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We present a catalog of 213 type-2 AGN selected from the zCOSMOS survey. The selected sample covers a wide redshift range (0.15, Comment: 20 pages, 11 figures, accepted for publication in Astronomy and Astrophysics

X-ray emission from AGN is believed to be produced via Comptonization of optical/UV seed photons emitted by the accretion disk, up-scattered by hot electrons in a corona surrounding the black hole. A critical compactness vs. temperature threshold is predicted above which any increase in the source luminosity, for a fixed size, would then generate positron-electron pairs rather than continue heating the coronal plasma. Current observations seem to confirm that all AGN populate the region below this critical line. These models, however, have never been probed by observations in the high-luminosity regime, where the critical line is expected to reach low temperatures. To fill this observational gap, we selected two luminous ($log(L_{bol})>47.5$ erg/s) quasars, 2MASSJ1614346+470420 ($z=1.86$) and B1422+231 ($z=3.62$), and obtained XMM-Newton and NuSTAR deep observations for them. We performed detailed spectral analysis of their quasi-simultaneous soft and hard X-ray data, in order to constrain the parameters of their coronae. Using a phenomenological cut-off power-law model, with the inclusion of a reflection component, we derived rest-frame values of the high energy cut-off of $E_{cut}=106^{+102}_{-37}$ keV and $E_{cut}=66^{+17}_{-12}$ keV, respectively. Comptonization models consistently give as best-fit parameters electron temperatures of $\sim45$ keV and $\sim28$ keV, respectively, and optically thick coronae ($��>1$). These low coronal temperatures fall in the limited region allowed at these luminosities to avoid runaway pair production., 8 pages, 2 figures, accepted for publication in ApJ Letter

Theoretical models of galaxy formation suggest that the presence of an active galactic nucleus (AGN) is required to regulate the growth of its host galaxy through feedback mechanisms, produced by e.g. AGN-driven outflows. Although such outflows are common both at low and high redshift, a comprehensive picture is still missing. The peak epoch of galaxy assembly (1, Comment: 28 pages, 8 figures, 5 tables. Accepted for publication in A&A

We analyse the properties of the CIV broad emission line in connection with the X-ray emission of 30 bright SDSS quasars at z~3.0-3.3 with pointed XMM-Newton observations, which were selected to test the suitability of AGN as cosmological tools. In our previous work, we found that a large fraction (~25%) of the quasars in this sample are X-ray underluminous by factors of >3-10. As absorbing columns of >10$^{23}$ cm$^{-2}$ can be safely ruled out, their weakness is most likely intrinsic. Here we explore possible correlations between the UV and X-ray features of these sources to investigate the origin of X-ray weakness. We fit their UV SDSS spectra and analyse their CIV properties (e.g., equivalent width, EW; line peak velocity, $\upsilon_{\rm peak}$) as a function of the X-ray photon index and 2-10 keV flux. We confirm the trends of CIV $\upsilon_{\rm peak}$ and EW with UV luminosity at 2500 angstrom for both X-ray weak and X-ray normal quasars, as well as the correlation between X-ray weakness and CIV EW. In contrast to some recent work, we do not observe any clear relation between the 2-10 keV luminosity and $\upsilon_{\rm peak}$. We find a correlation between the hard X-ray flux and the integrated CIV flux for X-ray normal quasars, whilst X-ray weak quasars deviate from the main trend by more than 0.5 dex. We argue that X-ray weakness might be interpreted in a starved X-ray corona picture associated with an ongoing disc-wind phase. If the wind is ejected in the vicinity of the black hole, the extreme-UV radiation that reaches the corona will be depleted, depriving the corona of seeds photons and generating an X-ray weak quasar. Yet, at the largest UV luminosities (>10$^{47}$ erg s$^{-1}$), there will still be an ample reservoir of ionising photons that can explain the excess CIV emission observed in the X-ray weak quasars with respect to normal sources of similar X-ray luminosities., Comment: 22 pages, 15 figures (with 3 more figures in the Appendix), abstract abridged. Accepted for publication in A&A

The discovery of hundreds of QSOs in the first Gyr of the Universe powered by already grown SMBHs challenges our knowledge of SMBH formation. In particular, investigations of $z>6$ QSOs presenting notable properties can provide unique information on the physics of fast SMBH growth in the early universe. We present the results of follow-up observations of the $z=6.515$ radio-quiet QSO PSO167-13, which is interacting with a close companion galaxy. The PSO167-13 system has been recently proposed to host the first heavily obscured X-ray source at high redshift. We observed PSO167-13 with Chandra/ACIS-S (177 ks), and obtained new spectroscopic observations (7.2 h) with Magellan/FIRE. No significant X-ray emission is detected from the PSO167-13 system, suggesting that the obscured X-ray source previously tentatively detected was either due to a strong background fluctuation or is highly variable. The upper limit (90% confidence level) on the X-ray emission of PSO167-13 ($L_{2-10\,\mathrm{keV}}6$ QSO. The ratio between the X-ray and UV luminosity of $\alpha_{ox}6$ times weaker than the expectation based on its UV luminosity. The new Magellan/FIRE spectrum of PSO167-13 is strongly affected by the unfavorable sky conditions, but the tentatively detected C IV and Mg II emission lines appear strongly blueshifted. The most plausible explanations for the X-ray weakness of PSO167-13 are intrinsic weakness or small-scale absorption by Compton-thick material. The possible strong blueshift of its emission lines hints at the presence of nuclear winds, which could be related to its X-ray weakness., Comment: Accepted for publication on A&A

