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

Daddi, E. and Alexander, D. M. and Dickinson, M. and Gilli, R. and Renzini, A. and Elbaz, D. and Cimatti, A. and Chary, R. and Frayer, D. and Bauer, F. E. and Brandt, W. N. and Giavalisco, M. and Grogin, N. A. and Huynh, M. and Kurk, J. and Mignoli, M. and Morrison, G. and Pope, A. and Ravindranath, S. (2007) Multiwavelength Study of Massive Galaxies at z ~ 2. II. Widespread Compton-thick Active Galactic Nuclei and the Concurrent Growth of Black Holes and Bulges. Astrophysical Journal, 670 (1). pp. 173-189. ISSN 0004-637X.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Daddi, E.0000-0002-3331-9590
Alexander, D. M.0000-0002-5896-6313
Dickinson, M.0000-0001-5414-5131
Renzini, A.0000-0002-7093-7355
Elbaz, D.0000-0002-7631-647X
Chary, R.0000-0001-7583-0621
Frayer, D.0000-0003-1924-1122
Bauer, F. E.0000-0002-8686-8737
Brandt, W. N.0000-0002-0167-2453
Giavalisco, M.0000-0002-7831-8751
Pope, A.0000-0001-8592-2706
Additional Information:© 2007 The American Astronomical Society. Received 2007 May 19; accepted 2007 July 22. We thank the rest of the GMASS team for allowing us to use the still unpublished spectroscopic redshifts, as well as the many other members of the GOODS team, who have helped to make these observations possible. We are grateful to Emily MacDonald, Daniel Stern, and Hy Spinrad for collecting some of the redshifts used in this work. We thank Gianni Zamorani and Susanne Madden for useful comments and discussions. E. D. gratefully acknowledges NASA support (at the beginning of this work) through the Spitzer Fellowship Program, award 1268429. D. M. A. thanks the Royal Society for funding. R. G. acknowledges financial support from the Italian Space Agency (ASI) under the contract ASI-INAF I /023/05/0. W. N. B. acknowledges Spitzer Space Telescope grant 1278940. J. K. acknowledges financial support from the German Science Foundation (DFG) under contract SFB-439. Support for this work, part of the Spitzer Space Telescope Legacy Science Program, was provided by NASA, contract 1224666 issued by the JPL, Caltech, under NASA contract 1407.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Agenzia Spaziale Italiana (ASI)ASI-INAF I /023/05/0
Deutsche Forschungsgemeinschaft (DFG)SFB-439
Subject Keywords:galaxies : active; galaxies : evolution; galaxies : formation; X-rays : galaxies
Issue or Number:1
Record Number:CaltechAUTHORS:20091109-110239525
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Official Citation:Multiwavelength Study of Massive Galaxies at z ~ 2. II. Widespread Compton-thick Active Galactic Nuclei and the Concurrent Growth of Black Holes and Bulges E. Daddi, D. M. Alexander, M. Dickinson, R. Gilli, A. Renzini, D. Elbaz, A. Cimatti, R. Chary, D. Frayer, F. E. Bauer, W. N. Brandt, M. Giavalisco, N. A. Grogin, M. Huynh, J. Kurk, M. Mignoli, G. Morrison, A. Pope, and S. Ravindranath 2007 ApJ 670 173-189 doi: 10.1086/521820
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16625
Deposited By: Tony Diaz
Deposited On:16 Nov 2009 17:11
Last Modified:17 Dec 2019 23:42

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