Abstract
Heavily obscured, Compton thick (CT, NH> 1024 cm-2) active galactic nuclei (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 at its peak. However, unambiguously identifying CT AGN beyond the local Universe is a challenging task even in the deepest X-ray surveys, and given the expected low spatial density of these sources in the 2−10 keV band, large area surveys are needed to collect sizable samples. Through direct X-ray spectra analysis, we selected 39 heavily obscured AGN (NH>3 × 1023 cm-2) at bright X-ray fluxes (F2−10 ≳ 10-14 erg s-1 cm-2) in the 2 deg2 XMM-COSMOS survey. After selecting CT AGN based on the fit of a simple absorbed two power law model to the shallow XMM-Newton data, the presence of bona fide CT AGN was confirmed in 80% of the sources using deeper Chandra data and more complex models. The final sample comprises ten CT AGN (six of them also have a detected Fe Kα line with EW ~ 1 keV), spanning a wide range of redshifts (z ~ 0.1−2.5) and luminosity (L2−10 ~ 1043.5−1045 erg s-1) and is complemented by 29 heavily obscured AGN spanning the same redshift and luminosity range. We collected the rich multi-wavelength information available for all these sources, in order to study the distribution of super massive black hole and host properties, such as black hole mass (MBH), Eddington ratio (λEdd), 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 MBH and higher λEdd with respect to unobscured sources, 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 redshifts. We also present and briefly discuss optical spectra, broadband spectral energy distribution (SED) and morphology for the sample of ten CT AGN. Both the optical spectra and SED agree with the classification as highly obscured sources: all the available optical spectra are dominated by the stellar component of the host galaxy, and to reproduce the broadband SED, a highly obscured torus component is needed for all the CT sources.Exploiting the high resolution Hubble-ACS images available, we are able to show that these highly obscured sources have a significantly larger merger fraction with respect to other X-ray selected samples of AGN. Finally we discuss the implications of our findings in the context of AGN/galaxy co-evolutionary models, and compare our results with the predictions of X-ray background synthesis models.
