X-ray variability in a deep, flux-limited sample of QSOs

We present an analysis of X-ray variability in a flux-limited sample of quasi-stellar objects (QSOs). Selected from our deep ROSAT survey, these QSOs span a wide range in redshift (0.1< z <3.2) and are typically very faint, so we have developed a method to constrain the amplitude of variability in ensembles of low signal-to-noise ratio light curves. We find evidence for trends in this variability amplitude with both redshift and luminosity. The mean variability amplitude declines sharply with luminosity, as seen in local active galactic nuclei (AGN), but with some suggestion of an upturn for the most powerful sources. We find tentative evidence that this is caused by redshift evolution, since the high-redshift QSOs ( z >0.5) do not show the anticorrelation with luminosity seen in local AGN. We speculate on the implications of these results for physical models of AGN and their evolution. Finally, we find evidence for X-ray variability in an object classified as a narrow-emission-line galaxy, suggesting the presence of an AGN.     

The relationship between X-ray variability and the central black hole mass

We assembled a sample of Seyfert 1 galaxies, quasi-stellar objects (QSOs) and low-luminosity active galactic nuclei (LLAGNs) observed by ASCA , the central black hole masses of which have been measured. We found that the X-ray variability (which is quantified by the 'excess variance' σ _rms²) is significantly anti-correlated with the central black hole mass, and it is likely that a linear relationship of σ _rms²∝ M _bh⁻¹ exists. It can be interpreted that the short time-scale X-ray variability is caused by some global coherent variations in the X-ray emission region, which is scaled by the size of the central black hole. Hence the central black hole mass is the driving parameter of the previously established relation between X-ray variability and luminosity. Our findings favour the hypothesis that the narrow-line Seyfert 1 galaxies and QSOs harbour smaller black holes than the broad-line objects, and can also easily explain the observational fact that high-redshift QSOs have greater variability than local AGNs at a given luminosity. Further investigations are needed to confirm our findings, and a large sample X-ray variability investigation can give constraints on the physical mechanisms and evolution of AGNs.     

Variability and spectral energy distributions of low-luminosity active galactic nuclei: a simultaneous X-ray/UV look with Swift

We have observed four low-luminosity active galactic nuclei (AGNs) classified as type 1 Low-Ionization Nuclear Emission-Line Regions (LINERs) with the X-Ray Telescope (XRT) and the Ultraviolet–Optical Telescope (UVOT) onboard Swift , in an attempt to clarify the main powering mechanism of this class of nearby sources. Among our targets, we detect X-ray variability in NGC 3998 for the first time. The light curves of this object reveal variations of up to 30 per cent amplitude in half a day, with no significant spectral variability on this time-scale. We also observe a decrease of ∼30 per cent over 9 d, with significant spectral softening. Moreover, the X-ray flux is ∼40 per cent lower than observed in previous years. Variability is detected in M81 as well, at levels comparable to those reported previously: a flux increase in the hard X-rays (1–10 keV) of 30 per cent in ∼3 h and variations by up to a factor of 2 within a few years. This X-ray behaviour is similar to that of higher luminosity, Seyfert-type objects. Using previous high-angular-resolution imaging data from the Hubble Space Telescope ( HST ), we evaluate the diffuse UV emission due to the host galaxy and isolate the nuclear flux in our UVOT observations. All sources are detected in the UV band, at levels similar to those of the previous observations with HST . The XRT (0.2–10 keV) spectra are well described by single power laws and the UV-to-X-ray flux ratios are again consistent with those of Seyferts and radio-loud AGNs of higher luminosity. The similarity in X-ray variability and broad-band energy distributions suggests the presence of similar accretion and radiation processes in low- and high-luminosity AGNs.     