We present a new catalogue of ~2,400 optically selected quasars with spectroscopic redshifts and X-ray observations from either Chandra or XMM-Newton. The sample can be used to investigate the non-linear relation between the UV and X-ray luminosity of quasars, and to build a Hubble diagram up to redshift z~7.5. We selected sources that are neither reddened by dust in the optical/UV nor obscured by gas in the X-rays, and whose X-ray fluxes are free from flux-limit related biases. After checking for any possible systematics, we confirm, in agreement with our previous works, that (i) the X-ray to UV relation provides distance estimates matching those from supernovae up to z~1.5, and (ii) its slope shows no redshift evolution up to z~5. We provide a full description of the methodology for testing cosmological models, further supporting a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat $\Lambda$CDM model up to z~1.5-2, but strong deviations emerge at higher redshifts. Since we have minimized all non-negligible systematic effects, and proven the stability of the $L_{\rm X}-L_{\rm UV}$ relation at high redshifts, we conclude that an evolution of the expansion rate of the Universe should be considered as a possible explanation for the observed deviation, rather than some systematic (redshift-dependent) effect associated with high-redshift quasars., Comment: 24 pages, 17 figures, accepted for publication in A&A

X-ray emission from QSOs has been used to assess SMBH accretion properties up to $z$~6. However, at $z>6$ only ~15 QSOs are covered by sensitive X-ray observations, preventing a statistically significant investigation of the X-ray properties of QSOs in the first Gyr of the Universe. We present new Chandra observations of 10 $z>6$ QSOs, selected to have virial black-hole mass estimates from Mg II line spectroscopy. Adding archival X-ray data for an additional 15 $z>6$ QSOs, we investigate the X-ray properties of the QSO population in the first Gyr of the Universe, focusing in particular on the $L_{UV}-L_{X}$ relation, which is traced by the $\alpha_{ox}$ parameter, and the shape of their X-ray spectra. We performed photometric analyses to derive estimates of the X-ray luminosities, and thus the $\alpha_{ox}$ values and bolometric corrections ($K_{bol}=L_{bol}/L_{X}$). We compared the resulting $\alpha_{ox}$ and $K_{bol}$ distributions with the results found for QSO samples at lower redshift. Finally, we performed a basic X-ray spectral analysis of the brightest $z>6$ QSOs to derive their individual photon indices, and joint spectral analysis of the whole sample to estimate the average photon index. We confirm a lack of significant evolution of $\alpha_{ox}$ with redshift, extending the results from previous works up to $z>6$, and the trend of an increasing bolometric correction with increasing luminosity found for QSOs at lower redshifts. The average power-law photon index of our sample ($\Gamma=2.20_{-0.34}^{+0.39}$ and $\Gamma=2.13_{-0.13}^{+0.13}$ for sources with $30$ net counts, respectively) is slightly steeper than, but still consistent with, typical QSOs at $z=1-6$. All these results point toward a lack of substantial evolution of the inner accretion-disk/hot-corona structure in QSOs from low redshift to $z>6$. Our data hint at generally high Eddington ratios at $z>6$., Comment: 15 pages. 10 figures. 7 tables. Accepted for publication in A&A

The existence of a large population of Compton thick (CT, $N_{H}>10^{24} cm^{-2}$) AGN is a key ingredient of most Cosmic X-ray background synthesis models. However, direct identification of these sources, especially at high redshift, is difficult due to the flux suppression and complex spectral shape produced by CT obscuration. We explored the Chandra COSMOS Legacy point source catalog, comprising 1855 sources, to select via X-ray spectroscopy, a large sample of CT candidates at high redshift. Adopting a physical model to reproduce the toroidal absorber, and a Monte-Carlo sampling method, we selected 67 individual sources with >5% probability of being CT, in the redshift range $0.0410^{24} cm^{-2}$, gives a total of 41.9 effective CT, corrected for classification bias. We derive number counts in the 2-10 keV band in three redshift bins. The observed logN-logS is consistent with an increase of the intrinsic CT fraction ($f_{CT}$) from $\sim0.30$ to $\sim0.55$ from low to high redshift. When rescaled to a common luminosity (log(L$_{\rm X}$/erg/s)$=44.5$) we find an increase from $f_{CT}=0.19_{-0.06}^{+0.07}$ to $f_{CT}=0.30_{-0.08}^{+0.10}$ and $f_{CT}=0.49_{-0.11}^{+0.12}$ from low to high z. This evolution can be parametrized as $f_{CT}=0.11_{-0.04}^{+0.05}(1+z)^{1.11\pm0.13}$. Thanks to HST-ACS deep imaging, we find that the fraction of CT AGN in mergers/interacting systems increases with luminosity and redshift and is significantly higher than for non-CT AGN hosts., Comment: 15 pages, 13 figures. Published in MNRAS 31 July 2018

We discuss the spectral analysis of a sample of 63 Active Galactic Nuclei (AGN) detected above a limiting flux of $S(8-24$ keV$)=7\times10^{-14}$ erg/s/cm$^2$ in the multi-tiered NuSTAR Extragalactic Survey program. The sources span a redshift range z=0-2.1 (median $\langle$z$\rangle=$0.58). The spectral analysis is performed over the broad 0.5-24 keV energy range, combining NuSTAR with Chandra and/or XMM-Newton data and employing empirical and physically motivated models. This constitutes the largest sample of AGN selected at $ >10$ keV to be homogeneously spectrally analyzed at these flux levels. We study the distribution of spectral parameters such as photon index, column density ($N_{\rm H}$), reflection parameter ($R$) and 10-40 keV luminosity ($L_{X}$). Heavily obscured ($\log [N_{\rm H}/cm^{-2}]\ge23$) and Compton Thick (CT; $\log [N_{\rm H}/cm^{-2}]\ge24$) AGN constitute $\sim$25% (15-17 sources) and $\sim$2-3% ( 1-2 sources) of the sample, respectively. The observed $N_{\rm H}$ distribution fairly agrees with predictions of Cosmic X-ray Background population synthesis models (CXBPSM). We estimate the intrinsic fraction of AGN as a function of $N_{H}$, accounting for the bias against obscured AGN in a flux-selected sample. The fraction of CT AGN relative to $\log [N_{\rm H}/cm^{-2}]=20-24$ AGN is poorly constrainted, formally in the range 2-56% (90% upper limit of 66%). We derived a fraction ($f_{abs}$) of obscured AGN ($\log [N_{\rm H}/cm^{-2}]=22-24$) as a function of $L_{X}$ in agreement with CXBPSM and previous z < 1 X-ray determinations. Furthermore $f_{abs}$ at z=0.1-0.5 and $\log (L_{X}/erg$ $s^{-1})\approx43.6-44.3$ agrees with observational measurements/trends obtained over larger redshift intervals. We report a significant anti-correlation of $R$ with $L_{X}$ (confirmed by our companion paper on stacked spectra) with considerable scatter around the median $R$ values., Comments: 38 pages, 17 figures, 8 tables. Accepted for publication in ApJ

