Showing papers by "Andrea Comastri published in 2007"

The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era

Abstract: We present a detailed and self-consistent modeling of the cosmic X-ray background (XRB) based on the most up-to-date X-ray luminosity functions (XLF) and evolution of Active Galactic Nuclei (AGN). The large body of observational results collected by soft (0.5-2 keV) and hard (2-10 keV) X-ray surveys are used to constrain at best the properties of the Compton-thin AGN population and its contribution to the XRB emission. The number ratio R between moderately obscured (Compton-thin) AGN and unobscured AGN is fixed by the comparison between the soft and hard XLFs, which suggests that R decreases from 4 at low luminosities to 1 at high luminosities. From the same comparison there is no clear evidence of an evolution of the obscured AGN fraction with redshift. The distribution of the absorbing column densities in obscured AGN is determined by matching the soft and hard source counts. A distribution rising towards larger column densities is able to reproduce the soft and hard AGN counts over about 6 dex in flux. The model also reproduces with excellent accuracy the fraction of obscured objects in AGN samples selected at different X-ray fluxes. The integrated emission of the Compton-thin AGN population is found to underestimate the XRB flux at about 30 keV, calling for an additional population of extremely obscured (Compton-thick) AGN. Since the number of Compton-thick sources required to fit the 30 keV XRB emission strongly depends on the spectral templates assumed for unobscured and moderately obscured AGN, we explored the effects of varying the spectral templates. In particular, in addition to the column density distribution, we also considered a distribution in the intrinsic powerlaw spectral indices of variable width. In our baseline model a Gaussian distribution of photon indices with mean (F) = 1.9 and dispersion or = 0.2 is assumed. This increases the contribution of the Compton-thin AGN population to the 30 keV XRB intensity by ∼30% with respect to the case of null dispersion (i.e. a single primary AGN powerlaw with I= 1.9) but is not sufficient to match the 30 keV XRB emission. Therefore a population of heavily obscured -Compton-thick- AGN, as large as that of moderately obscured AGN, is required to fit the residual background emission. Remarkably, the fractions of Compton-thick AGN observed in the Chandra Deep Field South and in the first INTEGRAL and Swift catalogs of AGN selected above 10 keV are in excellent agreement with the model predictions.

The First Release COSMOS Optical and Near-IR Data and Catalog

Abstract: We present imaging data and photometry for the COSMOS survey in 15 photometric bands between 0.3 and 2.4 μm. These include data taken on the Subaru 8.3 m telescope, the KPNO and CTIO 4 m telescopes, and the CFHT 3.6 m telescope. Special techniques are used to ensure that the relative photometric calibration is better than 1% across the field of view. The absolute photometric accuracy from standard-star measurements is found to be 6%. The absolute calibration is corrected using galaxy spectra, providing colors accurate to 2% or better. Stellar and galaxy colors and counts agree well with the expected values. Finally, as the first step in the scientific analysis of these data we construct panchromatic number counts which confirm that both the geometry of the universe and the galaxy population are evolving.

The First Release COSMOS Optical and Near-IR Data and Catalog

Abstract: We present imaging data and photometry for the COSMOS survey in 15 photometric bands between 0.3um and 2.4um. These include data taken on the Subaru 8.3m telescope, the KPNO and CTIO 4m telescopes, and the CFHT 3.6m telescope. Special techniques are used to ensure that the relative photometric calibration is better than 1% across the field of view. The absolute photometric accuracy from standard star measurements is found to be 6%. The absolute calibration is corrected using galaxy spectra, providing colors accurate to 2% or better. Stellar and galaxy colors and counts agree well with the expected values. Finally, as the first step in the scientific analysis of these data we construct panchromatic number counts which confirm that both the geometry of the universe and the galaxy population are evolving.

The Spitzer Legacy Survey of the HST-ACS 2 sq. deg. COSMOS Field: survey strategy and first analysis

Abstract: The Spitzer-COSMOS survey (S-COSMOS) is a Legacy program (Cycles 2+3) designed to carry out a uniform deep survey of the full 2 sq deg COSMOS field in all seven Spitzer bands (3.6, 4.5, 5.6, 8.0, 24.0, 70.0, 160.0 u). This paper describes the survey parameters, mapping strategy, data reduction procedures, achieved sensitivities to date, and the complete data set for future reference. We show that the observed infrared backgrounds in the S-COSMOS field are within 10% of the predicted background levels. The fluctuations in the background at 24u have been measured and do not show any significant contribution from cirrus, as expected. In addition, we report on the number of asteroid detections in the low galactic latitude COSMOS field. We use the Cycle 2 S-COSMOS data to determine preliminary number counts, and compare our results with those from previous Spitzer Legacy surveys (e.g. SWIRE, GOODS). The results from this "first analysis" confirm that the S-COSMOS survey will have sufficient sensitivity with IRAC to detect ~ L* disks and spheroids out to z ~ 3, and with MIPS to detect ultraluminous starbursts and AGN out to z ~3 at 24u and out to z ~1.5-2 at 70u and 160u.