XMM–Newton surveys of the Canada–France Redshift Survey Fields – III. The environments of X-ray selected active galactic nuclei at 0.4 < z < 0.6

The environmental properties of a sample of 31 hard X-ray selected active galactic nuclei (AGN) are investigated, from scales of 500 kpc down to 30 kpc, and are compared to a control sample of inactive galaxies. All the AGN lie in the redshift range  0.4 < z < 0.6  . The accretion luminosity density of the Universe peaks close to this redshift range, and the AGN in the sample have X-ray luminosities close to the knee in the hard X-ray luminosity function, making them representative of the population that dominated this important phase of energy conversion.
Using both the spatial clustering amplitude and near-neighbour counts, it is found that the AGN have environments that are indistinguishable from normal, inactive galaxies over the same redshift range and with similar optical properties. Typically, the environments are of subcluster richness, in contrast to similar studies of high- z quasars, which are often found in clusters with comparable richness to the Abell   R ≥ 0  clusters.
It is suggested that minor mergers with low-mass companions are a likely candidate for the mechanism by which these modest luminosity AGN are fuelled.     

The X-ray properties of optically selected z > 0.6 clusters in the European Southern Observatory Distant Cluster Survey

We present XMM–Newton observations of three optically selected   z > 0.6  clusters from the European Southern Observatory (ESO) Distant Cluster Survey (EDisCS), comprising the first results of a planned X-ray survey of the full EDisCS high-redshift sample. The EDisCS clusters were identified in the Las Campanas Distant Cluster Survey as surface brightness fluctuations in the optical sky and their masses and galaxy populations are well described by extensive photometric and spectroscopic observations. We detect two of the three clusters in the X-ray and place a firm upper limit on diffuse emission in the third cluster field. We are able to constrain the X-ray luminosity and temperature of the detected clusters and estimate their masses. We find that the X-ray properties of the detected EDisCS clusters are similar to those of X-ray-selected clusters of comparable mass and – unlike other high-redshift, optically selected clusters – are consistent with the T –σ and   L _X–σ  relations determined from X-ray-selected clusters at low redshift. The X-ray determined mass estimates are generally consistent with those derived from weak-lensing and spectroscopic analyses. These preliminary results suggest that the novel method of optical selection used to construct the EDisCS catalogue may, like selection by X-ray luminosity, be well suited for identification of relaxed, high-redshift clusters whose intracluster medium is in place and stable by   z ∼ 0.8  .     

Constraining the cosmological baryon density with X-ray clusters

We study the properties of X-ray galaxy clusters in four cold dark matter models with different baryon fractions Ω_BM, ranging from 5 to 20 per cent. By using an original three-dimensional hydrodynamic code based on the piecewise parabolic method, we run simulations on a box with a size of 64  h ⁻¹ Mpc and we identify the clusters by selecting the peaks in the X-ray luminosity field. We analyse these mock catalogues by computing the mass function, the luminosity function, the temperature distribution and the luminosity–temperature relation. By comparing the predictions of the different models to a series of recent observational results, we find that only the models with low baryonic content agree with the data, while models with larger baryon fraction are well outside the 1σ error bars. In particular, the analysis of the luminosity functions, both bolometric and in the energy band [0.5–2] keV, requires Ω_BM ≲ 0.05 when we fix the values h  = 0.5 and n  = 0.8 for the Hubble parameter and the primordial spectral index, respectively. Moreover we find that, independently of the cosmological scenario, all the considered quantities have very little redshift evolution, particularly between z  = 0.5 and 0.     

