Far-infrared-loud quasars – I. Disturbed and quiescent quasars in the PG survey

We use host galaxy imaging studies of the PG quasar survey to compare the far-infrared (FIR) properties of quasars with disturbed and undisturbed host galaxies. By using survival analysis, we show that the quasars with disturbed host galaxies, with morphologies classified from a homogeneous data set, have a 60-μm luminosity distribution that is different from that of those with undisturbed hosts with >97 per cent confidence. For morphological classifications using an inhomogeneous data set, including HST data for some objects, this confidence rises to >99 per cent confidence. The mean 60-μm luminosity of the disturbed-host quasars is several times greater than that of the undisturbed-host quasars. However, possible biases in the PG survey might affect these conclusions. Our results are interpreted as supporting the idea that quasars are related to at least some ultraluminous infrared galaxies. We discuss the implications of this result for studies of quasar and galaxy evolution.     

Clustering of galaxies around radio quasars at 0.5≤z≤0.8

We have observed the galaxy environments around a sample of 21 radio-loud, steep-spectrum quasars at 0.5≤ z ≤0.82, spanning several orders of magnitude in radio luminosity. The observations also include background control fields used to obtain the excess number of galaxies in each quasar field. The galaxy excess was quantified using the spatial galaxy–quasar correlation amplitude, B _gq, and an Abell-type measurement, N _0.5. A few quasars are found in relatively rich clusters, but on average, they seem to prefer galaxy groups or clusters of approximately Abell class 0. We have combined our sample with literature samples extending down to z ≈0.2 and covering the same range in radio luminosity. By using the Spearman statistic to disentangle redshift and luminosity dependences, we detect a weak, but significant, positive correlation between the richness of the quasar environment and the radio luminosity of the quasar. However, we do not find any epoch dependence in B _gq, as has previously been reported for radio quasars and galaxies. We discuss the radio luminosity–cluster richness link and possible explanations for the weak correlation that is seen.     

HST Planetary Camera images of quasar host galaxies

We present Hubble Space Telescope ( HST ) images of seven low-redshift quasars (six taken with the Planetary Camera, one with the Wide Field Camera). These complete the sample of 14 quasars observed by the Faint Object Camera Investigation Definition Team (FOC IDT). Following subtraction of the quasar nuclear light, host galaxies can be seen in all seven cases. A combination of the optical morphology and luminosity profiles of the residual host galaxies and the results of 2D cross-correlation model fitting implies that five of the objects have elliptical host galaxies and two have disc host galaxies. The luminosities vary from slightly fainter than L ^* to about 1.3 mag brighter than L ^*.   We discuss the properties of the complete sample of 14 quasars. Nine of the objects appear to have elliptical host galaxies (all six of the radio-loud quasars in the sample as well as three radio-quiet quasars). Two further radio-quiet quasars appear to lie in disc galaxies. The other three objects (radio-quiet, ultraluminous infrared quasars) all lie in violently interacting systems. The sample as a whole has an average luminosity about 0.8 mag brighter than L ^*, although the radio-loud objects have hosts on average 0.7 mag brighter than the radio-quiet objects.   We compare our results with those from HST imaging of quasars by other authors. Taken together, our observations are in broad agreement with those of Bahcall et al. Radio-loud quasars appear to lie in luminous elliptical galaxies whereas radio-quiet quasars are found to lie in either elliptical or spiral hosts. Host galaxy luminosities (of radio-quiet and radio-loud quasars) are much brighter than would be expected if they followed a Schechter luminosity function.     

Modelling the cosmological co-evolution of supermassive black holes and galaxies – II. The clustering of quasars and their dark environment

We use semi-analytic modelling on top of the Millennium simulation to study the joint formation of galaxies and their embedded supermassive black holes. Our goal is to test scenarios in which black hole accretion and quasar activity are triggered by galaxy mergers, and to constrain different models for the light curves associated with individual quasar events. In the present work, we focus on studying the spatial distribution of simulated quasars. At all luminosities, we find that the simulated quasar two-point correlation function is fit well by a single power law in the range  0.5 ≲ r ≲ 20  h ⁻¹ Mpc  , but its normalization is a strong function of redshift. When we select only quasars with luminosities within the range typically accessible by today's quasar surveys, their clustering strength depends only weakly on luminosity, in agreement with observations. This holds independently of the assumed light-curve model, since bright quasars are black holes accreting close to the Eddington limit, and are hosted by dark matter haloes with a narrow mass range of a few  10¹²  h ⁻¹ M_⊙  . Therefore, the clustering of bright quasars cannot be used to disentangle light-curve models, but such a discrimination would become possible if the observational samples can be pushed to significantly fainter limits. Overall, our clustering results for the simulated quasar population agree rather well with observations, lending support to the conjecture that galaxy mergers could be the main physical process responsible for triggering black hole accretion and quasar activity.     

