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Approximately 20-30% of 1.4<z<2.5 galaxies with K<22 (Vega) detected withSpitzer MIPS at 24um show excess mid-IR emission relative to that expectedbased on the rates of star formation measured from other multiwavelengthdata.These galaxies also display some near-IR excess in Spitzer IRAC data, witha spectral energy distribution peaking longward of 1.6um in the rest frame,indicating the presence of warm-dust emission usually absent in star forminggalaxies. Stacking Chandra data for the mid-IR excess galaxies yields asignificant hard X-ray detection at rest-frame energies >6.2 keV. The stackedX-ray spectrum rises steeply at >10 keV, suggesting that these sources hostCompton-thick Active Galactic Nuclei (AGNs) with column densities N_H~10^{24}cm^-2 and an average, unobscured X-ray luminosity L_{2-8 keV}~(1-4)x10^43erg/s. Their sky density(~3200 deg^-2) and space density (~2.6x10^-4 Mpc^-3)are twice those of X-ray detected AGNs at z~2, and much larger than those ofpreviously-known Compton thick sources at similar redshifts. The mid-IR excessgalaxies are part of the long sought-after population of distant heavilyobscured AGNs predicted by synthesis models of the X-ray background. Thefraction of mid-IR excess objects increases with galaxy mass, reaching ~50-60%for M~10^11 M_sun, an effect likely connected with downsizing in galaxyformation. The ratio of theinferred black hole growth rate from theseCompton-thick sources to the global star formation rate at z=2 is similar tothe mass ratio of black holes to stars inlocal spheroids, implying concurrentgrowth of both within the precursors oftoday's massive galaxies.

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