Using the wide multiband photometry available in the Cosmic Evolution Survey (COS-MOS) field, we explore the host galaxy properties of a large sample of active galactic nuclei (AGNs; ∼1700 objects) with L bol ranging from 10 43 to 10 47 erg s −1 , obtained by combining X-ray and optical spectroscopic selections. Based on a careful study of their spectral energy distributions, which have been parametrized using a two-component (AGN+galaxy) model fit, we have derived dust-corrected rest-frame magnitudes, colours and stellar masses of the obscured and unobscured AGN hosts up to high redshift (z 3). Moreover, for the sample of obscured AGNs, we have also derived reliable star formation rates (SFRs). We find that AGN hosts span a large range of stellar masses and SFRs. No colour-bimodality is seen at any redshift in the AGN hosts, which are found to be mainly massive, red galaxies. Once we have accounted for the colour-mass degeneracy in well-defined mass-matched samples, we find a residual (marginal) enhancement of the incidence of AGNs in redder galaxies with lower specific SFRs. We argue that this result might emerge because of our ability to properly account for AGN light contamination and dust extinction, compared to surveys with a more limited multiwavelength coverage. However, because these colour shifts are relatively small, systematic effects could still be considered responsible for some of the observed trends. Interestingly, we find that the probability for a galaxy to host a black hole that is growing at any given 'specific accretion rate' (i.e. the ratio of X-ray luminosity to the host stellar mass) is almost independent of the host galaxy mass, while it decreases as a power law with L X /M * . By analysing the normalization of such a probability distribution, we show how the incidence of AGNs increases with redshift as rapidly as (1 + z) 4 , which closely resembles the overall evolution of the specific SFR of the entire galaxy population. We provide analytical A. Bongiorno et al. fitting formulae that describe the probability of a galaxy of any mass (above the completeness limit of the COSMOS) to host an AGN of any given specific accretion rate as a function of redshift. These can be useful tools for theoretical studies of the growing population of black holes within galaxy evolution models. Although AGN activity and star formation in galaxies do appear to have a common triggering mechanism, at least in a statistical sense, within the COSMOS sample, we do not find any conclusive evidence to suggest that AGNs have a powerful influence on the star-forming properties of their host galaxies.

We report on the measurement of the physical properties (rest-frame K-band luminosity and total stellar mass) of the hosts of 89 broad-line (type-1) active galactic nuclei (AGNs) detected in the zCOSMOS survey in the redshift range 1 < z < 2.2. The unprecedented multi-wavelength coverage of the survey field allows us to disentangle the emission of the host galaxy from that of the nuclear black hole in their spectral energy distributions (SEDs). We derive an estimate of black hole masses through the analysis of the broad Mg ii emission lines observed in the mediumresolution spectra taken with VIMOS/VLT as part of the zCOSMOS project. We found that, as compared to the local value, the average black hole to host-galaxy mass ratio appears to evolve positively with redshift, with a best-fit evolution of the form (1 + z) 0.68±0.12 +0.6 −0.3 , where the large asymmetric systematic errors stem from the uncertainties in the choice of initial mass function, in the calibration of the virial relation used to estimate BH masses and in the mean QSO SED adopted. On the other hand, if we consider the observed rest-frame K-band luminosity, objects tend to be brighter, for a given black hole mass, than those on the local M BH -M K relation. This fact, together with more indirect evidence from the SED fitting itself, suggests that the AGN hosts are likely actively star-forming galaxies. A thorough analysis of observational biases induced by intrinsic scatter in the scaling relations reinforces the conclusion that an evolution of the M BH -M * relation must ensue for actively growing black holes at early times: either its overall normalization, or its intrinsic scatter (or both) appear to increase with redshift. This can be interpreted as signature of either a more rapid growth of supermassive black holes at high redshift, a change of structural properties of AGN hosts at earlier times, or a significant mismatch between the typical growth times of nuclear black holes and host galaxies. In any case, our results provide important clues on the nature of the early co-evolution of black holes and galaxies and challenging tests for models of AGN feedback and self-regulated growth of structures.

Context. The COSMOS survey is a multiwavelength survey aimed to study the evolution of galaxies, AGN and large scale structures. Within this survey XMM-COSMOS a powerful tool to detect AGN and galaxy clusters. The XMM-COSMOS is a deep X-ray survey over the full 2 deg 2 of the COSMOS area. It consists of 55 XMM-Newton pointings for a total exposure of ∼1.5 Ms with an average vignetting-corrected depth of 40 ks across the field of view and a sky coverage of 2.13 deg 2 . Aims. We present the catalogue of point-like X-ray sources detected with the EPIC CCD cameras, the log N − log S relations and the X-ray colour-colour diagrams. Methods. The analysis was performed using the XMM-SAS data analysis package in the 0.5-2 keV, 2-10 keV and 5-10 keV energy bands. Source detection has been performed using a maximum likelihood technique especially designed for raster scan surveys. The completeness of the catalogue as well as log N − log S and source density maps have been calibrated using Monte Carlo simulations. Results. The catalogs contains a total of 1887 unique sources detected in at least one band with likelihood parameter det_ml > 10. The survey, which shows unprecedented homogeneity, has a flux limit of ∼1.7×10 −15 erg cm −2 s −1 , ∼9.3×10 −15 erg cm −2 s −1 and ∼1.3×10 −14 erg cm −2 s −1 over 90% of the area (1.92 deg 2 ) in the 0.5-2 keV, 2-10 keV and 5-10 keV energy band, respectively. Thanks to the rather homogeneous exposure over a large area, the derived log N − log S relations are very well determined over the flux range sampled by XMM-COSMOS. These relations have been compared with XRB synthesis models, which reproduce the observations with an agreement of ∼10% in the 5-10 keV and 2-10 keV band, while in the 0.5-2 keV band the agreement is of the order of ∼20%. The hard X-ray colors confirmed that the majority of the extragalactic sources in a bright subsample are actually type I or type II AGN. About 20% of the sources have a X-ray luminosity typical of AGN (L X > 10 42 erg/s) although they do not show any clear signature of nuclear activity in the optical spectrum.