On quasar host galaxies as tests of non-cosmological redshifts

Despite a general consensus in the astronomical community that all quasars are located at the distances implied by their redshifts, a number of observations still challenge this interpretation, possibly indicating that some subpopulation of quasars may harbour significant redshift components not related to the expansion of the Universe. It has been suggested that these objects may have been ejected from local galaxies and are likely to evolve into new galaxies themselves. Here, a test of such exotic scenarios is proposed, based on the spectral energy distribution of the galaxies hosting quasars with suspected ejection origin. Provided that the time-scales over which the ejected objects manifest themselves as quasars is short, one would in the framework of the ejection scenarios expect to find either no quasar host galaxy, a pseudo-host consisting of gas ionized by the quasar, or a host galaxy consisting of young stars only. It is argued that the spectral energy distributions corresponding to the last two options should differ significantly from that of most quasar host galaxies detected at low redshift so far, thus providing a potential test of the claimed existence of ejected quasars. A minimal implementation of this test, involving optical and near-infrared broad-band photometry, is suggested.     

The KX method for producing K-band flux-limited samples of quasars

The longstanding question of the extent to which the quasar population is affected by dust extinction, within host galaxies or galaxies along the line of sight, remains open. More generally, the spectral energy distributions of quasars vary significantly, and flux-limited samples defined at different wavelengths include different quasars. Surveys employing flux measurements at widely separated wavelengths are necessary to characterize fully the spectral properties of the quasar population. The availability of panoramic near-infrared detectors on large telescopes provides the opportunity to undertake surveys capable of establishing the importance of extinction by dust on the observed population of quasars. We introduce an efficient method for selecting K -band, flux-limited samples of quasars, termed 'KX' by analogy with the UVX method. This method exploits the difference between the power-law nature of quasar spectra and the convex spectra of stars: quasars are relatively brighter than stars at both short wavelengths (the UVX method) and long wavelengths (the KX method). We consider the feasibility of undertaking a large-area KX survey for damped Ly α galaxies and gravitational lenses using the planned UKIRT wide-field near-infrared camera.     

The host galaxies of luminous radio-quiet quasars

We present the results of a deep K -band imaging study which reveals the host galaxies around a sample of luminous radio-quiet quasars. The K -band images, obtained at UKIRT, are of sufficient quality to allow accurate modelling of the underlying host galaxy. Initially, the basic structure of the hosts is revealed using a modified clean deconvolution routine optimized for this analysis. Two of the 14 quasars are shown to have host galaxies with violently disturbed morphologies which cannot be modelled by smooth elliptical profiles. For the remainder of our sample, 2D models of the host and nuclear component are fitted to the images using the χ ² statistic to determine goodness of fit. Host galaxies are detected around all of the quasars. The reliability of the modelling is extensively tested, and we find the host luminosity to be well constrained for nine quasars. The derived average K -band absolute K -corrected host galaxy magnitude for these luminous radio-quiet quasars is 〈 M _K 〉=25.15±0.04, slightly more luminous than an L * galaxy. The spread of derived host galaxy luminosities is small, although the spread of nuclear-to-host ratios is not. These host luminosities are shown to be comparable to those derived from samples of quasars of lower total luminosity, and we conclude that there is no correlation between host and nuclear luminosity for these quasars. Nuclear-to-host ratios break the lower limit previously suggested from studies of lower nuclear luminosity quasars and Seyfert galaxies. Morphologies are less certain but, on the scales probed by these images, some hosts appear to be dominated by spheroids while others appear to have disc-dominated profiles.     