S-COSMOS: The Spitzer Legacy Survey of the Hubble Space Telescope ACS 2 deg^2 COSMOS Field I: Survey Strategy and First Analysis

Abstract: The Spitzer-COSMOS survey (S-COSMOS) is a Legacy program (Cycles 2+3) designed to carry out a uniform deep survey of the full 2 sq deg COSMOS field in all seven Spitzer bands (3.6, 4.5, 5.6, 8.0, 24.0, 70.0, 160.0 u). This paper describes the survey parameters, mapping strategy, data reduction procedures, achieved sensitivities to date, and the complete data set for future reference. We show that the observed infrared backgrounds in the S-COSMOS field are within 10% of the predicted background levels. The fluctuations in the background at 24u have been measured and do not show any significant contribution from cirrus, as expected. In addition, we report on the number of asteroid detections in the low galactic latitude COSMOS field. We use the Cycle 2 S-COSMOS data to determine preliminary number counts, and compare our results with those from previous Spitzer Legacy surveys (e.g. SWIRE, GOODS). The results from this "first analysis" confirm that the S-COSMOS survey will have sufficient sensitivity with IRAC to detect ~ L* disks and spheroids out to z ~ 3, and with MIPS to detect ultraluminous starbursts and AGN out to z ~3 at 24u and out to z ~1.5-2 at 70u and 160u.

COSMOS: Hubble Space Telescope Observations

Abstract: The Cosmic Evolution Survey (COSMOS) was initiated with an extensive allocation (590 orbits in Cycles 12-13) using the Hubble Space Telescope (HST) for high-resolution imaging. Here we review the characteristics of the HST imaging with the Advanced Camera for Surveys (ACS) and parallel observations with NICMOS and WFPC2. A square field (1.8 deg^2) has been imaged with single-orbit ACS I-band F814W exposures with 50% completeness for sources 0.5" in diameter at I_(AB) = 26.0 mag. The ACS is a key part of the COSMOS survey, providing very high sensitivity and high-resolution (0.09" FWHM and 0.05" pixels) imaging and detecting a million objects. These images yield resolved morphologies for several hundred thousand galaxies. The small HST PSF also provides greatly enhanced sensitivity for weak-lensing investigations of the dark matter distribution.

The XMM-Newton Wide-Field Survey in the COSMOS Field. I. Survey Description

Abstract: We present the first set of XMM-Newton EPIC observations in the 2 deg^2 COSMOS field The strength of the COSMOS project is the unprecedented combination of a large solid angle and sensitivity over the whole multiwavelength spectrum The XMM-Newton observations are very efficient in localizing and identifying active galactic nuclei (AGNs) and clusters, as well as groups of galaxies One of the primary goals of the XMM-Newton Cosmos survey is to study the coevolution of active galactic nuclei as a function of their environment in the cosmic web Here we present the log of observations, images, and a summary of first research highlights for the first pass of 25 XMM-Newton pointings across the field In the existing data set we have detected 1416 new X-ray sources in the 05-2, 2-45, and 45-10 keV bands to an equivalent 05-2 keV flux limit of 7 × 10^(-16) erg cm^(-2) s^(-1) The number of sources is expected to grow to almost 2000 in the final coverage of the survey From an X-ray color-color analysis we identify a population of heavily obscured, partially leaky or reflecting absorbers, most of which are likely to be nearby, Compton-thick AGNs

The XMM-Newton Wide-Field Survey in the COSMOS Field: Statistical Properties of Clusters of Galaxies

Abstract: We present the results of a search for galaxy clusters in the first 36 XMM-Newton pointings on the Cosmic Evolution Survey (COSMOS) field. We reach a depth for a total cluster flux in the 0.5-2 keV band of 3 × 10^(-15) ergs cm^(-2) s^(-1), having one of the widest XMM-Newton contiguous raster surveys, covering an area of 2.1 deg^2. Cluster candidates are identified through a wavelet detection of extended X-ray emission. Verification of the cluster candidates is done based on a galaxy concentration analysis in redshift slices of thickness 0.1-0.2 in redshift, using the multiband photometric catalog of the COSMOS field and restricting the search to z S)-log S distribution compares well with previous results, although yielding a somewhat higher number of clusters at similar fluxes. The X-ray luminosity function of COSMOS clusters matches well the results of nearby surveys, providing a comparably tight constraint on the faint-end slope of α = 1.93 ± 0.04. For the probed luminosity range of (8 × 10^(42))-(2 × 10^(44)) ergs s^(-1), our survey is in agreement with and adds significantly to the existing data on the cluster luminosity function at high redshifts and implies no substantial evolution at these luminosities to z = 1.3.