We present the results and the source catalog of the NuSTAR survey in the UKIDSS Ultra Deep Survey (UDS) field, bridging the gap in depth and area between NuSTAR's ECDFS and COSMOS surveys. The survey covers a $\sim 0.6$ deg$^2$ area of the field for a total observing time of $\sim$ 1.75 Ms, to a half-area depth of $\sim$ 155 ks corrected for vignetting at $3-24$ keV, and reaching sensitivity limits at half-area in the full ($3-24$ keV), soft ($3-8$ keV) and hard ($8-24$ keV) bands of $2.2 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$, $1.0 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$, and $2.7 \times 10^{-14}$ erg cm$^{-2}$ s$^{-1}$, respectively. A total of 67 sources are detected in at least one of the three bands, 56 of which have a robust optical redshift with a median of $\langle z\rangle \sim 1.1$. Through a broadband ($0.5-24$ keV) spectral analysis of the whole sample combined with the NuSTAR hardness ratios, we compute the observed Compton-thick (CT; $N_{\rm H} > 10^{24}$ cm$^{-2}$) fraction. Taking into account the uncertainties on each $N_{\rm H}$ measurement, the final number of CT sources is $6.8\pm1.2$. This corresponds to an observed CT fraction of $(11.5\pm2.0)\%$, providing a robust lower limit to the intrinsic fraction of CT AGN and placing constraints on cosmic X-ray background synthesis models., 23 pages, accepted for publication in The Astrophysical Journal Supplement Series

Aims.We perform clustering measurements of 800 X-ray selectedChandraCOSMOS Legacy (CCL) Type 2 active galactic nuclei (AGN) with known spectroscopic redshift to probe the halo mass dependence on AGN host galaxy properties, such as galaxy stellar massMstar, star formation rate (SFR), and specific black hole accretion rate (BHAR;λBHAR) in the redshift rangez = [0−3].Methods.We split the sample of AGN with known spectroscopic redshits according toMstar, SFR andλBHAR, while matching the distributions in terms of the other parameters, including redshift. We measured the projected two-point correlation functionwp(rp) and modeled the clustering signal, for the different subsamples, with the two-halo term to derive the large-scale biasband corresponding typical mass of the hosting halo.Results.We find no significant dependence of the large-scale bias and typical halo mass on galaxy stellar mass and specific BHAR for CCL Type 2 AGN at meanz ∼ 1, while a negative dependence on SFR is observed, i.e. lower SFR AGN reside in richer environment. Mock catalogs of AGN, matched to have the same X-ray luminosity, stellar mass,λBHAR, and SFR of CCL Type 2 AGN, almost reproduce the observedMstar − Mh,λBHAR − Mhand SFR–Mhrelations, when assuming a fraction of satellite AGNfAGNsat∼ 0.15. This corresponds to a ratio of the probabilities of satellite to central AGN of being activeQ ∼ 2. Mock matched normal galaxies follow a slightly steeperMstar − Mhrelation, in which low mass mock galaxies reside in less massive halos than mock AGN of similar mass. Moreover, matched mock normal galaxies are less biased than mock AGN with similar specific BHAR and SFR, at least forQ > 1.

(abridged) We present the X-ray analysis of a sample of 30 luminous quasars at $z\simeq3.0-3.3$ with deep XMM-Newton observations, selected from the SDSS-DR7 to be representative of the most luminous, intrinsically blue quasar population. By construction, the sample boasts a unique degree of homogeneity in terms of optical/UV properties. In the X-rays, only four sources are too faint for a detailed spectral analysis. Neglecting a radio-loud object, the other 25 quasars are, as a whole, the most X-ray luminous ever observed, with rest-frame 2-10 keV luminosities of $0.5-7\times10^{45}$ erg/s. The continuum photon index distribution, centred at $\Gamma\sim1.85$, is in excellent agreement with those in place at lower redshift, luminosity and black-hole mass, confirming the universal nature of the X-ray emission mechanism in quasars. Even so, when compared against the well-known $L_{\rm X}-L_{\rm UV}$ correlation, our quasars unexpectedly split into two distinct subsets. About 2/3 of the sources are clustered around the relation with a minimal scatter of 0.1 dex, while the remaining 1/3 appear to be X-ray underluminous by factors of $>3-10$. Such a large incidence ($\approx25\%$) of X-ray weakness has never been reported in radio-quiet, non-BAL quasar samples. Several factors could contribute to enhance the X-ray weakness fraction among our $z\simeq3$ blue quasars. However, the X-ray weak objects also have, on average, flatter spectra, with no clear evidence of absorption. Indeed, column densities in excess of a few $\times10^{22}$ cm$^{-2}$ can be ruled out for most of the sample. We suggest that, at least in some of our X-ray weak quasars, the corona might experience a radiatively inefficient phase due to the presence of a powerful accretion-disc wind, which substantially reduces the accretion rate through the inner disc and so the availability of seed photons for Compton up-scattering., Comment: Version accepted for publication in Astronomy & Astrophysics (13+2 pages, 10+1 figures, 3 tables)