Optical and near-infrared imaging of faint Gigahertz Peaked Spectrum sources

A sample of 47 faint Gigahertz Peaked Spectrum (GPS) radio sources selected from the Westerbork Northern Sky Survey (WENSS) has been imaged in the optical and near-infrared, resulting in an identification fraction of 87 per cent. The R  −  I R  −  K colours of the faint optical counterparts are as expected for passively evolving elliptical galaxies, assuming that they follow the R -band Hubble diagram as determined for radio-bright GPS galaxies. We find evidence that the radio spectral properties of the GPS quasars are different from those of GPS galaxies. The observed distribution of radio spectral peak frequencies for GPS sources optically identified with bright stellar objects (presumably quasars) is shifted compared with GPS sources identified with faint or extended optical objects (presumably galaxies), in the sense that a GPS quasar is likely to have a higher peak frequency than a GPS galaxy. This means that the true peak frequency distribution is different for the GPS galaxies and quasars, because the sample selection effects are independent of optical identification. The correlation between peak frequency and redshift that has been suggested for bright sources has not been found in this sample; no correlation exists between R magnitude (and therefore redshift) and peak frequency for the GPS galaxies. We therefore believe that the claimed correlation is actually caused by the dependence of the peak frequency on optical host, because the GPS galaxies are generally at lower redshifts than the quasars. The difference in the peak frequency distributions of the GPS galaxies and quasars is further evidence against the hypothesis that they form a single class of object.     

The far-infrared–submillimetre spectral energy distribution of high-redshift quasars

We combine photometric observations of high-redshift     quasars, obtained at submillimetre to millimetre wavelengths, to obtain a mean far-infrared (rest-frame) spectral energy distribution (SED) of the thermal emission from dust, parametrized by a single temperature ( T ) and power-law emissivity index ( β ). The best-fitting values are     and     . Our method exploits the redshift spread of this set of quasars, which allows us to sample the SED at a larger number of rest wavelengths than is possible for a single object: the wavelength range extends down to ∼60 μm, and therefore samples the turnover in the greybody curve for these temperatures. This parametrization is of use to any studies that extrapolate from a flux at a single wavelength, for example to infer dust masses and far-infrared luminosities.
We interpret the cool, submillimetre component as arising from dust heated by star formation in the host galaxy of the quasar, although we do not exclude the presence of dust heated directly by the active galactic nucleus (AGN). Applying the mean SED to the data, we derive consistent star formation rates ∼1000 M_⊙ yr⁻¹ and dust masses ∼10⁹ M_⊙, and investigate a simple scheme of AGN and host galaxy co-evolution to account for these quantities. The time-scale for formation of the host galaxy is     , and the luminous quasar phase occurs towards the end of this period, just before the reservoir of cold gas is depleted. Given the youth of the Universe at     (1.6 Gyr), the coexistence of a massive black hole and a luminous starburst at high redshifts is a powerful constraint on models of quasar host galaxy formation.     

The ages of quasar host galaxies

We present the results of fitting deep off-nuclear optical spectra of radio-quiet quasars, radio-loud quasars and radio galaxies at z ≃0.2 with evolutionary synthesis models of galaxy evolution. Our aim was to determine the age of the dynamically dominant stellar populations in the host galaxies of these three classes of powerful active galactic nuclei (AGN). Some of our spectra display residual nuclear contamination at the shortest wavelengths, but the detailed quality of the fits longward of the 4000-Å break provides unequivocal proof, if further proof were needed, that quasars lie in massive galaxies with (at least at z ≃0.2) evolved stellar populations. By fitting a two-component model we have separated the very blue (starburst and/or AGN contamination) from the redder underlying spectral energy distribution, and find that the hosts of all three classes of AGN are dominated by old stars of age 8–14 Gyr. If the blue component is attributed to young stars, we find that, at most, 1 per cent of the visible baryonic mass of these galaxies is involved in star formation activity at the epoch of observation, at least over the region sampled by our spectroscopic observations. These results strongly support the conclusion reached by McLure et al. that the host galaxies of luminous quasars are massive ellipticals which have formed by the epoch of peak quasar activity at z ≃2.5.     

The quasar-frame velocity distribution of narrow C iv absorbers

We report on a survey for narrow (full widths at half-minimum <600 km s⁻¹) C  iv absorption lines in a sample of bright quasars at redshifts  1.8 ≤ z < 2.25  in the Sloan Digital Sky Survey. Our main goal is to understand the relationship of narrow C  iv absorbers to quasar outflows and, more generally, to quasar environments. We determine velocity zero-points using the broad Mg  ii emission line, and then measure the absorbers' quasar-frame velocity distribution. We examine the distribution of lines arising in quasar outflows by subtracting model fits to the contributions from cosmologically intervening absorbers and absorption due to the quasar host galaxy or cluster environment. We find that a substantial number (  ≥43 ± 6  per cent) of absorbers with   W ^λ1548₀ > 0.3  Å in the velocity range  +750 ≲ v ≲+ 12 000  km s⁻¹ are intrinsic to the active galactic nucleus outflow. This 'outflow fraction' peaks near   v =+2000  km s⁻¹ with a value of   f _outflow≃ 0.81 ± 0.13  . At velocities below   v ≈+ 2000  km s⁻¹, the incidence of outflowing systems drops, possibly due to geometric effects or to the over-ionization of gas that is nearer the accretion disc. Furthermore, we find that outflow absorbers are on average broader and stronger than cosmologically intervening systems. Finally, we find that ∼14 per cent of the quasars in our sample exhibit narrow, outflowing C  iv absorption with   W ^λ1548₀ > 0.3  Å, slightly larger than that for broad absorption line systems.     