The XMM-Newton wide-field survey in the COSMOS field - III. Optical identification and multiwavelength properties of a large sample of X-ray-selected sources

Abstract: We present the optical identification of a sample of 695 X-ray sources detected in the first 1.3 deg^2 of the COSMOS XMM-Newton survey, down to a 0.5-2 keV (2-10 keV) limiting flux of ~10^(-15) erg cm^(-2) s^(-1) (~5 × 10^(-15) erg cm^(-2) s^(-1)). In order to identify the correct optical counterparts and to assess the statistical significance of the X-ray-to-optical associations we have used the "likelihood ratio technique." Here we present the identification method and its application to the CFHT I-band and photometric catalogs. 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, with similar likelihoods of being the correct identification, 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 of this statistically meaningful sample of X-ray sources 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: the large majority of spectroscopically identified broad-line active galactic nuclei (BL AGNs) have a pointlike morphology on ACS data, blue optical colors in color-color diagrams, and an X-ray-to-optical flux ratio typical of optically selected quasars. Conversely, sources classified as narrow line AGNs or normal galaxies are on average associated with extended optical sources, have significantly redder optical-to-near-infrared colors, and span a larger range of X-ray-to-optical flux ratios. However, about 20% of the sources show an apparent mismatch between the morphological and spectroscopic classifications. All the "extended" BL AGNs lie at redshift <1.5, while the redshift distribution of the full BL AGN population peaks at z ~ 1.5. The most likely explanation is that in these objects the nuclear emission is not dominant with respect to the host galaxy emission in the observed ACS band. Our analysis also suggests that the type 2/type 1 ratio decreases toward high luminosities, in qualitative agreement with the results from X-ray spectral analysis and the most recent modeling of the X-ray luminosity function evolution.

The XMM-Newton Wide-Field Survey in the COSMOS Field. II. X-Ray Data and the log N-log S Relations*

Abstract: We present data analysis and X-ray source counts for the first season of XMM-Newton observations in the COSMOS field. The survey covers ~2 deg2 within the region of sky bounded by 09h57m30s < R.A. < 10h03m30s, 01°27'30'' < decl. < 02°57'30'' with a total net integration time of 504 ks. A maximum likelihood source detection was performed in the 0.5-2, 2-4.5, and 4.5-10 keV energy bands, and 1390 pointlike sources were detected in at least one band. Detailed Monte Carlo simulations were performed to fully test the source-detection method and to derive the sky coverage to be used in the computation of the log N-log S relations. These relations were then derived in the 0.5-2, 2-10, and 5-10 keV energy bands, down to flux limits of 7.2 × 10-16 ergs cm-2 s-1, 4.0 × 10-15 ergs cm-2 s-1, and 9.7 × 10-15 ergs cm-2 s-1, respectively. Thanks to the large number of sources detected in the COSMOS survey, the log N-log S curves are tightly constrained over a range of fluxes which were poorly covered by previous surveys, especially in the 2-10 and 5-10 keV bands. The 0.5-2 and 2-10 keV differential log N-log S relations were fitted with a broken power-law model which revealed a Euclidean slope at the bright end and a flatter slope (α ~ 1.5) at faint fluxes. In the 5-10 keV energy band a single power law provides an acceptable fit to the observed source counts with a slope α ~ 2.4. A comparison with the results of previous surveys shows good agreement in all the energy bands under investigation in the overlapping flux range. We also notice a remarkable agreement between our log N-log S relations and the most recent model of the X-ray background.

The XMM-Newton wide-field survey in the COSMOS field II: X-ray data and the logN-logS

Abstract: We present the data analysis and the X-ray source counts for the first season of XMM-Newton observations in the COSMOS field. The survey covers ~2 deg^2 within the region of sky bounded by 9^h57.5^m

The XMM-Newton Wide-Field Survey in the COSMOS Field. IV. X-Ray Spectral Properties of Active Galactic Nuclei

Abstract: We present a detailed spectral analysis of pointlike X-ray sources in the XMM-NewtonCOSMOS field. Our sample of 135 sources only includes those that have more than 100 net counts in the 0.3-10 keV energy band and have been identified through optical spectroscopy. The majority of the sources are well described by a simple power-law model with either no absorption (76%) or a significant intrinsic, absorbing column (20%). The remaining ~4% of the sources require a more complex modeling by incorporating additional components to the power law. For sourceswith more than 180 net counts (bright sample), we allowed both the photon spectral index Γ and the equivalent hydrogen column N_H to be free parameters. For fainter sources, we fix Γ to the average value and allow N_H to vary. The mean spectral index of the 82 sources in the bright sample is = 2:06 ± 0:08, with an intrinsic dispersion of ~0.24. Each of these sources has fractional errors on the value of Γ below 20%. As expected, the distribution of intrinsic absorbing column densities is markedly different between AGNs with or without broad optical emission lines. We find within our sample four type 2 QSO candidates (L_X >10^(44) ergs s^(-1), N_H >10^(22) cm^(-2)), with a spectral energy distribution well reproduced by a composite Seyfert 2 spectrum, that demonstrates the strength of the wide-field COSMOS XMM-Newton survey to detect these rare and underrepresented sources. In addition, we have identified a Compton-thick (N_H >1:5 ; 10^(24) cm^(-)2) AGN at z = 0:1248. Its X-ray spectrum is well fitted by a pure reflection model and a significant Fe Kα line at rest-frame energy of 6.4 keV.

