Multiwavelength study of massive galaxies at z ∼ 2.: II.: Widespread compton-thick active galactic nuclei and the concurrent growth of black holes and bulges

Approximately 20%-30% of 1.4 less than or similar to z less than or similar to 2.5 galaxies with K-Vega < 22 detected with Spitzer MIPS at 24 mu m show excess mid-IR emission relative to that expected based on the rates of star formation measured from other multiwavelength data. These galaxies also display some near-IR excess in Spitzer IRAC data, with an SED peaking longward of 1.6 mu m in the rest frame, indicating the presence of warm dust emission usually absent in star-forming galaxies. Stacking Chandra data for the mid-IR excess galaxies yields a significant hard X-ray detection at rest-frame energies > 6.2 keV. The stacked X-ray spectrum rises steeply at > 10 keV, suggesting that these sources host Comptonthick AGNs with column densities N-H greater than or similar to 10(24) cm(-2) and an average, unobscured X-ray luminosity L2-8 (keV) approximate to (1-4); 10(43) ergs s(-1). Their sky density (similar to 3200 deg(-2)) and space density (similar to 2.6 x 10(-4) Mpc(-3)) are twice those of X-ray-detected AGNs at z approximate to 2, and much larger than those of previously known Compton-thick sources at similar redshifts. The mid-IR excess galaxies are part of the long sought after population of distant heavily obscured AGNs predicted by synthesis models of the X-ray background. The fraction of mid-IR excess objects increases with galaxy mass, reaching similar to 50%-60% for M similar to 10(11) M-circle dot, an effect likely connected with downsizing in galaxy formation. The ratio of the inferred black hole growth rate from these Compton-thick sources to the global star formation rate at z = 2 is similar to the mass ratio of black holes to stars in local spheroids, implying concurrent growth of both within the precursors of today's massive galaxies.