2007ApJ...670..173D

Approximately 20%-30% of 1.4≲z≲2.5 galaxies with K_Vega<22 detected with Spitzer MIPS at 24 µm 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 µm 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 Compton-thick AGNs with column densities N_H≳10²⁴/cm2 and an average, unobscured X-ray luminosity L_2–8keV~(1-4)x10⁴³ ergs/s. Their sky density (∼3200/deg2) and space density (∼2.6x10^–4/Mpc3) are twice those of X-ray-detected AGNs at z~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 ∼50%-60% for M∼10¹¹ M_☉, 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.