The Cosmic X-Ray Background and the Population of the Most Heavily Obscured AGNs

Abstract: We report on an accurate measurement of the CXB in the 15-50 keV range performed with the Phoswich Detection System (PDS) instrument aboard the BeppoSAX satellite. We establish that the most likely CXB intensity level at its emission peak (26-28 keV) is ≈40 keV cm-2 s-1 sr-1, a value consistent with that derived from the best available CXB measurement obtained over 25 years ago with the first High Energy Astronomical Observatory (HEAO-1) satellite mission (Gruber et al.), whose intensity, lying well below the extrapolation of some lower energy measurements performed with focusing telescopes, was questioned in recent years. We find that 90% of the acceptable solutions of our best-fit model to the PDS data give a 20-50 keV CXB flux lower than 6.5 × 10-8 ergs cm-2 s-1 sr-1, which is 12% higher than that quoted by Gruber et al. when we use our best calibration scale. In combination with the CXB synthesis models we infer that about 25% of the intensity at ~30 keV arises from extremely obscured, Compton-thick AGNs (absorbing column density NH > 1024 cm-2), while a much larger population would be implied by the highest intensity estimates. We also infer a mass density of supermassive black holes of ~3 × 105 M☉ Mpc-3. The summed contribution of resolved sources (Moretti et al.) in the 2-10 keV band exceeds our best-fit CXB intensity extrapolated to lower energies, but it is within our upper limit, so that any significant contribution to the CXB from sources other than AGNs, such as star-forming galaxies and diffuse warm-hot intergalactic medium (WHIM), is expected to be mainly confined below a few keV.

The Cosmic Evolution Survey (COSMOS): A Large-Scale Structure at z = 0.73 and the Relation of Galaxy Morphologies to Local Environment*

Abstract: We have identified a large-scale structure at z ≃ 0.73 in the COSMOS field, coherently described by the distribution of galaxy photometric redshifts, an ACS weak-lensing convergence map, and the distribution of extended X-ray sources in a mosaic of XMM-Newton observations. The main peak seen in these maps corresponds to a rich cluster with T_X = 3.51^(+0.60)_(-0.46) keV and L_X = (1.56 ± 0.04) × 10^(44) ergs s^(-1) (0.1-2.4 keV band). We estimate an X-ray mass within r_(500) corresponding to M_(500) ≃ 1.6 × 10^(14) M_⊙ and a total lensing mass (extrapolated by fitting a NFW profile) M_(NFW) = (6 ± 3) × 10^(15) M_⊙. We use an automated morphological classification of all galaxies brighter than I_(AB) = 24 over the structure area to measure the fraction of early-type objects as a function of local projected density Σ_(10), based on photometric redshifts derived from ground-based deep multiband photometry. We recover a robust morphology-density relation at this redshift, indicating, for comparable local densities, a smaller fraction of early-type galaxies than today. Interestingly, this difference is less strong at the highest densities and becomes more severe in intermediate environments. We also find, however, local "inversions" of the observed global relation, possibly driven by the large-scale environment. In particular, we find direct correspondence of a large concentration of disk galaxies to (the colder side of) a possible shock region detected in the X-ray temperature map and surface brightness distribution of the dominant cluster. We interpret this as potential evidence of shock-induced star formation in existing galaxy disks, during the ongoing merger between two subclusters. Our analysis reveals the value of combining various measures of the projected mass density to locate distant structures and their potential for elucidating the physical processes at work in the transformation of galaxy morphologies.

