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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 358773.0, MPI für Astronomie / Publikationen_mpia
Multiwavelength study of massive galaxies at z~2. II. Widespread compton-thick active galactic nuclei and the concurrent growth of black holes and bulges
Authors:Daddi, E.; Alexander, D. M.; Dickinson, M.; Gilli, R.; Renzini, A.; Elbaz, D.; Cimatti, A.; Chary, R.; Frayer, D.; Bauer, F. E.; Brandt, W. N.; Giavalisco, M.; Grogin, N. A.; Huynh, M.; Kurk, J.; Mignoli, M.; Morrison, G.; Pope, A.; Ravindranath, S.
Date of Publication (YYYY-MM-DD):2007
Title of Journal:The Astrophysical Journal
Start Page:173
End Page:189
Audience:Not Specified
Abstract / Description:Approximately 20%-30% of 1.4<~z<~2.5 galaxies with KVega<22 detected with Spitzer MIPS at 24 mum 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 mum 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.
Free Keywords:Galaxies: Active; Galaxies: Evolution; Galaxies: Formation; X-Rays: Galaxies
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
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