Multiwavelength Study of Massive Galaxies at z ~ 2. II. Widespread Compton-thick Active Galactic Nuclei and the Concurrent Growth of Black Holes and Bulges

Abstract

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 NH ≳ 10^(24) cm^(-2) and an average, unobscured X-ray luminosity L_(2-8 keV) ≈ (1-4) × 10^(43) ergs s^(-1). Their sky density (~3200 deg^(-2)) and space density (~2.6 × 10^(-4) Mpc^(-3)) 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 ~ 10^(11) 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.

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