CHASING HIGHLY OBSCURED QSOs IN THE COSMOS FIELD

Abstract

(abridged) We take advantage of the deep Chandra and Spitzer coverage of a\nlarge area (more than 10 times the area covered by the Chandra deep fields,\nCDFs in the COSMOS field, to extend the search of highly obscured,\nCompton-thick active nuclei to higher luminosity. These sources have low\nsurface density and large samples can be provided only through large area\nsurveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS\nsources with 24$\\mu$m fluxes higher than 550$\\mu$Jy. For the MIPS sources not\ndirectly detected in the Chandra images we produce stacked images in soft and\nhard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS\nsource population we compare the observed stacked count rates and hardness\nratios to those predicted by detailed Monte Carlo simulations including both\nobscured AGN and star-forming galaxies. The density of lower luminosity\nCompton-thick AGN (logL(2-10keV)=43.5-44) at z=0.7--1.2 is $(3.7\\pm1.1)\n\\times10^{-5}$ Mpc$^{-3}$, corresponding to $\\sim67%$ of that of X-ray selected\nAGN. The comparison between the fraction of infrared selected, Compton thick\nAGN to the X-ray selected, unobscured and moderately obscured AGN at high and\nlow luminosity suggests that Compton-thick AGN follow a luminosity dependence\nsimilar to that discovered for Compton-thin AGN, becoming relatively rarer at\nhigh luminosities. We estimate that the fraction of AGN (unobscured, moderately\nobscured and Compton thick) to the total MIPS source population is $49\\pm10%$,\na value significantly higher than that previously estimated at similar 24$\\mu$m\nfluxes. We discuss how our findings can constrain AGN feedback models.\n