We define a quasar-galaxy mixing diagram using the slopes of their spectral energy distributions (SEDs) from 1\mu m to 3000\AA\ and from 1\mu m to 3\mu m in the rest frame. The mixing diagram can easily distinguish among quasar-dominated, galaxy-dominated and reddening-dominated SED shapes. By studying the position of the 413 XMM selected Type 1 AGN in the wide-field "Cosmic Evolution Survey" (COSMOS) in the mixing diagram, we find that a combination of the Elvis et al. (1994, hereafter E94) quasar SED with various contributions from galaxy emission and some dust reddening is remarkably effective in describing the SED shape from 0.3-3\mu m for large ranges of redshift, luminosity, black hole mass and Eddington ratio of type 1 AGN. In particular, the location in the mixing diagram of the highest luminosity AGN is very close (within 1\sigma) to that of the E94 SED. The mixing diagram can also be used to estimate the host galaxy fraction and reddening in quasar. We also show examples of some outliers which might be AGN in different evolutionary stages compared to the majority of AGN in the quasar-host galaxy co-evolution cycle., Comment: 19 pages, 13 figures, 2 tables, accepted by MNRAS

We present the X-ray spectral analysis of the 390 brightest extragalactic sources in the Chandra-COSMOS catalog, showing at least 70 net counts in the 0.5-7 keV band. This sample has a 100% completeness in optical-IR identification, with 75% of the sample having a spectroscopic redshift and 25% a photometric redshift. Our analysis allows us to accurately determine the intrinsic absorption, the broad band continuum shape ({\Gamma}) and intrinsic L(2-10) distributions, with an accuracy better than 30% on the spectral parameters for 95% of the sample. The sample is equally divided in type-1 (49.7%) and type-2 AGN (48.7%) plus few passive galaxies at low z. We found a significant difference in the distribution of {\Gamma} of type-1 and type-2, with small intrinsic dispersion, a weak correlation of {\Gamma} with L(2-10) and a large population (15% of the sample) of high luminosity, highly obscured (QSO2) sources. The distribution of the X ray/Optical flux ratio (Log(FX /Fi)) for type-1 is narrow (0 < X/O < 1), while type-2 are spread up to X/O = 2. The X/O correlates well with the amount of X-ray obscuration. Finally, a small sample of Compton thick candidates and peculiar sources is presented. In the appendix we discuss the comparison between Chandra and XMM-Newton spectra for 280 sources in common. We found a small systematic difference, with XMM-Newton spectra that tend to have softer power-laws and lower obscuration., Comment: 20 pages, 16 figures. Accepted for Pubblication in MNRAS, 2013 February 5

The co-evolution of host galaxies and the active black holes which reside in their centre is one of the most important topics in modern observational cosmology. Here we present a study of the properties of obscured Active Galactic Nuclei (AGN) detected in the CDFS 1Ms observation and their host galaxies. We limited the analysis to the MUSIC area, for which deep K-band observations obtained with ISAAC@VLT are available, ensuring accurate identifications of the counterparts of the X-ray sources as well as reliable determination of photometric redshifts and galaxy parameters, such as stellar masses and star formation rates. In particular, we: 1) refined the X-ray/infrared/optical association of 179 sources in the MUSIC area detected in the Chandra observation; 2) studied the host galaxies observed and rest frame colors and properties. We found that X-ray selected (L_X>10^{42} erg s^{-1}) AGN show Spitzer colors consistent with both AGN and starburst dominated infrared continuum; the latter would not have been selected as AGN from infrared diagnostics. The host galaxies of X-ray selected obscured AGN are all massive (M_*>10^{10} M_sun) and, in 50% of the cases, are also actively forming stars (1/SSFR1 and M_*>3x10^{11} M_sun, a fraction significantly higher than in the local Universe for AGN of similar luminosities., Comment: 19 pages, 11 figures, to match with the pubslihed version, A&A, 507, 1277

We present the catalog of optical and infrared counterparts of the Chandra COSMOS-Legacy Survey, a 4.6 Ms Chandra program on the 2.2 square degrees of the COSMOS field, combination of 56 new overlapping observations obtained in Cycle 14 with the previous C-COSMOS survey. In this Paper we report the i, K, and 3.6 micron identifications of the 2273 X-ray point sources detected in the new Cycle 14 observations. We use the likelihood ratio technique to derive the association of optical/infrared (IR) counterparts for 97% of the X-ray sources. We also update the information for the 1743 sources detected in C-COSMOS, using new K and 3.6 micron information not available when the C-COSMOS analysis was performed. The final catalog contains 4016 X-ray sources, 97% of which have an optical/IR counterpart and a photometric redshift, while 54% of the sources have a spectroscopic redshift. The full catalog, including spectroscopic and photometric redshifts and optical and X-ray properties described here in detail, is available online. We study several X-ray to optical (X/O) properties: with our large statistics we put better constraints on the X/O flux ratio locus, finding a shift towards faint optical magnitudes in both soft and hard X-ray band. We confirm the existence of a correlation between X/O and the the 2-10 keV luminosity for Type 2 sources. We extend to low luminosities the analysis of the correlation between the fraction of obscured AGN and the hard band luminosity, finding a different behavior between the optically and X-ray classified obscured fraction., Comment: 28 pages, 14 figures, accepted to ApJ

We present the X-ray spectral analysis of the 1855 extragalactic sources in the Chandra COSMOS-Legacy survey catalog having more than 30 net counts in the 0.5-7 keV band. 38% of the sources are optically classified Type 1 active galactic nuclei (AGN), 60% are Type 2 AGN and 2% are passive, low-redshift galaxies. We study the distribution of AGN photon index and of the intrinsic absorption N(H,z) based on the sources optical classification: Type 1 have a slightly steeper mean photon index than Type 2 AGN, which on the other hand have average intrinsic absorption ~3 times higher than Type 1 AGN. We find that ~15% of Type 1 AGN have N(H,z)>1E22 cm^(-2), i.e., are obscured according to the X-ray spectral fitting; the vast majority of these sources have L(2-10keV)>$1E44 erg/s. The existence of these objects suggests that optical and X-ray obscuration can be caused by different phenomena, the X-ray obscuration being for example caused by dust-free material surrounding the inner part of the nuclei. ~18% of Type 2 AGN have N(H,z), Comment: 23 pages, 16 figures. Accepted for publication on ApJ

