Chasing Highly Obscured Qsos in the Cosmos Field

A large population of heavily obscured, Compton-thick AGN is predicted by AGN synthesis models for the cosmic X-ray background and by the “relic” super-massive black-hole mass function measured from local bulges. However, even the deepest X-ray surveys are inefficient to search for these elusive AGN. Alternative selection criteria, combining mid-infrared with near-infrared and optical photometry, have instead been successful to pin-point a large population of Compton thick AGN. We take advantage of the deep Chandra and Spitzer coverage of a large area (more than 10 times the area covered by the Chandra deep fields, CDFs) in the COSMOS field, to extend the search of highly obscured, Compton-thick active nuclei to higher luminosity. These sources have low surface density and large samples can be provided only through large area surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS sources with 24μm fluxes higher than 550μJy. For the MIPS sources not directly detected in the Chandra images we produce stacked images in soft and hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS source population we compare the observed stacked count rates and hardness ratios to those predicted by detailed Monte Carlo simulations including both obscured AGN and star-forming galaxies. The volume density of Compton thick QSOs (logL(2-10keV)=44-45 ergs s, or logλLλ(5.8μm)=44.79-46.18 ergs s −1 for a typical infrared to X-ray luminosity ratio) evaluated in this way is (4.8 ± 1.1) × 10 Mpc in the redshift bin 1.2–2.2. This density is ∼ 44% of that of all X-ray selected QSOs in the same redshift and luminosity bin, and it is consistent with the expectation of most up-to-date AGN synthesis models for the Cosmic X-ray background (Gilli et al. 2007). The density of lower luminosity Compton-thick AGN (logL(2-10keV)=43.5-44) at z=0.7–1.2 is (3.7 ± 1.1) × 10 Mpc, corresponding to ∼ 67% of that of X-ray selected AGN. The comparison between the fraction of infrared selected, Compton thick AGN to the X-ray selected, unobscured and moderately obscured AGN at high and low luminosity suggests that Compton-thick AGN follow a luminosity dependence similar to that discovered for Compton-thin AGN, becoming relatively rarer at high luminosities. We estimate that the fraction of AGN (unobscured, moderately obscured and Compton thick) to the total MIPS source population is 49 ± 10%, a value significantly higher than that previously estimated at similar 24μm fluxes. We discuss how our findings can constrain AGN feedback models. Subject headings: Active Galactic Nuclei Electronic address: [email protected] 1 INAF–Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio (Rm), I00040, Italy 2 ASI Science data Center, via Galileo Galilei, 00044 Frascati, Italy 3 Max Planck Institut für extraterrestrische Physik, Giessenbachstrasse 1, D–85748 Garching, Germany 4 California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125 5 INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, I– 40127 Bologna, Italy 6 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 7 CEA/DSM-CNRS, Universite’ Paris Diderot, DAPNIA/SAp, Orme des Merisiers, 91191, Gif-sur-Yvette, France 8 Space Telescope Science Institute, 3700 SanMartin Drive, Baltimore, MD 21218 9 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI, 96822 USA 10 Steward Observatory, University of Arizona, Tucson, AZ 85721 11 Department of Physics, Eidgenossiche Technische Hochschule (ETH), CH-8093 Zurich, Switzerland 12 European Southern Observatory, Karl-Schwarzschild-str. 2,