The Cosmic Evolution Survey (COSMOS): a large-scale structure at z=0.73 and the relation of galaxy morphologies to local environment

Abstract: We have identified a large-scale structure at z~0.73 in the COSMOS field, coherently described by the distribution of galaxy photometric redshifts, an ACS weak-lensing convergence map and the distribution of extended X-ray sources in a mosaic of XMM observations. The main peak seen in these maps corresponds to a rich cluster with Tx= 3.51+0.60/-0.46 keV and Lx=(1.56+/-0.04) x 10^{44} erg/s ([0.1-2.4] keV band). We estimate an X-ray mass within $r500$ corresponding to M500~1.6 x 10^{14} Msun and a total lensing mass (extrapolated by fitting a NFW profile) M(NFW)=(6+/-3) x 10^15 Msun. We use an automated morphological classification of all galaxies brighter than I_AB=24 over the structure area to measure the fraction of early-type objects as a function of local projected density Sigma_10, based on photometric redshifts derived from ground-based deep multi-band photometry. We recover a robust morphology-density relation at this redshift, indicating, for comparable local densities, a smaller fraction of early-type galaxies than today. Interestingly, this difference is less strong at the highest densities and becomes more severe in intermediate environments. We also find, however, local "inversions'' of the observed global relation, possibly driven by the large-scale environment. In particular, we find direct correspondence of a large concentration of disk galaxies to (the colder side of) a possible shock region detected in the X-ray temperature map and surface brightness distribution of the dominant cluster. We interpret this as potential evidence of shock-induced star formation in existing galaxy disks, during the ongoing merger between two sub-clusters.

The XMM-Newton Wide-Field Survey in the COSMOS Field. V. Angular Clustering of the X-Ray Point Sources

Abstract: We present the first results of the measurements of angular auto-correlation functions (ACFs) of X-ray point sources detected in the XMM-Newton observations of the ~2 deg2 COSMOS field (COSMOS XMM-Newton). Significant positive signals have been detected in the 0.5-2 (SFT) band, in the angle range of 0.5'-24', while the positive signals were at the ~2 and ~3 σ levels in the 2-4.5 (MED) and 4.5-10 (UHD) keV bands, respectively. Correctly taking integral constraints into account is a major limitation in interpreting our results. With power-law fits to the ACFs without the integral constraint term, we find correlation lengths of θc = 1.9'' ± 0.3'', 0.8'' , and 6'' ± 2'' for the SFT, MED, and UHD bands, respectively, for γ = 1.8. The inferred comoving correlation lengths, also taking into account the bias by the source merging due to XMM-Newton PSF, are rc ≈ 9.8 ± 0.7, 5.8, and 12 ± 2 h-1 Mpc at the effective redshifts of eff ≈ 1.1, 0.9, and 0.6 for the SFT, MED, and UHD bands, respectively. If we include the integral constraint term in the fitting process, assuming that the power law extends to the scale length of the entire COSMOS XMM-Newton field, the correlation lengths become larger by ~20%-90%. Comparing the inferred rms fluctuations of the spatial distribution of AGNs σ8,AGN with those of the underlying mass, the bias parameters of the X-ray source clustering at these effective redshifts are in the range bAGN = 1.5-4.

The HELLAS2XMM survey: XI. Unveiling the nature of X-ray Bright Optically Normal Galaxies ⋆

Abstract: Aims. X-ray bright optically normal galaxies (XBONGs) constitute a small but significant fraction of hard X-ray selected sources in recent Chandra and XMM-Newton surveys. Even though several possibilities were proposed to explain why a relatively luminous hard X-ray source does not leave any significant signature of its presence in terms of optical emission lines, the nature of XBONGs is still subject of debate. We aim to better understand their nature by means of a multiwavelength and morphological analysis of a small sample of these sources. Methods. Good-quality photometric near-infrared data (ISAAC/VLT) of four low-redshift (z = 0.1−0.3) XBONGs, selected from the HELLAS2XMM survey, have been used to search for the presence of the putative nucleus, applying the surface-brightness decomposition technique through the least-squares fitting program GALFIT. Results. The surface brightness decomposition allows us to reveal a nuclear point-like source, likely to be responsible for the X-ray emission, in two out of the four sources. The results indicate that moderate amounts of gas and dust, covering a large solid angle (possibly 4π) at the nuclear source, combined with the low nuclear activity, may explain the lack of optical emission lines. The third XBONG is associated with an X-ray extended source and no nuclear excess is detected in the near infrared at the limits of our observations. The last source is associated to a close (d ≤ 1 arcsec) double system and the fitting procedure cannot achieve a firm conclusion.

