Showing papers by "Andrea Comastri published in 2004"

Compton-Thick AGN: The Dark Side of the X-Ray Background

Abstract: The spectrum of the hard X-ray background records the history of accretion processes integrated over the cosmic time. Several pieces of observational and theoretical evidence indicate that a significant fraction of the energy density is obscured by large columns of gas and dust. The absorbing matter is often very thick, with column densities exceeding N H ≃ 1.5 × 1024 cm-2, the value corresponding to unity optical depth for Compton scattering. These sources are called “Compton thick” and appear to be very numerous, at least in the nearby universe. Although Compton-thick Active Galactic Nuclei (AGN) are thought to provide an important contribution to the overall cosmic energy budget, their space density and cosmological evolution are poorly known. The properties of Compton-thick AGN are reviewed here, with particular emphasis on their contributions to the extragalactic background light in the hard X-ray and infrared bands.

The HELLAS2XMM survey - VI. X-ray absorption in the 1df AGN sample through a spectral analysis

Abstract: The spectroscopic analysis of 117 serendipitous sources in the HELLAS2XMM 1df (1 degree field) survey is de- scribed. Of these, 106 sources, of which 86% have a spectroscopic redshift, are used to evaluate the fraction of X-ray absorbed (log NH > 22) Active Galactic Nuclei (AGN) in the 2−10 keV flux range 0.8−20 × 10 −14 erg cm −2 s −1 . This fraction turns out lower than what is predicted by two well known Cosmic X-Ray Background synthesis models, and the discrepancy is signifi- cant at the 99.999% level. This result consolidates the findings recently obtained by other authors. In the flux interval explored, the data are consistent with an intrinsic distribution of the absorbing columns (flat per decade above log NH > 21) independent of luminosity and redshift, together with an AGN luminosity function evolving purely in luminosity. It is shown that, on the other hand, extrapolation to lower fluxes fails to reproduce the results inferred from the Chandra Deep Field North survey. It is found that about 40% of the high luminosity sources in our sample have best fit log NH > 22, and the surface density of these X-ray obscured QSOs can then be estimated at about 48 per square degree, at the flux limit of ∼10 −14 erg cm −2 s −1 of the HELLAS2XMM 1df survey. As a side issue, 5 or 6 out of 60 sources, that is about 10%, identified with broad line AGN, turn out to be affected by log NH > 22 absorption.

The HELLAS2XMM survey: VI. X-ray absorption in the 1dfAGN sample through a spectral analysis

Abstract: The spectroscopic analysis of 117 serendipitous sources in the HELLAS2XMM 1df (1 degree field) survey is described. Of these, 106 sources, of which 86% have a spectroscopic redshift, are used to evaluate the fraction of X-ray absorbed (log NH>22) Active Galactic Nuclei (AGN) in the 2--10 keV flux range 0.8-20E-14 erg/cm2/s. This fraction turns out lower than what is predicted by two well known Cosmic X-Ray Background synthesis models, and the discrepancy is significant at the 99.999% level. This result consolidates the findings recently obtained by other authors. In the flux interval explored, the data are consistent with an intrinsic distribution of the absorbing columns (flat per decade above logNH>21) independent of luminosity and redshift, together with an AGN luminosity function evolving purely in luminosity. It is shown that, on the other hand, extrapolation to lower fluxes fails to reproduce the results inferred from the Chandra Deep Field North survey. It is found that about 40% of the high luminosity sources in our sample have best fit logNH>22, and the surface density of these X-ray obscured QSOs can then be estimated at about 48 per square degree, at the flux limit of ~1E-14} erg/cm2/s of the HELLAS2XMM 1df survey. As a side issue, 5 or 6 out of 60 sources, that is about 10%, identified with broad line AGN, turn out to be affected by logNH>22 absorption.

The X-Ray-derived Cosmological Star Formation History and the Galaxy X-Ray Luminosity Functions in the Chandra Deep Fields North and South