A large population of heavily obscured, Compton-thick AGN is predicted by AGN synthesis models for the cosmic X-ray background and by the "relic" super-massive black-hole mass function measured from local bulges. However, even the deepest X-ray surveys are inefficient to search for these elusive AGN. Alternative selection criteria, combining mid-infrared with near-infrared and optical photometry, have instead been successful to pin-point a large population of Compton thick AGN. We take advantage of the deep Chandra and Spitzer coverage of a large area (more than 10 times the area covered by the Chandra deep fields, CDFs) in the COSMOS field, to extend the search of highly obscured, Compton-thick active nuclei to higher luminosity. These sources have low surface density and large samples can be provided only through large area surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS sources with 24µm fluxes higher than 550µJy. For the MIPS sources not directly detected in the Chandra images we produce stacked images in soft and hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS source population we compare the observed stacked count rates and hardness ratios to those predicted by detailed Monte Carlo simulations including both obscured AGN and star-forming galaxies. The volume density of Compton thick QSOs (logL(2-10keV)=44-45 ergs s −1 , or logλL λ (5.8µm)=44.79-46.18 ergs s −1 for a typical infrared to X-ray luminosity ratio) evaluated in this way is (4.8 ± 1.1) × 10 −6 Mpc −3 in the redshift bin 1.2-2.2. This density is ∼ 44% of that of all X-ray selected QSOs in the same redshift and luminosity bin, and it is consistent with the expectation of most up-to-date AGN synthesis models for the Cosmic X-ray background ). The density of lower luminosity Compton-thick AGN (logL(2-10keV)=43.5-44) at z=0.7-1.2 is (3.7 ± 1.1) × 10 −5 Mpc −3 , corresponding to ∼ 67% of that of X-ray selected AGN. The comparison between the fraction of infrared selected, Compton thick AGN to the X-ray selected, unobscured and moderately obscured AGN at high and low luminosity suggests that Compton-thick AGN follow a luminosity dependence similar to that discovered for Compton-thin AGN, becoming relatively rarer at high luminosities. We estimate that the fraction of AGN (unobscured, moderately obscured and Compton thick) to the total MIPS source population is 49 ± 10%, a value significantly higher than that previously estimated at similar 24µm fluxes. We discuss how our findings can constrain AGN feedback models.

Aims. To study the cosmological evolution of active galactic nuclei (AGN) is one of the main goals of X-ray surveys. To accurately determine the intrinsic (before absorption) X-ray luminosity function, it is essential to constrain the evolutionary properties of AGN and therefore the history of the formation of supermassive black holes with cosmic time. Methods. In this paper we investigate the X-ray luminosity function of absorbed (log N H > 22) and unabsorbed AGN in three energy bands (soft: 0.5−2 keV, hard: 2−10 keV and ultrahard: 4.5−7.5 keV). For the hard and ultrahard sources we have also studied the N H function and the dependence of the fraction of absorbed AGN on luminosity and redshift. This investigation is carried out using the XMS survey along with other highly complete flux-limited deeper and shallower surveys in all three bands for a total of 1009, 435, and 119 sources in the soft, hard and ultrahard bands, respectively. We modelled the instrinsic absorption of the hard and ultrahard sources (N H function) and computed the X-ray luminosity function in all bands using two methods. The first makes use of a modified version of the classic 1/V a technique, while the second performs a maximum likelihood (ML) fit using an analytic model and all available sources without binning. Results. We find that the X-ray luminosity function (XLF) is best described by a luminosity-dependent density evolution (LDDE) model. Our results show good overall agreement with previous results in the hard band, although with slightly weaker evolution. Our model in the soft band present slight discrepancies with other works in this band, the shape of our present day XLF being significantly flatter. We find faster evolution in the AGN detected in the ultrahard band than those in the hard band. Conclusions. The results reported here show that the fraction of absorbed AGN in the hard and ultrahard bands is dependent on the X-ray luminosity. We find evidence that this fraction evolves with redshift in the hard band, whereas there is none in the ultrahard band, possibly due to the low statistics. Our best-fit XLF shows that the high-luminosity AGN, detected in all bands, exhibit a similar behaviours and are fully formed earlier than the less luminous AGN. The latter sources account for the vast majority of the accretion rate and mass density of the Universe, according to an anti-hierarchical black hole growth scenario.