Measuring the black hole masses of high-redshift quasars

A new technique is presented for determining the black hole masses of high-redshift quasars from optical spectroscopy. The new method utilizes the full-width at half-maximum (FWHM) of the low-ionization Mg  ii emission line and the correlation between the broad-line region (BLR) radius and the continuum luminosity at 3000 Å. Using archival ultraviolet (UV) spectra it is found that the correlation between BLR radius and 3000-Å luminosity is tighter than the established correlation with 5100-Å luminosity. Furthermore, it is found that the correlation between BLR radius and 3000-Å continuum luminosity is consistent with a relation of the form   R _BLR∝λ L ^1/2_λ  , as expected for a constant ionization parameter. Using a sample of objects with broad-line radii determined from reverberation mapping it is shown that the FWHM of Mg  ii and Hβ are consistent with following an exact one-to-one relation, as expected if both Hβ and Mg  ii are emitted at the same radius from the central ionizing source. The resulting virial black hole mass estimator based on rest-frame UV observables is shown to reproduce black hole mass measurements based on reverberation mapping to within a factor of 2.5 (1σ). Finally, the new UV black hole mass estimator is shown to produce identical results to the established optical (Hβ) estimator when applied to 128 intermediate-redshift  (0.3 < z < 0.9)  quasars drawn from the Large Bright Quasar Survey and the radio-selected Molonglo quasar sample. We therefore conclude that the new UV virial black hole mass estimator can be reliably used to estimate the black hole masses of quasars from   z ∼ 0.25  through to the peak epoch of quasar activity at   z ∼ 2.5  via optical spectroscopy alone.     

Quasar–galaxy associations revisited

Gravitational lensing predicts an enhancement of the density of bright, distant quasi-stellar objects (QSOs) around foreground galaxies. We measure this QSO–galaxy correlation w _qg for two complete samples of radio-loud quasars, the southern 1 Jy and half-Jansky samples. The existence of a positive correlation between     quasars and     galaxies is confirmed at a     per cent significance level (>99.9 per cent if previous measurements on the northern hemisphere are included). A comparison with the results obtained for incomplete quasar catalogues (e.g. the Veron-Cetty and Veron compilation) suggests the existence of an 'identification bias', which spuriously increases the estimated amplitude of w _qg for incomplete samples. This effect may explain many of the surprisingly strong quasar–galaxy associations found in the literature. Nevertheless, the value of w _qg that we measure in our complete catalogues is still considerably higher than the predictions from weak lensing. We consider two effects which could help to explain this discrepancy: galactic dust extinction and strong lensing.     

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-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 M _R =−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 M _R ≃− 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.     

Ultraviolet Fe ii emission in z∼ 2 quasars

We present spectra of six luminous quasars at   z ∼ 2  , covering rest wavelengths 1600−3200 Å. The fluxes of the UV Fe  ii emission lines and Mg  ii λ2798 doublet, the line widths of Mg  ii and the 3000 Å luminosity were obtained from the spectra. These quantities were compared with those of low-redshift quasars at   z = 0.06–0.55  studied by Tsuzuki et al. In a plot of the Fe  ii (UV)/Mg  ii flux ratio as a function of the central black hole mass, Fe  ii (UV)/Mg  ii in our   z ∼ 2  quasars is systematically greater than in the low-redshift quasars. We confirmed that luminosity is not responsible for this excess. It is unclear whether this excess is caused by rich Fe abundance at   z ∼ 2  over low-redshift or by non-abundance effects such as high gas density, strong radiation field and high microturbulent velocity.     

