that star formation occurs in molecular gas associated with the emission-line nebulae and with evidence that the emission-line nebulae are mainly powered by ongoing star formation. We find a correlation between mass deposition rates (Ṁ X ) estimated from the X-ray emission and the star formation rates estimated from the IR luminosity. The star formation rates are 1/10 to 1/100 of the mass deposition rates, suggesting that the reheating of the intracluster medium is generally very effective in reducing the amount of mass cooling from the hot phase but not eliminating it completely.

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.