We present a galaxy group catalog spanning the redshift range 0.1 z 1 in the ∼1.7 deg 2 COSMOS field, based on the first ∼10,000 zCOSMOS spectra. The performance of both the Friends-of-Friends (FOF) and Voronoi-Delaunay method (VDM) approaches to group identification has been extensively explored and compared using realistic mock catalogs. We find that the performance improves substantially if groups are found by progressively optimizing the group-finding parameters for successively smaller groups, and that the highest fidelity catalog, in terms of completeness and purity, is obtained by combining the independently created FOF and VDM catalogs. The final completeness and purity of this catalog, both in terms of the groups and of individual members, compares favorably with recent results in the literature. The current group catalog contains 102 groups with N 5 spectroscopically confirmed members, with a further ∼700 groups with 2 N 4. Most of the groups can be assigned a velocity dispersion and a dark-matter mass derived from the mock catalogs, with quantifiable uncertainties. The fraction of zCOSMOS galaxies in groups is about 25% at low redshift and decreases toward ∼15% at z ∼ 0.8. The zCOSMOS group catalog is broadly consistent with that expected from the semianalytic evolution model underlying the mock catalogs. Not least, we show that the number density of groups with a given intrinsic richness increases from redshift z ∼ 0.8 to the present, consistent with the hierarchical growth of structure.

We present an investigation into how well the properties of the accretion flow onto a supermassive black hole may be coupled to those of the overlying hot corona. To do so, we specifically measure the characteristic spectral index, Γ, of a power-law energy distribution, over an energy range of 2 to 10 keV, for X-ray selected, broad-lined radio-quiet active galactic nuclei (AGN) up to z∼2 in COSMOS and E-CDF-S. We test the previously reported dependence between Γ and black hole mass, FWHM and Eddington ratio using a sample of AGN covering a broad range in these parameters based on both the Mg ii and Hα emission lines with the later afforded by recent near infrared spectroscopic observations using Subaru/FMOS. We calculate the Eddington ratios, λ Edd , for sources where a bolometric luminosity (L Bol ) has been presented in the literature, based on SED fitting, or, for sources where these data do not exist, we calculate L Bol using a bolometric correction to the X-ray luminosity, derived from a relationship between the bolometric correction, and L X /L 3000 . From a sample of 69 X-ray bright sources (> 250 counts), where Γ can be measured with greatest precision, with an estimate of L Bol , we find a statistically significant correlation between Γ and λ Edd , which is highly significant with a chance probability of 6.59×10 −8 . A statistically significant correlation between Γ and the FWHM of the optical lines is confirmed, but at lower significance than with λ Edd indicating that λ Edd is the key parameter driving conditions in the corona. Linear regression analysis reveals that Γ = (0.32 ± 0.05)log 10 λ Edd +(2.27 ± 0.06) and Γ = (−0.69 ± 0.11)log 10 (FWHM/km s −1 )+(4.44 ± 0.42). Our results on Γ-λ Edd are in very good agreement with previous results. While the Γ-λ Edd relationship means that X-ray spectroscopy may be used to estimate black hole accretion rate, considerable dispersion in the correlation does not make this viable for single sources, however could be valuable however for large X-ray spectral samples, such as those to be produced by eROSITA.

We present a detailed study of a peculiar source detected in the COSMOS survey at z=0.359. Source CXOC J100043.1+020637, also known as CID-42, presents two compact optical sources embedded in the same galaxy. The distance between the two, measured in the HST/ACS image, is 0.495"±0.005" that, at the redshift of the source, corresponds to a projected separation of 2.46±0.02 kpc. A large (∼1200 km/s) velocity offset between the narrow and broad components of Hβ has been measured in three different optical spectra from the VLT/VIMOS and Magellan/IMACS instruments. CID-42 is also the only X-ray source in COSMOS having in its X-ray spectra a strong redshifted broad absorption iron line, and an iron emission line, drawing an inverted P-Cygni profile. The Chandra and XMM-Newton data show that the absorption line is variable in energy by ∆E=500 eV over 4 years and that the absorber has to be highly ionized, in order not to leave a signature in the soft X-ray spectrum. That these features, the morphology, the velocity offset and the inverted P-Cygni profile, occur in the same source is unlikely to be a coincidence. We envisage two possible explanations, both exceptional, for this system: (1) a gravitational wave recoiling black hole (BH), caught 1-10 Myr after merging, (2) a Type 1/ Type 2 system in the same galaxy where the Type 1 is recoiling due to slingshot effect produced by a triple BH system. The first possibility gives us a candidate gravitational waves recoiling BH with both spectroscopic and imaging signatures. In the second case, the Xray absorption line can be explained as a BAL-like outflow from the foreground nucleus (a Type 2 AGN) at the rearer one (a Type 1 AGN), which illuminates the otherwise undetectable wind, giving us the first opportunity to show that fast winds are present in obscured AGN, and possibly universal in AGNs. Subject headings: