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.
We present the first direct measurement of the mean Halo Occupation Distribution (HOD) of X-ray selected AGN in the COSMOS field at z < 1, based on the association of 41 XMM and 17 C-COSMOS AGN with member galaxies of 189 X-ray detected galaxy groups
from XMM and Chandra data. We model the mean AGN occupation in the halo mass range logM_200[Msun] = 13-14.5 with a rolling-off power-law with the best fit index alpha = 0.06(-0.22;0.36) and normalization parameter f_a = 0.05(0.04;0.06). We find the mean HOD of AGN among central galaxies to be modelled by a softened step function at logMh > logMmin = 12.75 (12.10,12.95) Msun while for the satellite AGN HOD we find a preference for an increasing AGN fraction with Mh suggesting that the average number of AGN in satellite galaxies grows slower (alpha_s < 0.6) than the linear proportion (alpha_s = 1) observed for the satellite HOD of samples of galaxies. We present an estimate of the projected auto correlation function (ACF) of galaxy groups over the range of r_p = 0.1-40 Mpc/h at <z> = 0.5. We use the large-scale clustering signal to verify the agreement between the group bias estimated by using the observed galaxy groups ACF and the value derived from the group mass estimates. We perform a measurement of the projected AGN-galaxy group cross-correlation function, excluding from the analysis AGN that are within galaxy groups and we model the 2-halo term of the clustering signal with the mean AGN HOD based on our results.
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.
Our analysis is aimed at characterizing the properties of the integrated spectrum of active galactic nuclei (AGNs) such as the ubiquity of the Fe K{alpha} emission in AGNs and the dependence of the spectral parameters on the X-ray luminosity and reds
hift. We selected 2646 point sources from the 2XMM catalogue at high galactic latitude (|BII| > 25 degrees) and with the sum of EPIC-PN and EPIC-MOS 0.2-12 keV counts greater than 1000. Redshifts were obtained for 916 sources from the NED. The final sample consists of 507 AGN. Individual source spectra have been summed in the observed frame to compute the integrated spectra in different redshift and luminosity bins over the range 0<z<5. Detailed analysis of these spectra has been performed. We find that the narrow Fe K{alpha} line at 6.4 keV is significantly detected up to z=1. The line equivalent width decreases with increasing X-ray luminosity in the 2-10 keV band (IT effect). The anti-correlation is characterized by the relation log(EWFe) = (1.66 +/- 0.09) + (-0.43 +/- 0.07) log(LX,44), where EWFe is the rest frame equivalent width of the neutral iron K{alpha} line in eV and LX,44 is the 2-10 keV X-ray luminosity in units of 10^{44} erg s^{-1}. The equivalent width is nearly independent of redshift up to z ~ 0.8 with an average value of 101+/-40 (rms dispersion) eV in the luminosity range 43.5<= logLX <= 44.5. Our analysis also confirmed the hardening of the spectral indices at low luminosities implying a dependence of obscuration on luminosity. We confirm that the neutral narrow Fe K{alpha} line is an almost ubiquitous feature of AGNs. We find compelling evidence for the IT effect over a redshift interval larger than probed in any previous study. We detect no evolution of the average rest frame equivalent width of the Fe K{alpha} line with redshift.
Intrinsic absorption is a fundamental physical property to understand the evolution of active galactic nuclei (AGN). Here a sample of 1290 AGN, selected in the 2-10 keV band from different flux-limited surveys with very high optical identification co
mpleteness is studied. The AGN are grouped into two classes, unabsorbed (type-1) and absorbed (type-2), depending on their optical spectroscopic classification and X-ray absorption properties, using hardness ratios. Utilizing the optical to X-ray flux ratios, a rough correction for the ~8% redshift incompleteness still present in the sample is applied. A strong decrease of the absorbed fraction with X-ray luminosity is found. This can be represented by an almost linear decrease from ~80% to ~20% in the luminosity range log L_X=42-46 and is consistent with similar derivations in the optical and MIR bands. A significant increase of the absorbed fraction with redshift is found, which can be described by a power law with a slope ~(1+z)^{0.62+/-0.11}, saturating at a redshift of z~2. A simple power law fit ~(1+z)^{0.48+/-0.08} over the whole redshift is also marginally consistent with the data. The variation of the AGN absorption with luminosity and redshift is described with higher statistical accuracy and smaller systematic errors than previous results. The findings have important consequences for the broader context of AGN and galaxy co-evolution. Here it is proposed that the cosmic downsizing in the AGN population is due to two different feeding mechanisms: a fast process of merger driven accretion at high luminosities and high redshifts versus a slow process of gas accretion from gravitational instabilities in galactic disks rebuilding around pre-formed bulges and black holes.
