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 KVega<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>~1024 cm-2 and an average, unobscured X-ray luminosity L2-8keV~(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% for M~1011 Msolar, 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.
- Publication:
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The Astrophysical Journal
- Pub Date:
- November 2007
- DOI:
- 10.1086/521820
- arXiv:
- arXiv:0705.2832
- Bibcode:
- 2007ApJ...670..173D
- Keywords:
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- Galaxies: Active;
- Galaxies: Evolution;
- Galaxies: Formation;
- X-Rays: Galaxies;
- Astrophysics
- E-Print:
- 18 pages, 10 figures. ApJ in press. Minor changes only. See also the companion paper arXiv:0705.2831