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%Y30% of 1:4P zP 2:5 galaxies with KVega < 22 detected with Spitzer MIPS at 24 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 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 NHk 10 cm 2 and an average, unobscured X-ray luminosity L2Y8 keV (1Y4) ; 10 ergs s . Their sky density ( 3200 deg ) and space density ( 2:6 ; 10 4 Mpc ) are twice those of X-rayY 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%Y60% forM 10 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 1⁄4 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. Subject headingg s: galaxies: active — galaxies: evolution — galaxies: formation — X-rays: galaxies Online material: color figures