Compton thick AGN in the XMM-COSMOS survey^⋆

¹ Institute of Astronomy & Astrophysics, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
e-mail: giolanzu@gmail.com
² INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
⁴ Dipartimento di Fisica e di Astronomia, Universitá di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
⁶ Yale Center for Astronomy and Astrophysics, 260 Whitney Avenue, CT 06520 New Haven, USA
⁷ Institute for Astronomy, 2680 Woodlawn Drive, HI 96822-1839 Honolulu, USA
⁸ ICREA and Institut de Ciències del Cosmos, Universitat de Barcelona, IEEC-UB, Martí i Franquès, 1, 08028 Barcelona, Spain
⁹ Institut de Ciéncies del Cosmos (ICCUB), Universitat de Barcelona, Martí i Franqués, 1 08028 Barcelona, Spain
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, MD 21218 Baltimore, USA
¹¹ Max Planck Institut fur Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
¹³ Kavli Institute for the Physics and Mathematics of the Universe (IPMU) 5-1-5 Kashiwanoha Kashiwa, 277-8583 Chiba, Japan
¹⁴ Department of Physics, Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland

Received: 5 September 2014
Accepted: 25 November 2014

Abstract

Heavily obscured, Compton thick (CT, N_H> 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 (N_H>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 ~ 10^43.5−10⁴⁵ 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 (M_BH), Eddington ratio (λ_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 λ_Edd 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.