The intrinsic column density (NH) distribution of quasars is poorly known. At the high obscuration end of the quasar population and for redshifts z 1.5e24 cm^-2) type 2 quasars (CTQSO2s); five new NuSTAR observations are reported herein, and four have been previously published. The candidate CTQSO2s lie at z~ 90 net source counts at 8-24 keV). For these NuSTAR-detected sources direct (i.e., X-ray spectral) constraints on the intrinsic AGN properties are feasible, and we measure column densities ~2.5-1600 times higher and intrinsic (unabsorbed) X-ray luminosities ~10-70 times higher than pre-NuSTAR constraints from Chandra and XMM-Newton. Assuming the NuSTAR-detected type 2 quasars are representative of other Compton-thick candidates, we make a correction to the NH distribution for optically selected type 2 quasars as measured by Chandra and XMM-Newton for 39 objects. With this approach, we predict a Compton-thick fraction of f_CT = 36^{+14}_{-12} %, although higher fractions (up to 76%) are possible if indirect absorption diagnostics are assumed to be reliable., Comment: 19 pages, 13 figures, 5 tables. Accepted for publication in ApJ

Heavily obscured, Compton Thick (CT, NH>10^24 cm^-2) AGN may represent an important phase in AGN/galaxy co-evolution and are expected to provide a significant contribution to the cosmic X-ray background (CXB). Through direct X-ray spectra analysis, we selected 39 heavily obscured AGN (NH>3x10^23 cm^-2) in the 2 deg^2 XMM-COSMOS survey. After selecting CT AGN based on the fit of a simple absorbed two power law model to the XMM data, the presence of CT AGN was confirmed in 80% of the sources using deeper Chandra data and more complex models. The final sample of CT AGN comprises 10 sources spanning a large range of redshift and luminosity. We collected the multi-wavelength information available for all these sources, in order to study the distribution of SMBH and host properties, such as BH mass (M_BH), Eddington ratio (\lambda_Edd), stellar mass (M*), specific star formation rate (sSFR) in comparison with a sample of unobscured AGN. We find that highly obscured sources tend to have significantly smaller M_BH and higher \lambda_edd with respect to unobscured ones, while a weaker evolution in M* is observed. The sSFR of highly obscured sources is consistent with the one observed in the main sequence of star forming galaxies, at all redshift. We also present optical spectra, spectral energy distribution (SED) and morphology for the sample of 10 CT AGN: all the available optical spectra are dominated by the stellar component of the host galaxy, and a highly obscured torus component is needed in the SED of the CT sources. Exploiting the high resolution Hubble-ACS images available, we conclude that these highly obscured sources have a significantly larger merger fraction with respect to other X-ray selected samples of AGN. Finally we discuss implications in the context of AGN/galaxy co-evolutionary models, and compare our results with the predictions of CXB synthesis models., Comment: Revised version after referee comments. Accepted for publication in Astronomy & Astrophysics on 25 November 2014. 23 pages, 2 tables, 16 figures

We construct the rest-frame 2--10 keV intrinsic X-ray luminosity function of Active Galactic Nuclei (AGNs) from a combination of X-ray surveys from the all-sky Swift BAT survey to the Chandra Deep Field-South. We use ~3200 AGNs in our analysis, which covers six orders of magnitude in flux. The inclusion of the XMM and Chandra COSMOS data has allowed us to investigate the detailed behavior of the XLF and evolution. In deriving our XLF, we take into account realistic AGN spectrum templates, absorption corrections, and probability density distributions in photometric redshift. We present an analytical expression for the overall behavior of the XLF in terms of the luminosity-dependent density evolution, smoothed two power-law expressions in 11 redshift shells, three-segment power-law expression of the number density evolution in four luminosity classes, and binned XLF. We observe a sudden flattening of the low luminosity end slope of the XLF slope at z>~0.6. Detailed structures of the AGN downsizing have been also revealed, where the number density curves have two clear breaks at all luminosity classes above log LX>43. The two break structure is suggestive of two-phase AGN evolution, consisting of major merger triggering and secular processes., 39 Pages, 9 figures. ApJ in press

We report NuSTAR observations of NuSTAR J033202-2746.8, a heavily obscured, radio-loud quasar detected in the Extended Chandra Deep Field-South, the deepest layer of the NuSTAR extragalactic survey (~400 ks, at its deepest). NuSTAR J033202-2746.8 is reliably detected by NuSTAR only at E>8 keV and has a very flat spectral slope in the NuSTAR energy band (Gamma=0.55^{+0.62}_{-0.64}; 3-30 keV). Combining the NuSTAR data with extremely deep observations by Chandra and XMM-Newton (4 Ms and 3 Ms, respectively), we constrain the broad-band X-ray spectrum of NuSTAR J033202-2746.8, indicating that this source is a heavily obscured quasar (N_H=5.6^{+0.9}_{-0.8}x10^23 cm^-2) with luminosity L_{10-40 keV}~6.4x10^44 erg s^-1. Although existing optical and near-infrared (near-IR) data, as well as follow-up spectroscopy with the Keck and VLT telescopes, failed to provide a secure redshift identification for NuSTAR J033202-2746.8, we reliably constrain the redshift z=2.00+/-0.04 from the X-ray spectral features (primarily from the iron K edge). The NuSTAR spectrum shows a significant reflection component (R=0.55^{+0.44}_{-0.37}), which was not constrained by previous analyses of Chandra and XMM-Newton data alone. The measured reflection fraction is higher than the R~0 typically observed in bright radio-loud quasars such as NuSTAR J033202-2746.8, which has L_{1.4 GHz}~10^27 W Hz^-1. Constraining the spectral shape of AGN, including bright quasars, is very important for understanding the AGN population, and can have a strong impact on the modeling of the X-ray background. Our results show the importance of NuSTAR in investigating the broad-band spectral properties of quasars out to high redshift., 13 pages, 5 figures; accepted for publication in ApJ

