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.
|Original language||English (US)|
|Number of pages||17|
|State||Published - Nov 20 2007|
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science