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 Comptonthick AGNs with column densities NH ≳ 1024 cm-2 and an average, unobscured X-ray luminosity L2-8 kev ≈ (1-4) × 1043 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% forM ∼ 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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science