We investigate, for the first time at z ∼ 3, the clustering properties of 189 Type 1 and 157 Type 2 X-ray active galactic nuclei (AGNs) of moderate luminosity ((L {sub bol}) = 10{sup 45.3} erg s{sup –1}), with photometric or spectroscopic redshifts in the range 2.2 < z < 6.8. These samples are based on Chandra and XMM-Newton data in COSMOS. We find that Type 1 and Type 2 COSMOS AGNs at z ∼ 3 inhabit DMHs with typical mass of log M{sub h} = 12.84{sub −0.11}{sup +0.10} and 11.73{sub −0.45}{sup +0.39} h {sup –1} M {sub ☉}, respectively. This result requires a drop in the halo masses of Type 1 and 2 COSMOS AGNs at z ∼ 3 compared to z ≲ 2 XMM-COSMOS AGNs with similar luminosities. Additionally, we infer that unobscured COSMOS AGNs at z ∼ 3 reside in 10 times more massive halos compared to obscured COSMOS AGNs, at the 2.6σ level. This result extends to z ∼ 3 the results found in COSMOS at z ≲ 2, and rules out the picture in which obscuration is purely an orientation effect. A model which assumes that the AGNs activity is triggered by major mergers ismore » quite successful in predicting both the low halo mass of COSMOS AGNs and the typical mass of luminous SDSS quasars at z ∼ 3, with the latter inhabiting more massive halos respect to moderate luminosity AGNs. Alternatively we can argue, at least for Type 1 COSMOS AGNs, that they are possibly representative of an early phase of fast (i.e., Eddington limited) BH growth induced by cosmic cold flows or disk instabilities. Given the moderate luminosity, these new fast growing BHs have masses of ∼10{sup 7-8} M {sub ☉} at z ∼ 3 which might evolve into ∼10{sup 8.5-9} M {sub ☉} mass BHs at z = 0. Following our clustering measurements, we argue that this fast BH growth at z ∼ 3 in AGNs with moderate luminosity occurs in DMHs with typical mass of ∼ 6× 10{sup 12} h {sup –1} M {sub ☉}.« less

We present a detailed spectral analysis of new XMM-Newton data of the source CXOC J100043.1+020637, also known as CID-42, detected in the COSMOS survey at z = 0.359. Previous works suggested that CID-42 is a candidate recoiling supermassive black holes showing also an inverted P-Cygni profile in the X- ray spectra at ~6 keV (rest) with an iron emission line plus a redshifted absorption line (detected at 3sigma in previous XMM-Newton and Chandra observations). Detailed analysis of the absorption line suggested the presence of ionized material inflowing into the black hole at high velocity. In the new long XMM-Newton observation, while the overall spectral shape remains constant, the continuum 2-10 keV flux decreased of ~20% with respect to previous observation and the absorption line is undetected. The upper limit on the intensity of the absorption line is EW, 11 pages, 7 figures, 2 tables; Accepted for publication on ApJ on October 3rd

We present the measurement of the projected and redshift space 2-point correlation function (2pcf) of the new catalog of Chandra COSMOS-Legacy AGN at 2.9$\leq$z$\leq$5.5 ($\langle L_{bol} \rangle \sim$10$^{46}$ erg/s) using the generalized clustering estimator based on phot-z probability distribution functions (Pdfs) in addition to any available spec-z. We model the projected 2pcf estimated using $\pi_{max}$ = 200 h$^{-1}$ Mpc with the 2-halo term and we derive a bias at z$\sim$3.4 equal to b = 6.6$^{+0.60}_{-0.55}$, which corresponds to a typical mass of the hosting halos of log M$_h$ = 12.83$^{+0.12}_{-0.11}$ h$^{-1}$ M$_{\odot}$. A similar bias is derived using the redshift-space 2pcf, modelled including the typical phot-z error $\sigma_z$ = 0.052 of our sample at z$\geq$2.9. Once we integrate the projected 2pcf up to $\pi_{max}$ = 200 h$^{-1}$ Mpc, the bias of XMM and \textit{Chandra} COSMOS at z=2.8 used in Allevato et al. (2014) is consistent with our results at higher redshift. The results suggest only a slight increase of the bias factor of COSMOS AGN at z$\gtrsim$3 with the typical hosting halo mass of moderate luminosity AGN almost constant with redshift and equal to logM$_h$ = 12.92$^{+0.13}_{-0.18}$ at z=2.8 and log M$_h$ = 12.83$^{+0.12}_{-0.11}$ at z$\sim$3.4, respectively. The observed redshift evolution of the bias of COSMOS AGN implies that moderate luminosity AGN still inhabit group-sized halos at z$\gtrsim$3, but slightly less massive than observed in different independent studies using X-ray AGN at z$\leq2$., Comment: Accepted for publication in The Astrophysical Journal

X-ray surveys contain sizable numbers of star forming galaxies, beyond the AGN which usually make the majority of detections. Many methods to separate the two populations are used in the literature, based on X-ray and multiwavelength properties. We aim at a detailed test of the classification schemes and to study the X-ray properties of the resulting samples. We build on a sample of galaxies selected at 1.4 GHz in the VLA-COSMOS survey, classified by Smolcic et al. (2008) according to their optical colours and observed with Chandra. A similarly selected control sample of AGN is also used for comparison. We review some X-ray based classification criteria and check how they affect the sample composition. The efficiency of the classification scheme devised by Smolcic et al. (2008) is such that ~30% of composite/misclassified objects are expected because of the higher X-ray brightness of AGN with respect to galaxies. The latter fraction is actually 50% in the X-ray detected sources, while it is expected to be much lower among X-ray undetected sources. Indeed, the analysis of the stacked spectrum of undetected sources shows, consistently, strongly different properties between the AGN and galaxy samples. X-ray based selection criteria are then used to refine both samples. The radio/X-ray luminosity correlation for star forming galaxies is found to hold with the same X-ray/radio ratio valid for nearby galaxies. Some evolution of the ratio may be possible for sources at high redshift or high luminosity, tough it is likely explained by a bias arising from the radio selection. Finally, we discuss the X-ray number counts of star forming galaxies from the VLA- and C-COSMOS surveys according to different selection criteria, and compare them to the similar determination from the Chandra Deep Fields. The classification scheme proposed here may find application in future works and surveys., Comment: 15 pages, 7 figures, 3 tables