Abstract: The cosmological star formation rate in the combined Chandra Deep Fields North and South is derived from our X-ray luminosity function for galaxies in these deep fields. Mild evolution is seen up to redshift order unity with star formation rate ~ (1 + z)2.7. This is the first directly observed normal star-forming galaxy X-ray luminosity function (XLF) at cosmologically interesting redshifts (z > 0). This provides the most direct measure yet of the X-ray-derived cosmic star formation history of the universe. We make use of Bayesian statistical methods to classify the galaxies and the two types of active galactic nuclei (AGNs), finding the most useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and X-ray to optical flux ratio. There is some residual AGN contamination in the sample at the bright end of the luminosity function. Incompleteness slightly flattens the XLF at the faint end of the luminosity function. The XLF has a lognormal distribution and agrees well with the radio and infrared luminosity functions. However, the XLF does not agree with the Schechter luminosity function for the Hα LF, indicating that, as discussed in the text, additional and different physical processes may be involved in the establishment of the lognormal form of the XLF. The agreement of our star formation history points with the other star formation determinations in different wavebands (IR, radio, Hα) gives an interesting constraint on the initial mass function (IMF). The X-ray emission in the Chandra band is most likely due to binary stars, although X-ray emission from nonstellar sources (e.g., intermediate-mass black holes and/or low-luminosity AGNs) remains a possibility. Under the assumption that it is due to binary stars, the overall consistency and correlations between single-star effects and binary-star effects indicate that not only is the one-parameter IMF (M) constant but also the bivariate IMF(M1, M2) must be constant, at least at the high-mass end. Another way to put this, quite simply, is that X-ray observations may be measuring directly the binary-star formation history of the universe. X-ray studies will continue to be useful for probing the star formation history of the universe by avoiding problems of obscuration. Star formation may therefore be measured in more detail by deep surveys with future X-ray missions.

The HELLAS2XMM 1dF Survey: On the Nature of High X-Ray/Optical Flux Sources

Abstract: We present results from the photometric and spectroscopic identification of 122 X-ray sources recently discovered by XMM-Newton in the 2–10 keV band (the HELLAS2XMM 1dF sample). One of the most interesting results (which is found also in deeper surveys) is that ∼ 20% of the sources have an X-ray to optical flux ratio (X/O) ten times or more higher than that of optically selected AGN. Unlike the faint sources found in the ultra-deep Chandra and XMM-Newton surveys, which reach X-ray (and optical) fluxes ≳ 10 times lower than in the HELLAS2XMM sample, many of the extreme X/O sources in our sample have R≲25 and are therefore accessible to optical spectroscopy. We report the identification of 13 sources with extreme X/O values. While four of these sources are broad line QSO, eight of them are narrow line QSO, seemingly the extension to very high luminosity of the type 2 Seyfert galaxies.

The HELLAS2XMM survey V. Near-Infrared observations of X-ray sources with extreme X/O ratios

Abstract: We present the results of deep near-infrared observations (with ISAAC at VLT) of eleven hard X-ray selected sources in the Hellas2XMM survey, with faint optical magnitude (R > 24) and high X-ray-to-optical flux ratio. All but one of the sources have been detected in the Ks band, with bright counterparts (Ks 5), and therefore belong to the ERO population. The quality of the near-infrared data is such that we can take advantage of the sub-arcsec seeing to obtain accurate morphological information. A detailed analysis of the surface brightness profiles allows us to classify all of the near-infrared counterparts. There are two point-like objects, seven elliptical (bulge) galaxies and one source with an exponential profile. None of the extended sources shows any evidence of the presence of a central unresolved object tracing the putative X-ray emitting AGN. Using both the R − K colors and the morphological information, we have estimated for all the sources a "minimum photometric redshift", ranging between 0.8 and 2.4; the elliptical hosts have zmin = 0.9−1.4. We computed the X-ray properties using these redshifts: most of the sources have NH > 10 22 cm −2 , with unabsorbed X-ray luminosities up to 10 45 erg s −1 in the intrinsic 2−10 keV band. These objects therefore belong to the long-sought population of obscured (type II) quasars and, from a statistical point of view, they are a non-negligible fraction (about 10%) of the most luminous AGN. Selecting the high X/O sources for a follow-up study in the near-infrared is therefore a powerful technique aimed at studying at high redshift the hosts of Type II AGN, whose obscured nuclei do not affect the host galaxy morphologies. Overall, our results seem to indicate that the hosts are mostly elliptical galaxies at z ∼ 1, and that these near-IR bright objects would be among the most massive spheroids at these epochs.

Stellar and gaseous abundances in m82

Abstract: The near-infrared (near-IR) absorption spectra of starburst galaxies show several atomic and molecular lines from red supergiants that can be used to infer reliable stellar abundances. The metals locked in stars give a picture of the galaxy metallicity prior to the last burst of star formation. The enrichment of the new generation of stars born in the last burst can be traced by measuring the hot gas in the X-rays. For the first time, detailed stellar abundances in the nuclear region of the starburst galaxy M82 have been obtained. They are compared with those of the hot gas, as derived from an accurate reanalysis of the XMM-Newton and Chandra nuclear X-ray spectra. The cool stars and the hot gas suggest = -0.35 ± 0.2 dex and an overall [Si/Fe] and [Mg/Fe] enhancement by ~0.4 and 0.5 dex, respectively. This is consistent with a major chemical enrichment by Type II supernova explosions in recursive bursts on short timescales. Oxygen is more puzzling to interpret, since it is enhanced by ~0.3 dex in stars and depleted by ~0.2 dex in the hot gas. None of the standard enrichment scenarios can fully explain such a behavior compared with that of the other α-elements.