Implications for unified schemes from submillimetre and far-infrared follow-up of radio-selected samples

We extend our previous analysis which used generalized luminosity functions (GLFs) to predict the number of quasars and galaxies in low-radio-frequency-selected samples as a function of redshift, radio luminosity, narrow-emission-line luminosity and type of unified scheme. Our extended analysis incorporates the observed submillimetre (850-μm) flux densities of radio sources, employs a new method which allows us to deal with non-detections, and focuses on the high-luminosity population. First, we conclude that the submillimetre luminosity L ₈₅₀ of low-frequency-selected radio sources is correlated with the bolometric luminosity L _bol of their quasar nuclei via an approximate scaling relation   L ₈₅₀∝ L ^0.7±0.2_bol  . Secondly, we conclude that there is quantitative evidence for a receding-torus-like physical process for the high-luminosity population within a two-population unified scheme for radio sources; this evidence comes from the fact that radio quasars are brighter in both narrow emission lines and submillimetre luminosity than radio galaxies matched in radio luminosity and redshift. Thirdly, we note that the combination of a receding-torus-like scheme and the assumption that the observed submillimetre emission is dominated by quasar-heated dust yields a scaling relation   L ₈₅₀∝ L ^1/2_bol  which is within the errors of that determined here for radio-selected quasars, and consistent with that inferred for radio-quiet quasars.     

Nuclear spectra of polar-ring galaxies

We report the results of spectroscopic observations of eight southern polar-ring galaxies (PRGs), in the wavelength range 5900–7300 Å. We find that five out of eight galaxies contain LINERs or Sy nuclei. Taking into consideration all PRGs with available spectral data, we estimate that about half of all PRGs and PRG candidates have either LINER or Seyfert nuclei. The observed widths of the [N  ii ] λ 6583 line in the nuclei of early-type PRGs follow the linewidth–absolute luminosity relation for normal E/S0 galaxies. We found that one of the observed galaxies – ESO 576-G69 – is a new kinematically-confirmed polar-ring galaxy with a spiral host.     

A simple model for the evolution of supermassive black holes and the quasar population

An empirically motivated model is presented for accretion-dominated growth of supermassive black holes (SMBH) in galaxies, and the implications are studied for the evolution of the quasar population in the Universe. We investigate the core aspects of the quasar population, including space density evolution, evolution of the characteristic luminosity, plausible minimum masses of quasars, the mass function of SMBH and their formation epoch distribution. Our model suggests that the characteristic luminosity in the quasar luminosity function arises primarily as a consequence of a characteristic mass scale above which there is a systematic separation between the black hole and the halo merging rates. At lower mass scales, black hole merging closely tracks the merging of dark haloes. When combined with a declining efficiency of black hole formation with redshift, the model can reproduce the quasar luminosity function over a wide range of redshifts. The observed space density evolution of quasars is well described by formation rates of SMBH above  ∼10⁸  M_⊙  . The inferred mass density of SMBH agrees with that found independently from estimates of the SMBH mass function derived empirically from the quasar luminosity function.     

The properties of Lyα emitting galaxies in hierarchical galaxy formation models

We present detailed predictions for the properties of Lyα-emitting galaxies in the framework of the Λ cold dark matter cosmology, calculated using the semi-analytical galaxy formation model galform . We explore a model that assumes a top-heavy initial mass function in starbursts and that has previously been shown to explain the sub-millimetre number counts and the luminosity function of Lyman-break galaxies at high redshift. We show that this model, with the simple assumption that a fixed fraction of Lyα photons escape from each galaxy, is remarkably successful at explaining the observed luminosity function of Lyα emitters (LAEs) over the redshift range  3 < z < 6.6  . We also examine the distribution of Lyα equivalent widths and the broad-band continuum magnitudes of emitters, which are in good agreement with the available observations. We look more deeply into the nature of LAEs, presenting predictions for fundamental properties such as the stellar mass and radius of the emitting galaxy and the mass of the host dark matter halo. The model predicts that the clustering of LAEs at high redshifts should be strongly biased relative to the dark matter, in agreement with observational estimates. We also present predictions for the luminosity function of LAEs at   z > 7  , a redshift range that is starting to be be probed by near-infrared surveys and using new instruments such as the Dark Ages Z Lyman Explorer (DAzLE).     

ROSAT X-ray observations of 3CRR radio sources

Over half the 3CRR sample of radio galaxies and quasars has been observed in X-rays with ROSAT pointed observations, and we present results from these observations, discussing many of the sources in detail. The improved spatial resolution of ROSAT over earlier missions allows a better separation of the nuclear and extended components of the X-ray emission. We investigate the relationship between nuclear X-ray and core radio luminosity, and show that our results support a model in which every radio galaxy and quasar has a beamed nuclear soft X-ray component directly related to the radio core. We report evidence for rich cluster environments around several powerful quasars. These X-ray environments are comparable to those of high-redshift radio galaxies.