With this paper, we release accurate photometric redshifts for 1692 counterparts to Chandra sources in the central square degree of the COSMOS field. The availability of a large training set of spectroscopic redshifts that extends to faint magnitudes enabled photometric redshifts comparable to the highest quality results presently available for normal galaxies. We demonstrate that morphologically extended, faint X-ray sources without optical variability are more accurately described by a library of normal galaxies (corrected for emission lines) than by AGN-dominated templates, even if these sources have AGN-like X-ray luminosities. Preselecting the library on the bases of the source properties allowed us to reach an accuracy sigma_(Delta z/(1+z_spec)) \sim0.015 with a fraction of outliers of 5.8% for the entire Chandra-COSMOS sample. In addition, we release revised photometric redshifts for the 1735 optical counterparts of the XMM-detected sources over the entire 2 sq. deg.of COSMOS. For 248 sources, our updated photometric redshift differs from the previous release by Delta z>0.2. These changes are predominantly due to the inclusion of newly available deep H-band photometry H_AB=24 mag. We illustrate once again the importance of a spectroscopic training sample and how an assumption about the nature of a source together with the number and the depth of the available bands influence the accuracy of the photometric redshifts determined for AGN. These considerations should be kept in mind when defining the observational strategies of upcoming large surveys targeting AGN, such as eROSITA at X-ray energies and ASKAP/EMU in the radio band., ApJ, accepted for publication. Data also available at http://www.ipp.mpg.de/~msalv/PHOTOZ_XCOSMOS

We present the high-redshift (3 3. Eighty-one sources are selected in the 0.5-2 keV band, fourteen are selected in the 2-10 keV and six in the 0.5-10 keV bands. We sample the high-luminosity (log L_((2-10keV)) > 44.15 erg s^(–1)) space density up to z ~ 5 and a fainter luminosity range (43.5 erg s^(–1) < log L_((2-10keV)) < 44.15 erg s^(–1)) than previous studies, up to z = 3.5. We weighted the contribution to the number counts and the space density of the sources with photometric redshift by using their probability of being at z > 3. We find that the space density of high-luminosity AGNs declines exponentially at all the redshifts, confirming the trend observed for optically selected quasars. At lower luminosity, the measured space density is not conclusive, and a larger sample of faint sources is needed. Comparisons with optical luminosity functions and black hole formation models are presented together with prospects for future surveys.

We present a study of the multi-wavelength properties, from the mid-infrared to the hard X-rays, of a sample of 255 spectroscopically identified X-ray selected Type-2 AGN from the XMM-COSMOS survey. Most of them are obscured the X-ray absorbing column density is determined by either X-ray spectral analyses (for the 45% of the sample), or from hardness ratios. Spectral Energy Distributions (SEDs) are computed for all sources in the sample. The average SEDs in the optical band is dominated by the host-galaxy light, especially at low X-ray luminosities and redshifts. There is also a trend between X-ray and mid-infrared luminosity: the AGN contribution in the infrared is higher at higher X-ray luminosities. We calculate bolometric luminosities, bolometric corrections, stellar masses and star formation rates (SFRs) for these sources using a multi-component modeling to properly disentangle the emission associated to stellar light from that due to black hole accretion. For 90% of the sample we also have the morphological classifications obtained with an upgraded version of the Zurich Estimator of Structural Types (ZEST+). We find that on average Type-2 AGN have lower bolometric corrections than Type-1 AGN. Moreover, we confirm that the morphologies of AGN host-galaxies indicate that there is a preference for these Type-2 AGN to be hosted in bulge-dominated galaxies with stellar masses greater than 10^10 solar masses., 16 pages, 15 figures, accepted for publication in A&A

Based on broad/narrow-band photometry and Keck DEIMOS spectroscopy we report a redshift of z=4.64-0.08+0.06 for AzTEC/COSMOS 1, the brightest sub-mm galaxy in the AzTEC/COSMOS field. In addition to the COSMOS-survey X-ray to radio data, we report observations of the source with Herschel/PACS (100, 160 micron), CSO/SHARC II (350 micron), CARMA and PdBI (3 mm). We do not detect CO(5-4) line emission in the covered redshift ranges, 4.56-4.76 (PdBI/CARMA) and 4.94-5.02 (CARMA). If the line is within this bandwidth, this sets 3sigma upper limits on the gas mass to, Comment: 7 pages, 4 figures, 1 table, accepted for publication in ApJL; one reference corrected

We present the first results of the spectroscopy of distant, obscured AGN as obtained with the ultra-deep (~3.3 Ms) XMM-Newton survey in the Chandra Deep Field South (CDFS). One of the primary goals of the project is to characterize the X-ray spectral properties of obscured and heavily obscured Compton-thick AGN over the range of redhifts and luminosities that are relevant in terms of their contribution to the X-ray background. The ultra-deep exposure, coupled with the XMM detector's spectral throughput, allowed us to accumulate good quality X-ray spectra for a large number of X-ray sources and, in particular, for heavily obscured AGN at cosmological redshifts. Specifically we present the X-ray spectral properties of two high-redshift - z= 1.53 and z=3.70 - sources. The XMM spectra of both are very hard, with a strong iron Kalpha line at a rest-frame energy of 6.4 keV. A reflection-dominated continuum provides the best description of the X-ray spectrum of the z=1.53 source, while the intrinsic continuum of the z=3.70 AGN is obscured by a large column N_H ~ 10^24 cm-2 of cold gas. Compton-thick absorption, or close to it, is unambiguously detected in both sources. Interestingly, these sources would not be selected as candidate Compton thick AGN by some multiwavelength selection criteria based on the mid-infrared to optical and X-ray to optical flux ratios., Comment: 6 pages, 3 figures, 1 table. Minor changes applied to match the published version [A&A 526, L9 (2011)]