The X-Ray Derived Cosmological Star Formation History and the Galaxy X-Ray Luminosity Functions in the Chandra Deep Fields North and South

Abstract: The cosmological star formation rate in the combined Chandra Deep Fields North and South is derived from our X-Ray Luminosity Function for Galaxies in these Deep Fields. Mild evolution is seen up to redshift order unity with SFR ~ (1 + z)^{2.7}. This is the first directly observed normal star-forming galaxy X-ray luminosity function (XLF) at cosmologically interesting redshifts (z>0). This provides the most direct measure yet of the X-ray derived cosmic star-formation history of the Universe. We make use of Bayesian statistical methods to classify the galaxies and the two types of AGN, finding the most useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and X-ray to optical flux ratio. There is some residual AGN contamination in the sample at the bright end of the luminosity function. Incompleteness slightly flattens the XLF at the faint end of the luminosity function. The XLF has a lognormal distribution and agrees well with the radio and infrared luminosity functions. However, the XLF does not agree with the Schechter luminosity function for the H-alpha LF indicating that additional and different physical processes may be involved in the establishment of the lognormal form of the XLF. The agreement of our star formation history points with the other star formation determinations in different wavebands (IR, Radio, H-alpha) gives an interesting constraint on the IMF, and X-rays may be measuring directly the binary star formation history of the Universe. X-ray studies will continue to be useful for probing the star formation history of the universe by avoiding problems of obscuration. Star formation may therefore be measured in more detail by deep surveys with future x-ray missions.

The HELLAS2XMM survey. V. Near-Infrared observations of X-ray sources with extreme X/O ratios

Abstract: We present the results of deep near-infrared observations of 11 hard X-ray selected sources in the Hellas2XMM survey, with faint optical magnitude (R>24) and high X-ray-to-optical flux ratio. All but one of the sources have been detected in the Ks band, with bright counterparts (Ks 5), and therefore belong to the ERO population. A detailed analysis of the surface brightness profiles allows us to classify all of the NIR counterparts. There are 2 point-like objects, 7 elliptical galaxies and one source with a disky profile. None of the extended sources shows any evidence for the presence of a central unresolved object tracing the putative X-ray emitting AGN. Using both the R-K colors and the morphological information, we have estimated for all the sources a ``minimum photometric redshift'', ranging between 0.8 and 2.4; the elliptical hosts have zmin=0.9-1.4. We computed the X-ray properties using these redshifts: most of the sources have NH>10^{22}, with unabsorbed X-ray luminosities up to 10^{45}erg s^{-1}. These objects therefore belong to the population of obscured (Type II) quasars and, from a statistical point of view, they turn out to be a non-negligible fraction (~10%) of the most luminous AGN. Selecting the high X/O sources for a follow-up study in the NIR is therefore a powerful technique aimed at studying at high redshift the hosts of Type II AGN. Overall, our results seem to indicate that the hosts are mostly elliptical galaxies at z~1, and that these near-IR bright objects would be among the most massive spheroids at these epochs.

Evidence for X-ray Obscuration in Type II Quasar Candidates from the Sloan Digital Sky Survey

Abstract: Recently, Zakamska et al. selected 291 high-ionization narrow-emission-line active galactic nuclei (AGN) in the redshift range 0.3-0.83 from the Sloan Digital Sky Survey (SDSS) spectroscopic data. The sample includes both type 2 Seyfert galaxies and their higher-luminosity 'cousins', Type II quasar candidates. Here we present the results on the X-ray properties of 17 of these objects for which archival X-ray data (ROSAT and XMM-Newton) are available. Three sources have been significantly (≥6a) detected, one being the most radio-loud source of the sample; its X-ray emission, possibly enhanced by jet emission, is consistent with the absence of absorption. Another source has 6σ detection in the ROSAT All-Sky Survey (RASS), possibly complex radio structure, and no evidence for strong X-ray absorption. For the third X-ray detection, the XMM-Newton spectrum indicates a column density of N H = 1.26 +0.75 -0.51 x 10 22 cm -2 ; this result, coupled with the 2-10 keV luminosity of 4 × 10 44 erg s -1 , makes this source a genuine Type II quasar. Using the [O III] λ5007 line luminosities, we estimated the intrinsic X-ray power of the AGN and found that ≥47 per cent of the observed sample shows indications of X-ray absorption, with column densities ≥10 22 cm -2 . This provides further evidence that a considerable fraction are obscured quasars. Support for our conclusions also comes from the very recent analysis of RASS data performed by Zakamska et al., who found five additional lower-significance (2.1σ-3.6σ) X-ray matches.