High-redshift galaxies, their active nuclei and central black holes

We demonstrate that the luminosity function of the recently detected population of actively star-forming galaxies at redshift z  = 3 and the B -band luminosity function of quasi-stellar objects (QSOs) at the same redshift can both be matched with the mass function of dark matter haloes predicted by standard variants of hierarchical cosmogonies for lifetimes of optically bright QSOs anywhere in the range 10⁶ to 10⁸ yr. There is a strong correlation between the lifetime and the required degree of non-linearity in the relation between black hole and halo mass. We suggest that the mass of supermassive black holes may be limited by the back-reaction of the emitted energy on the accretion flow in a self-gravitating disc. This would imply a relation of black hole to halo mass of the form M _bh ∝  v ⁵_halo ∝  M ^5/3_halo and a typical duration of the optically bright QSO phase of a few times 10⁷ yr. The high integrated mass density of black holes inferred from recent black hole mass estimates in nearby galaxies may indicate that the overall efficiency of supermassive black holes for producing blue light is smaller than previously assumed. We discuss three possible accretion modes with low optical emission efficiency: (i) accretion at far above the Eddington rate, (ii) accretion obscured by dust, and (iii) accretion below the critical rate leading to an advection-dominated accretion flow lasting for a Hubble time. We further argue that accretion with low optical efficiency might be closely related to the origin of the hard X-ray background and that the ionizing background might be progressively dominated by stars rather than QSOs at higher redshift.     

On the lack of cold dust in IRAS P09104+4109 and IRAS F15307+3252: their spectral energy distributions and implications for finding dusty AGNs at high redshift

We present upper limits on the 850-μm and 450-μm fluxes of the warm hyperluminous (bolometric luminosity     galaxies IRAS P09104+4109     and IRAS F15307+3252     , derived from measurements using the SCUBA bolometer array on the James Clerk Maxwell Telescope. Hot luminous infrared sources like these are thought to differ from more normal cold ultraluminous infrared     galaxies in that they derive most of their bolometric luminosities from dusty active galactic nuclei (AGNs) as opposed to starbursts. Such hot, dusty AGNs at high redshift are thought to be responsible for much of the mass accretion of the Universe that is in turn responsible for the formation of the supermassive black holes seen in the centres of local galaxies. The galaxy IRAS P09104+4109 is also unusual in that it is a cD galaxy in the centre of a substantial cooling-flow cluster, not an isolated interacting galaxy like most ultraluminous infrared galaxies. Previously it was known to have large amounts of hot     dust from IRAS observations. We now show that the contribution of cold dust to the bolometric luminosity is less than 3 per cent. Most ultraluminous infrared galaxies possess large amounts of cold dust, and it is now known that some cooling-flow cluster cD galaxies do as well. Yet this object, which is an extreme example of both, does not have enough cold gas to contribute significantly to the bolometric luminosity. We outline physical reasons why this could have happened. We then provide a discussion of strategies for finding hot dusty AGNs, given the limitations on submillimetre surveys implied by this work.     

XMM–Newton and Chandra observations of SHEEP sources

We present Chandra and XMM–Newton observations of 12 bright  [ f (2–10 keV) > 10⁻¹³ erg cm⁻² s⁻¹]  sources from the ASCA search for the High Energy Extragalactic Population (SHEEP) survey. Most of these have been either not observed or not detected previously with the ROSAT mission, and therefore they constitute a sample biased towards hard sources. The Chandra observations are important in locating the optical counterpart of the X-ray sources with accuracy. Optical spectroscopic observations show that our sample is associated with both narrow-line (NL) (six objects) and broad-line (BL) active galactic nuclei (AGN) (five objects), with one source remaining unidentified. Our sources cover the redshift range 0.04–1.29, spanning luminosities from 10⁴² to  10⁴⁵ erg s⁻¹  (2–10 keV). The NL sources have preferentially lower redshift (and luminosity) compared to the BL ones. This can be most easily explained in a model where the NL AGN are intrinsically less luminous than the BL ones in line with the results of Steffen et al. The X-ray spectral fittings show a roughly equal number of obscured  ( N _H > 10²² cm⁻²)  and unobscured  ( N _H < 10²² cm⁻²)  sources. There is a clear tendency for obscured sources to be associated with NL AGN and unobscured sources with BL ones. However, there is a marked exception with the highest obscuring column observed at a BL AGN at a redshift of z = 0.5.     

Host galaxy morphologies of X-ray selected AGN: assessing the significance of different black hole fuelling mechanisms to the accretion density of the Universe at z∼ 1.