We present a detailed study of a peculiar source in the COSMOS survey at z=0.359. Source CXOCJ100043.1+020637 (CID-42) presents two compact optical sources embedded in the same galaxy. The distance between the 2, measured in the HST/ACS image, is 0.495" that, at the redshift of the source, corresponds to a projected separation of 2.46 kpc. A large (~1200 km/s) velocity offset between the narrow and broad components of Hbeta 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 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 data show that the absorption line is variable in energy by 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 occur in the same source is unlikely to be a coincidence. We envisage two possible explanations: (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 X-ray 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., 13 figures; submitted to ApJ. Sent back to the referee after the first interaction and awaiting the final comments

Using Chandra observations we have identified a sample of seven off-nuclear X-ray sources, in the redshift range z=0.072-0.283, located within optically bright galaxies in the COSMOS Survey. Using the multi-wavelength coverage available in the COSMOS field, we study the properties of the host galaxies of these ULXs. In detail, we derived their star formation rate from H_alpha measurements and their stellar masses using SED fitting techniques with the aim to compute the probability to have an off-nuclear source based on the host galaxy properties. We divide the host galaxies in different morphological classes using the available ACS/HST imaging. We find that our ULXs candidates are located in regions of the SFR versus M$_star$ plane where one or more off-nuclear detectable sources are expected. From a morphological analysis of the ACS imaging and the use of rest-frame colours, we find that our ULXs are hosted both in late and early type galaxies. Finally, we find that the fraction of galaxies hosting a ULX ranges from ~0.5% to ~0.2% going from L[0.5-2 keV]=3 x 10^39 erg s^-1 to L[0.5-2 keV]= 2 x 10^40 erg s^-1., Comment: 10 pages, 9 figures, accepted for publication in Astronomy & Astrophysics

We present a study of the X-ray to optical properties of a sample of 545 X-ray selected Type 1 AGN, from the XMM-COSMOS survey, over a wide range of redshifts ($0.04, Comment: 14 pages, 15 figures, to appear in A&A

Recent X-ray surveys have provided a large number of high-luminosity, obscured Active Galactic Nuclei (AGN), the so-called Type 2 quasars. Despite the large amount of multi-wavelength supporting data, the main parameters related to the black holes harbored in such AGN are still poorly known. Here we present the results obtained for a sample of eight Type 2 quasars in the redshift range 0.9-2.1 selected from the HELLAS2XMM survey, for which we used Ks-band, Spitzer IRAC and MIPS data at 24 micron to estimate bolometric corrections, black hole masses, and Eddington ratios., 6 pages, to appear in "The Multicoloured Landscape of Compact Objects and their Explosive Progenitors: Theory vs Observations" (Cefalu, Sicily, June 2006). Eds. L. Burderi et al. (New York: AIP)

We exploit very deep mid-IR (MIR) and X-ray observations by Spitzer and Chandra in the GOODS North to identify signs of hidden (either starburst or AGN) activity in spheroidal galaxies between z~0.3 and 1. Our reference is a complete sample of 168 morphologically classified spheroidal galaxies with z[AB], Paper accepted for publication in MNRAS. A few figures have been degraded. A full resolution version can be found at http://www.astro.unipd.it/giulia/ELL/MN-06-0969-MJ.pdf

[ABRIGED] We present the optical identification of a sample of 695 X-ray sources detected in the first 1.3 deg^2 of the XMM-COSMOS survey, down to a 0.5-2 keV (2-10 keV) limiting flux of ~10^-15 erg cm-2 s-1 (~5x10^-15 erg cm^-2 s-1). We were able to associate a candidate optical counterpart to ~90% (626) of the X-ray sources, while for the remaining ~10% of the sources we were not able to provide a unique optical association due to the faintness of the possible optical counterparts (I_AB>25) or to the presence of multiple optical sources within the XMM-Newton error circles. We also cross-correlated the candidate optical counterparts with the Subaru multicolor and ACS catalogs and with the Magellan/IMACS, zCOSMOS and literature spectroscopic data; the spectroscopic sample comprises 248 objects (~40% of the full sample). Our analysis reveals that for ~80% of the counterparts there is a very good agreement between the spectroscopic classification, the morphological parameters as derived from ACS data, and the optical to near infrared colors. About 20% of the sources show an apparent mismatch between the morphological and spectroscopic classifications. All the ``extended'' BL AGN lie at redshift, 17 pages, ApJS COSMOS Special Issue, 2007 in press. Full resolution version is available at http://www.mpe.mpg.de/XMMCosmos/PAPERS/

The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that has imaged the central 0.9 deg^2 of the COSMOS field down to limiting depths of 1.9 10^-16 erg cm^-2 s-1 in the 0.5-2 keV band, 7.3 10^-16 erg cm^-2 s^-1 in the 2-10 keV band, and 5.7 10^-16 erg cm^-2 s-1 in the 0.5-10 keV band. In this paper we report the i, K and 3.6micron identifications of the 1761 X-ray point sources. We use the likelihood ratio technique to derive the association of optical/infrared counterparts for 97% of the X-ray sources. For most of the remaining 3%, the presence of multiple counterparts or the faintness of the possible counterpart prevented a unique association. For only 10 X-ray sources we were not able to associate a counterpart, mostly due to the presence of a very bright field source close by. Only 2 sources are truly empty fields. Making use of the large number of X-ray sources, we update the "classic locus" of AGN and define a new locus containing 90% of the AGN in the survey with full band luminosity >10^42 erg/s. We present the linear fit between the total i band magnitude and the X-ray flux in the soft and hard band, drawn over 2 orders of magnitude in X-ray flux, obtained using the combined C-COSMOS and XMM-COSMOS samples. We focus on the X-ray to optical flux ratio (X/O) and we test its known correlation with redshift and luminosity, and a recently introduced anti-correlation with the concentration index (C). We find a strong anti-correlation (though the dispersion is of the order of 0.5 dex) between C and X/O, computed in the hard band, and that 90% of the obscured AGN in the sample with morphological information live in galaxies with regular morphology (bulgy and disky/spiral), suggesting that secular processes govern a significant fraction of the BH growth at X-ray luminosities of 10^43- 10^44.5 erg/s., 21 pages, 17 figures, 4 tables; accepted for publication in ApJS. The catalog is available at the urls listed in the paper