Anisotropic inverse Compton emission in the radio galaxy 3C 265

Abstract: We present the results from a Chandra observation of the powerful radio galaxy 3C 265. We detect X-ray emission from the nucleus, the radio hotspots and lobes. In particular, the lobe X-ray emission is well explained as anisotropic inverse Compton scattering of the nuclear photons by the relativistic electrons in the radio lobes; the comparison between radio synchrotron and IC emission yields a magnetic field strength a factor ≃ 2 lower than that calculated under minimum energy conditions. The X-ray spectrum of the nucleus is consistent with that of a powerful, strongly absorbed quasar and the X-ray emission of the south-eastern hotspot can be successfully reproduced by a combination of synchro-self Compton and inverse Compton emission assuming a magnetic field slightly lower than equipartition.

HEXIT-SAT: a mission concept for x-ray grazing incidence telescopes from 0.5 to 70 keV

Abstract: While the energy density of the Cosmic X-ray Background (CXB) provides a statistical estimate of the super massive black hole (SMBH) growth and mass density in the Universe, the lack, so far, of focusing instrument in the 20-60 keV (where the CXB energy density peaks), frustrates our effort to obtain a comprehensive picture of the SMBH evolutionary properties HEXIT-SAT (High Energy X-ray Imaging Telescope SATellite) is a mission concept capable of exploring the hard X-ray sky with focusing/imaging instrumentation, to obtain an unbiased census of accreting SMBH up to the redshifts where galaxy formation peaks, and on extremely wide luminosity ranges This will represent a leap forward comparable to that achieved in the soft X-rays by the Einstein Observatory in the late 70' In addition to accreting SMBH, and very much like the Einstein Observatory, this mission would also have the capabilities of investigating almost any type of the celestial X-ray sources HEXIT-SAT is based on high throughput (>400 cm2 @ 30 keV; >1200 cm2 @ 1 keV), high quality (15 arcsec Half Power Diameter) multi-layer optics, coupled with focal plane detectors with high efficiency in the full 05-70keV range Building on the BeppoSAX experience, a low-Earth, equatorial orbit, will assure a low and stable particle background, and thus an extremely good sensitivity for faint hard X-ray sources At the flux limits of 1/10 microCrab (10-30 keV) and 1/3 microCrab (20-40 keV) (reachable in one Msec observation) we should detect ~100 and ~40 sources in the 15 arcmin FWHM Field of View respectively, thus resolving >80% and ~65% of the CXB where its energy density peaks

A broad iron K alpha line at z=1.146

Abstract: We report the discovery of a strong iron K alpha line in the hard X-ray selected source CXOJ 123716.7+621733 in the Chandra Deep Field North survey at z=1.146. The analysis is made possible by the very deep exposure ~2 Ms and low background of the ACIS detector. The line profile seems to be inconsistent with a narrow feature. The best fit solution is achieved with a broad line. Most of the flux in the broad component originates at energies below 6.4 keV with a shape similar to that expected from emission in the innermost regions of the accretion disk.

A broad iron Kα line at z= 1.146

Abstract: We report the discovery of a strong iron Ka line in the hard X-ray selected source CXOJ 123716.7+621733 in the Chandra Deep Field-North survey at z = 1.146. The analysis is made possible by the very deep exposure of ∼2 Ms and the low background of the Advanced CCD Imaging Spectrometer (ACIS). The line profile seems to be inconsistent with a narrow feature. The best-fitting solution is achieved with a broad line. Most of the flux in the broad component originates at energies below 6.4 keV with a shape similar to that expected from emission in the innermost regions of the accretion disc.

The Density and Masses of Obscured Black Holes

Abstract: Recent Chandra and XMM-Newton surveys have uncovered a large fraction of the obscured AGN responsible of the hard X-ray background. One of the most intriguing results of extensive programs of follow-up observations concerns the optical and near-infrared properties of the hard X-ray sources counterparts. More specifically, for a significant fraction of hard X-ray obscured sources the AGN responsible of the high X-ray luminosity remains elusive over a wide range of wavelengths from soft X-rays to near-infrared. This very observational result opens the possibility to investigate the host of bright obscured quasars in some detail. Here we briefly report on some preliminar results obtained for a small sample of elusive AGN in the HELLAS2XMM survey.