We use morphological information of X-ray selected active galactic nuclei (AGN) hosts to set limits on the fraction of the accretion density of the Universe at   z ≈ 1  that is not likely to be associated with major mergers. Deep X-ray observations are combined with high-resolution optical data from the Hubble Space Telescope in the All-wavelength Extended Groth strip International Survey, Great Observatories Origins Deep Survey (GOODS) North and GOODS South fields to explore the morphological breakdown of X-ray sources in the redshift interval  0.5 < z < 1.3  . The sample is split into discs, early-type bulge-dominated galaxies, peculiar systems and point sources in which the nuclear source outshines the host galaxy. The X-ray luminosity function and luminosity density of AGN at   z ≈ 1  are then calculated as a function of morphological type. We find that disc-dominated hosts contribute  30 ± 9  per cent to the total AGN space density and  23 ± 6  per cent to the luminosity density at   z ≈ 1  . We argue that AGN in disc galaxies are most likely fuelled not by major merger events but by minor interactions or internal instabilities. We find evidence that these mechanisms may be more efficient in producing luminous AGN     compared to predictions for the stochastic fuelling of massive black holes in disc galaxies.     

The distribution of absorption in AGN detected in the XMM–Newton observations of the CDFS

We have used very deep XMM–Newton observations of the Chandra Deep Field -South to examine the spectral properties of the faint active galactic nucleus (AGN) population. Crucially, redshift measurements are available for 84 per cent (259/309) of the XMM–Newton sample. We have calculated the absorption and intrinsic luminosities of the sample using an extensive Monte Carlo technique incorporating the specifics of the XMM–Newton observations. 23 sources are found to have substantial absorption and intrinsic X-ray luminosities greater than 10⁴⁴ erg s⁻¹, putting them in the 'type-2' QSO regime. We compare the redshift, luminosity and absorption distributions of our sample to the predictions of a range of AGN population models. In contrast to recent findings from ultradeep Chandra surveys, we find that there is little evidence that the absorption distribution is dependent on either redshift or intrinsic X-ray luminosity. The pattern of absorption in our sample is best reproduced by models in which ∼75 per cent of the AGN population is heavily absorbed at all luminosities and redshifts.     

Is there an upper limit to black hole masses?

We make a case for the existence for ultra-massive black holes (UMBHs) in the Universe, but argue that there exists a likely upper limit to black hole (BH) masses of the order of   M ∼ 10¹⁰ M_⊙  . We show that there are three strong lines of argument that predicate the existence of UMBHs: (i) expected as a natural extension of the observed BH mass bulge luminosity relation, when extrapolated to the bulge luminosities of bright central galaxies in clusters; (ii) new predictions for the mass function of seed BHs at high redshifts predict that growth via accretion or merger-induced accretion inevitably leads to the existence of rare UMBHs at late times; (iii) the local mass function of BHs computed from the observed X-ray luminosity functions of active galactic nuclei predict the existence of a high-mass tail in the BH mass function at   z = 0  . Consistency between the optical and X-ray census of the local BH mass function requires an upper limit to BH masses. This consistent picture also predicts that the slope of the   M _bh–σ  relation will evolve with redshift at the high-mass end. Models of self-regulation that explain the co-evolution of the stellar component and nuclear BHs naturally provide such an upper limit. The combination of multiwavelength constraints predicts the existence of UMBHs and simultaneously provides an upper limit to their masses. The typical hosts for these local UMBHs are likely the bright, central cluster galaxies in the nearby Universe.     

Variability of extragalactic objects in relation to redshift, color, radio spectral index and absorption lines

Optical variability of extragalactic objects, viz., QSOs, BL Lacs and Seyfert galaxies has been monitored systematically over an appreciable period of time and a large amount of data have accumulated. The present work reports results of investigations involving statistical analysis of updated data on relationships between variability and various observed properties of the objects, viz., redshift, color indices, radio spectral index and absorption lines. It is found that at high frequencies (rest frame) radio spectral index does not change significantly with the degree of variability. However, the degree of variability depends on redshifts. On the other hand, presence or absence of absorption lines is significantly associated with variability for QSOs with larger redshifts (z > 1.0), while no such relationship exists for QSOs at smaller redshifts (z < 1.0) or other objects. Correlation between color indices and redshifts depends on the degree of variability and the sample chosen for the color index.     

