Multiwavelength study of massive galaxies at z~2. II. Widespread Compton thick AGN and the concurrent growth of black holes and bulges

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Abstract
We investigate a population of z~2 galaxies that shows a distinct mid-IR (24um) excess flux over what expected from the SFR at radio, UV, 70um, soft X-ray, and submm wavelengths in GOODS. These mid-IR excess galaxies, represent ~20-30% of all z~2 star forming galaxies to K10 keV, suggesting that these sources host AGNs obscured by Compton-thick absorption with column densities N_H>10^{24}/cm^2. Their unobscured X-ray luminosity is estimated to be L_{2-8 keV}~(1-4)x10^{43}erg/s. Their sky density (~3200/deg^2) and space density (~2.6x10^-4/Mpc^3) are twice those of individually X-ray detected AGNs at z~2, and much larger than those of previously known Compton thick sources at high-z. These objects are part of the long sought-after population of distant heavily obscured AGNs predicted by models of the X-ray background. The ratio of their BH growth rate density to the ongoing SFR density in the full z=2 galaxy population is similar to the ratio of BH to stellar mass in local spheroids. We are likely witnessing the concurrent growth of BH and stellar mass in the precursors of today's massive galaxies. The fraction of mid-IR excess objects increases sharply with the galaxy mass, reaching ~50-60% for M~10^11Msun, and we argue that the absorption of hard X-rays in these Compton thick AGNs may be related to the AGN feedback postulated in galaxy formation models. (Abridged)

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