The HELLAS2XMM survey: VIII. Optical identifications of the extended sample. ⋆

Abstract: Aims. Hard X-ray, large-area surveys are a fundamental complement to ultra-deep, pencil-beam surveys in obtaining more complete coverage of the AGN luminosity-redshift plane and finding sizeable samples of “rare” AGN. Methods. We present the results of the photometric and spectroscopic identification of 110 hard X-ray selected sources from 5 additional XMM-Newton fields, nearly doubling the original HELLAS2XMM sample. Their 2–10 keV fluxes cover the range 6 × 10 −15 −4 × 10 −13 erg cm −2 s −1 and the total area surveyed is ∼0. 5d eg 2 at the bright flux limit. We spectroscopically identified 59 new sources, bringing the spectroscopic completeness of the full HELLAS2XMM sample to almost 70% over a total area of ∼1. 4d eg 2 at the bright flux limit. We found optical counterparts for 214 out of the 232 X-ray sources of the full sample down to R ∼ 25. We measured the flux and luminosity of the [OIII]λ5007 emission line for 59 of these sources. Results. Assuming that most high X-ray-to-optical flux ratio sources are obscured QSOs, we used the full HELLAS2XMM sample and the CDF samples to estimate their log N− logS . We find obscured QSOs surface density of 45 ± 15 and 100–350 deg −2 down to flux limits of 10 −14 and 10 −15 erg cm −2 s −1 , respectively. At these flux limits, the fraction of X-ray-selected obscured QSOs turns out to be similar to that of unobscured QSOs. Since X-ray selection misses most Compton-thick AGN, the number of obscured QSOs may well outnumber the unobscured QSOs. We find that hard X-ray selected AGNs with a detected [OIII] emission span a wide range of L2−10 keV/L[OIII] with a logarithmic median of 2.14 and interquartile range of 0.38. This is marginally higher than for a sample of optically selected AGNs (median 1.69 and interquatile range 0.30), suggesting that optically selected samples are at least partly incomplete and/or that [OIII] emission is not a perfect isotropic indicator of the nuclear power. The seven X-ray bright, optically normal galaxy (XBONG) candidates in the sample have L2−10 keV/L[OIII] > 1000, while their X-ray and optical luminosities and obscuring column density are similar to those of narrow-line AGNs in the same redshift interval (0.075–0.32). This suggests that, while the central engine of narrow-line AGNs and XBONGs looks similar, the narrow-line region in XBONGs could be strongly inhibited or obscured.

The HELLAS2XMM survey X. The bolometric output of luminous obscured quasars: The Spitzer perspective

Abstract: Aims. We aim at estimating the spectral energy distributions (SEDs) and the physical parameters related to the black holes harbored in eight high X-ray-to-optical (FX/FR > 10) obscured quasars at z > 0.9 selected in the 2–10 keV band from the HELLAS2XMM survey. Methods. We use IRAC and MIPS 24 µm observations, along with optical and Ks-band photometry, to obtain the SEDs of the sources. The observed SEDs are modeled using a combination of an elliptical template and torus emission (using the phenomenological templates of Silva et al. 2004) for six sources associated with passive galaxies; for two point-like sources, the empirical SEDs of red quasars are adopted. The bolometric luminosities and the MBH − LK relation are used to provide an estimate of the masses and Eddington ratios of the black holes residing in these AGN. Results. All of our sources are detected in the IRAC and MIPS (at 24 µm) bands. The SED modeling described above is in good agreement with the observed near- and mid-infrared data. The derived bolometric luminosities are in the range ≈10 45 −10 47 erg s −1 , and the median 2–10 keV bolometric correction is ≈25, consistent with the widely adopted value derived by Elvis et al. (1994). For the objects with elliptical-like profiles in the Ks band, we derive high stellar masses (0.8−6.2) × 10 11 M� , black hole masses in the range (0.2−2.5) × 10 9 M� , and Eddington ratios L/LEdd < 0.1, suggesting a low-accretion phase.

The XMM-Newton view of IRAS 09104+4109: evidence for a changing-look Type 2 quasar?

Abstract: Aims. We report on a 14 ks XMM-Newton observation of the hyperluminous infrared galaxy IRAS 09104+4109, which harbors a type 2 quasar in its nucleus. Our analysis was aimed at studying the properties of the absorbing matter and the Fe K complex at 6–7 keV in this source. Methods. We analyzed the spectroscopic data from the PN and the MOS cameras in the 0.4–10 keV band. We also used an archival BeppoSAX 1–50 keV observation of IRAS 09104+4109 to investigate possible variations of the quasar emission. Results. The X-ray emission in the EPIC band is dominated by the intra-cluster medium thermal emission. We found that the quasar contributes ∼35% of the total flux in the 2–10 keV band. Both a transmission- (through a Compton-thin absorber with a Compton optical depth of τC ∼ 0.3, i.e. NH ∼ 5 × 10 23 cm −2 ) and a reflection-dominated (τC > 1) model provide an excellent fit to the quasar continuum emission. However, the value measured for the EW of Fe Kα emission line is only marginally consistent with the presence of a Compton-thick absorber in a reflection-dominated scenario, which had been suggested by a previous, marginal (i.e. 2.5σ) detection with the hard X-ray (15–50 keV), non-imaging BeppoSAX/PDS instrument. Moreover, the value of luminosity in the 2–10 keV band measured by the transmission-dominated model is fully consistent with that expected on the basis of the bolometric luminosity of IRAS 09104+4109. From the analysis of the XMM-Newton data we therefore suggest the possibility that the absorber along the line of sight to the nucleus of IRAS 09104+4109 is Compton-thin. Alternatively, the absorber column density could have changed from Compton-thick to -thin in the five years elapsed between the observations. If this is the case, then IRAS 09104+4109 is the first “changing-look” quasar ever detected.