The density and masses of obscured Black Holes

Abstract: Recent Chandra and XMM-Newton surveys have uncovered a large fraction of the obscured AGN responsible of the hard X-ray background. One of the most intriguing results of extensive programs of follow-up observations concerns the optical and near-infrared properties of the hard X-ray sources counterparts. More specifically, for a significant fraction of hard X-ray obscured sources the AGN responsible of the high X-ray luminosity remains elusive over a wide range of wavelengths from soft X-rays to near-infrared. This very observational result opens the possibility to investigate the host of bright obscured quasars in some detail. Here we briefly report on some preliminar results obtained for a small sample of elusive AGN in the HELLAS2XMM survey.

The Missing X-ray Background

Abstract: The fraction of the hard X-ray background (XRB) resolved into individual sources by the deep Chandra and XMM-Newton surveys strongly depends on the adopted energy range and decreases with increasing energy. As a consequence, the nature of the sources of the even harder (>10 keV) XRB remains observationally poorly constrained. I will briefly discuss the need for X-ray observations above 10 keV.

The Missing X-Ray Background

Abstract: The fraction of the hard X-ray background (XRB) resolved into individual sources by the deep Chandra and XMM-Newton surveys strongly depends on the adopted energy range and decreases with increasing energy. As a consequence, the nature of the sources of the even harder (>10 keV) XRB remains observationally poorly constrained. I will briefly discuss the need for X-ray observations above 10 keV.

Compton Thick AGN: the dark side of the X-ray background

Abstract: The spectrum of the hard X-ray background records the history of accretion processes integrated over the cosmic time. Several pieces of observational and theoretical evidence indicate that a significant fraction of the energy density is obscured by large columns of gas and dust. The absorbing matter is often very thick, with column densities exceeding N_H > 1.5 10^24 cm-2, the value corresponding to unity optical depth for Compton scattering. These sources are called ``Compton thick'' and appear to be very numerous, at least in the nearby universe. Although Compton thick Active Galactic Nuclei (AGN) are thought to provide an important contribution to the overall cosmic energy budget, their space density and cosmological evolution are poorly known. The properties of Compton thick AGN are reviewed here, with particular emphasis on their contributions to the extragalactic background light in the hard X-ray and infrared bands.

Restless quasar activity: from BeppoSAX to Chandra and XMM-Newton

Abstract: We briefly review some of the progress made in the last decade in the study of the X-ray properties of the quasar population from the luminous, local objects observed by BeppoSAX to the large, rapidly increasing population of z >4 quasars detected by Chandra and XMM- Newton in recent years.

X-ray number counts of star forming galaxies

Abstract: The behaviour of the X-ray number counts of normal galaxies at faint (-18

X-Ray Number Counts of Star Forming Galaxies

Abstract: The behaviour of the X-ray number counts of normal galaxies at faint (-18

XMM-Newton observations of Extremely Red Objects and the link with luminous, X-ray obscured Quasars

Abstract: We present the results of a deep (about 80 ks) XMM-Newton survey of the largest sample of near-infrared selected Extremely Red Objects (R-K>5) available to date to K_S 4x10^{-15} cgs) and near-infrared magnitude probed by the present observations, the fraction of AGN (i.e., X-ray detected) among the ERO population is small (~3.5%); conversely, the fraction of EROs among hard X-ray selected sources is much higher (~20%). The X-ray properties of EROs detected in our XMM-Newton observation indicate absorption in excess of 10^{22} cm^{-2} in a large fraction of them. We have also considered additional samples of X-ray detected EROs available in the literature. X-ray spectral analysis of the highest S/N sources unambiguously indicates that large columns of cold gas (even >10^{23} cm^{-2}) are the rule rather than the exception. The X-ray, optical, and near-infrared properties of those X-ray selected EROs with a spectroscopic or photometric redshift nicely match those expected for quasars 2, the high-luminosity, high-redshift obscured AGNs predicted in baseline XRB synthesis models. A close correlation is detected between X- and K-band fluxes. For the AGN EROs this is consistent, under reasonable assumptions, with the relation established locally between the host galaxies and their central black holes. This suggest that the majority of EROs are powered by massive black holes accreting, on average, at about 0.03-0.1 of the Eddington limit.