The link between SCUBA and Spitzer: cold galaxies at z≲ 1

We show that the far-IR properties of distant Luminous and UltraLuminous InfraRed Galaxies (LIRGs and ULIRGs, respectively) are on average divergent from analogous sources in the local Universe. Our analysis is based on Spitzer Multiband Imaging Photometer (MIPS) and Infrared Array Camera (IRAC) data of   L _IR > 10¹⁰ L_⊙, 70 μm  selected objects in the  0.1 < z < 2  redshift range and supported by a comparison with the IRAS Bright Galaxy Sample. The majority of the objects in our sample are described by spectral energy distributions (SEDs) which peak at longer wavelengths than local sources of equivalent total infrared luminosity. This shift in SED peak wavelength implies a noticeable change in the dust and/or star-forming properties from   z ∼ 0  to the early Universe, tending towards lower dust temperatures, indicative of strong evolution in the cold dust, 'cirrus', component. We show that these objects are potentially the missing link between the well-studied local IR-luminous galaxies, Spitzer IR populations and SCUBA sources – the   z < 1  counterparts of the cold   z > 1  SubMillimetre Galaxies (SMGs) discovered in blank-field submillimetre surveys. The Herschel Space Observatory is well placed to fully characterize the nature of these objects, as its coverage extends over a major part of the far-IR/sub-mm SED for a wide redshift range.     

Multiwavelength study of the nuclei of a volume-limited sample of galaxies – I. X-ray observations

We discuss ROSAT HRI X-ray observations of 33 very nearby galaxies, sensitive to X-ray sources down to a luminosity of approximately 10³⁸ erg s⁻¹. The galaxies are selected from a complete, volume-limited sample of 46 galaxies with     for which we have extensive multiwavelength data. For an almost complete subsample with     (29/31 objects) we have HRI images. Contour maps and source lists are presented within the central region of each galaxy, together with nuclear upper limits where no nuclear source was detected. Nuclear X-ray sources are found to be very common, occurring in ∼35 per cent of the sample. Nuclear X-ray luminosity is statistically connected to host galaxy luminosity – there is not a tight correlation, but the probability of a nuclear source being detected increases strongly with galaxy luminosity, and the distribution of nuclear luminosities seems to show an upper envelope that is roughly proportional to galaxy luminosity. While these sources do seem to be a genuinely nuclear phenomenon rather than nuclear examples of the general X-ray source population, it is far from obvious that they are miniature Seyfert nuclei. The more luminous nuclei are very often spatially extended, and H  ii region nuclei are detected just as often as LINERs. Finally, we also note the presence of fairly common superluminous X-ray sources in the off-nuclear population – out of 29 galaxies we find nine sources with a luminosity greater than 10³⁹ erg s⁻¹. These show no particular preference for more luminous galaxies. One is already known to be a multiple SNR system, but most have no obvious optical counterpart and their nature remains a mystery.     

Richness dependence of the recent evolution of clusters of galaxies

We revisit the issue of the recent dynamical evolution of clusters of galaxies using a sample of Abell, Corwin & Olowin (ACO) clusters with   z < 0.14  , which has been selected such that it does not contain clusters with multiple velocity components nor strongly merging or interacting clusters, as revealed in X-rays. We use as proxies of the cluster dynamical state the projected cluster ellipticity, velocity dispersion and X-ray luminosity. We find indications for a recent dynamical evolution of this cluster population, which however strongly depends on the cluster richness. Poor clusters appear to be undergoing their primary phase of virialization, with their ellipticity increasing with redshift with a rate  dε/d z ≃ 2.5 ± 0.4  , while the richest clusters show an ellipticity evolution in the opposite direction (with  dε/d z ≃−1.2 ± 0.1  ), which could be due to secondary infall. When taking into account sampling effects due to the magnitude-limited nature of the ACO cluster catalogue we find no significant evolution of the cluster X-ray luminosity, while the velocity dispersion increases with decreasing redshift, independent of the cluster richness, at a rate  dσ _v /d z ≃−1700 ± 400 km s⁻¹  .     