Hidden activity in high-redshift spheroidal galaxies from mid-infrared and X-ray observations in the GOODS-North field

Abstract: We exploit very deep mid-infrared (MIR) and X-ray observations by Spitzer and Chandra in the GOODS-North field to identify signs of hidden [either starburst or active galactic nucleus (AGN)] activity in spheroidal galaxies between z≃ 0.3 and 1. Our reference is a complete sample of 168 morphologically classified spheroidal (elliptical/lenticular) galaxies with zAB < 22.5 selected from GOODS Advanced Camera for Surveys imaging. Of these 19 have 24-μm detections in the GOODS catalogue, half of which have an X-ray counterpart in the 2-Ms Chandra catalogue (six detected in the 2–10 keV X-rays hard band), while about 25 per cent have 1.4-GHz fluxes larger than 40 μJy. Traces of hidden activity in the spheroidal population are also searched for in the deep X-ray images and 14 additional galaxies are detected in X-rays only. The nature of the observed MIR emissions is investigated by modelling their spectral energy distributions based on the available multiwavelength photometry, including X-ray, ultraviolet, optical, near-infrared, MIR and radio fluxes, and optical spectroscopy. The amount of dust derived from the infrared emission observed by Spitzer appears in excess of that expected by mass loss from evolved stars. When the available independent diagnostics are compared, in general, they provide consistent classifications about the nature of the activity in our spheroidal population. Given that, in principle, none of these diagnostics alone can be considered as conclusive, only trough a panchromatic comparison of them, we can reach an accurate comprehension of the underlying physical processes. In particular, our multiwavelength analysis of the X-ray and MIR properties leads us to conclude that at least eight of the 19 24-μm bright sources should hide an obscured AGN, while the X-ray undetected sources are more likely dominated by star formation. We conclude that ∼30 objects (∼20 per cent) of the original flux-limited sample of 168 spheroidal galaxies in the GOODS-North are detected during phases of prominent activity, of both stellar and quasar origin. Due to the short expected lifetimes of the infrared and X-ray emissions, this fraction might imply a significant level of activity in this class of galaxies during the relatively recent cosmic epochs –z∼ 0.3–1– under investigation.

Compton Thick AGN in the Suzaku Era

Abstract: Suzaku observations of two hard X-ray (> 10 keV) selected nearby Seyfert 2 galaxies are presented. Both sources were clearly detected with the pin Hard X-ray Detector up to several tens of keV, allowing for a fairly good characterization of the broad band X-ray continuum. Both sources are heavily obscured, one of which (NGC 5728 )b eing Compton thick, while at lower energies the shape and intensity of the scattered/reflected continuum is very different. Strong iron Kα lines are detected in both sources. There are also hints for the presence of a broad relativistic iron line in NGC 4992.

Unveiling obscured accretion in the Chandra Deep Field South

Abstract: A large population of heavily obscured, Compton Thick AGNs is predicted by models of galaxy formation, models of Cosmic X-ray Background and by the ``relic'' super-massive black-hole mass function measured from local bulges. However, so far only a handful of Compton thick AGNs have been possibly detected using even the deepest Chandra and XMM surveys. Compton-thick AGNs can be recovered thanks to the reprocessing of the AGN UV emission in the infrared by selecting sources with AGN luminosity's in the mid-infrared and faint near-infrared and optical emission. To this purpose, we make use of deep HST, VLT, Spitzer and Chandra data on the Chandra Deep Field South to constrain the number of Compton thick AGN in this field. We show that sources with high 24$\mu$m to optical flux ratios and red colors form a distinct source population, and that their infrared luminosity is dominated by AGN emission. Analysis of the X-ray properties of these extreme sources shows that most of them (80$\pm15%$) are indeed likely to be highly obscured, Compton thick AGNs. The number of infrared selected, Compton thick AGNs with 5.8$\mu$m luminosity higher than $10^{44.2}$ erg s$^{-1}$ turns out to be similar to that of X-ray selected, unobscured and moderately obscured AGNs with 2-10 keV luminosity higher than $10^{43}$ erg s$^{-1}$ in the redshift bin 1.2-2.6. This ``factor of 2'' source population is exactly what it is needed to solve the discrepancies between model predictions and X-ray AGN selection.

Compton thick AGN in the Suzaku era

Abstract: Suzaku observations of two hard X-ray (> 10 keV) selected nearby Seyfert 2 galaxies are presented. Both sources were clearly detected with the PIN Hard X-ray Detector up to several tens of keV, allowing for a fairly good characterization of the broad band X-ray continuum. Both sources are heavily obscured, one of which (NGC 5728) being Compton thick, while at lower energies the shape and intensity of the scattered/reflected continuum is very different. Strong iron Kalpha lines are detected in both sources. There are also hints for the presence of a broad relativistic iron line in NGC 4992.

Extragalactic Surveys: AGN physics and evolution

Abstract: We review the most important findings on AGN physics and cosmological evolution as obtained by extragalactic X-ray surveys and associated multiwavelength observations. We briefly discuss the perspectives for future enterprises and in particular the scientific case for an extremely deep (2-3 Ms) XMM survey.

Heavily obscured AGN with BeppoSAX, INTEGRAL, SWIFT, XMM and Chandra: prospects for Simbol-X

Abstract: According to the latest versions of synthesis modeling of the Cosmic X-ray Background, Compton Thick AGN should represent ~50% of the total absorbed AGN population. However, despite their importance in the cosmological context, only a few dozens of Compton Thick AGN have been found and studied so far. We will briefly review this topic and discuss the improvement in this field offered by the Simbol-X mission with its leap in sensitivity (E>10 keV) of more than a factor 500 with respect to previous X-ray missions.

The HELLAS2XMM survey. X. The bolometric output of luminous obscured quasars: The Spitzer perspective

Abstract: Aims: We aim at estimating the spectral energy distributions (SEDs) and the physical parameters related to the black holes harbored in eight high X-ray-to-optical (F_X/F_R>10) obscured quasars at z>0.9 selected in the 2--10 keV band from the HELLAS2XMM survey. Methods: We use IRAC and MIPS 24 micron observations, along with optical and Ks-band photometry, to obtain the SEDs of the sources. The observed SEDs are modeled using a combination of an elliptical template and torus emission (using the phenomenological templates of Silva et al. 2004) for six sources associated with passive galaxies; for two point-like sources, the empirical SEDs of red quasars are adopted. The bolometric luminosities and the M_BH-L_K relation are used to provide an estimate of the masses and Eddington ratios of the black holes residing in these AGN. Results: All of our sources are detected in the IRAC and MIPS (at 24 micron) bands. The SED modeling described above is in good agreement with the observed near- and mid-infrared data. The derived bolometric luminosities are in the range ~10^45-10^47 erg s^-1, and the median 2--10 keV bolometric correction is ~25, consistent with the widely adopted value derived by Elvis et al. (1994). For the objects with elliptical-like profiles in the K_s band, we derive high stellar masses (0.8-6.2)X10^11 Mo, black hole masses in the range (0.2-2.5)X10^9 Mo, and Eddington ratios L/L_Edd<0.1, suggesting a low-accretion phase.

EDGE: Explorer of diffuse emission and gamma-ray burst explosions

Abstract: How structures of various scales formed and evolved from the early Universe up to present time is a fundamental question of astrophysical cosmology. EDGE (Piro et al., 2007) will trace the cosmic history of the baryons from the early generations of massive stars by Gamma-Ray Burst (GRB) explosions, through the period of galaxy cluster formation, down to the very low redshift Universe, when between a third and one half of the baryons are expected to reside in cosmic filaments undergoing gravitational collapse by dark matter (the so-called warm hot intragalactic medium). In addition EDGE, with its unprecedented capabilities, will provide key results in many important fields. These scientific goals are feasible with a medium class mission using existing technology combined with innovative instrumental and observational capabilities by: (a) observing with fast reaction Gamma-Ray Bursts with a high spectral resolution. This enables the study of their star-forming and host galaxy environments and the use of GRBs as back lights of large scale cosmological structures; (b) observing and surveying extended sources (galaxy clusters, WHIM) with high sensitivity using two wide field of view X-ray telescopes (one with a high angular resolution and the other with a high spectral resolution). The mission concept includes four main instruments: a Wide-field Spectrometer (0.1–2.2 eV) with excellent energy resolution (3 eV at 0.6 keV), a Wide-Field Imager (0.3–6 keV) with high angular resolution (HPD = 15”) constant over the full 1.4 degree field of view, and a Wide Field Monitor (8–200 keV) with a FOV of ¼ of the sky, which will trigger the fast repointing to the GRB. Extension of its energy response up to 1 MeV will be achieved with a GRB detector with no imaging capability. This mission is proposed to ESA as part of the Cosmic Vision call. We will outline the science drivers and describe in more detail the payload of this mission.

Compton-Thick AGN and the Synthesis of the Cosmic X-Ray Background: The Suzaku Perspective

Abstract: We discuss the abundance of Compton-thick AGN as estimated by the most recent population synthesis models of the cosmic X-ray background. Only a small fraction of these elusive objects have been detected so far, in line with the model expectations. The advances expected by the broad band detectors on board Suzaku are briefly reviewed.