The 2dF-SDSS LRG and QSO Survey: the spectroscopic QSO catalogue

We present the final spectroscopic QSO catalogue from the 2dF-SDSS LRG (luminous red galaxy) and QSO (2SLAQ) survey. This is a deep,  18 < g < 21.85  (extinction corrected), sample aimed at probing in detail the faint end of the broad line active galactic nuclei luminosity distribution at   z ≲ 2.6  . The candidate QSOs were selected from SDSS photometry and observed spectroscopically with the 2dF spectrograph on the Anglo-Australian Telescope. This sample covers an area of 191.9 deg² and contains new spectra of 16 326 objects, of which 8764 are QSOs and 7623 are newly discovered [the remainder were previously identified by the 2dF QSO Redshift Survey (2QZ) and SDSS]. The full QSO sample (including objects previously observed in the SDSS and 2QZ surveys) contains 12 702 QSOs. The new 2SLAQ spectroscopic data set also contains 2343 Galactic stars, including 362 white dwarfs, and 2924 narrow emission-line galaxies with a median redshift of   z = 0.22  .
We present detailed completeness estimates for the survey, based on modelling of QSO colours, including host-galaxy contributions. This calculation shows that at   g ≃ 21.85  QSO colours are significantly affected by the presence of a host galaxy up to redshift   z ∼ 1  in the SDSS ugriz bands. In particular, we see a significant reddening of the objects in   g − i   towards the fainter g -band magnitudes. This reddening is consistent with the QSO host galaxies being dominated by a stellar population of age at least 2–3 Gyr.
The full catalogue, including completeness estimates, is available on-line at http://www.2slaq.info/ .     

The detection and X-ray evolution of galaxy groups at high redshift

We describe some of the first X-ray detections of groups of galaxies at high redshifts  ( z ∼0.4)  , based on the UK deep X-ray survey of M^cHardy et al. Combined with other deep ROSAT X-ray surveys with nearly complete optical identifications, we investigate the X-ray evolution of these systems. We find no evidence for evolution of the X-ray luminosity function up to   z =0.5  at the low luminosities of groups of galaxies and poor clusters  ( L _X≳10^42.5 erg s^-1)  , although the small sample size precludes very accurate measurements. This result confirms and extends to lower luminosities current results based on surveys at brighter X-ray fluxes. The evolution of the X-ray luminosity function of these low-luminosity systems is more sensitive to the thermal history of the intragroup medium (IGM) than to cosmological parameters. Energy injection into the IGM (from, for example, supernovae or active galactic nuclei winds) is required to explain the X-ray properties of nearby groups. The observed lack of evolution suggests that the energy injection occurred at redshifts   z >0.5  .     

Cross-correlation of the unresolved X‐ray background with faint galaxies

At the faint end of the deepest X-ray surveys, a population of X-ray luminous galaxies is seen. In this paper, we present the results of a cross-correlation between the residual, unresolved X-ray photons in a very deep X-ray survey and the positions of faint galaxies, in order to examine the importance of these objects at even fainter flux levels. We measure a significant correlation on all angular scales up to ∼1 arcmin. This signal could account for a significant fraction of the unresolved X‐ray background, approximately 35 per cent if the clustering is similar to optically selected galaxies. However, the angular form of the correlation is seen to be qualitatively similar to that expected for clusters of galaxies and the X-ray emission could be associated with hot gas in clusters or with QSOs within galaxy clusters rather than emission from individual faint galaxies. The relative contribution from each of these possibilities cannot be determined with the current data.