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Chasing highly obscured QSOs in the COSMOS field

Fiore, F. and Puccetti, S. and Brusa, M. and Salvato, M. and Zamorani, G. and Aldcroft, T. and Aussel, H. and Brunner, H. and Capak, P. and Cappelluti, N. and Civano, F. and Comastri, A. and Elvis, M. and Feruglio, C. and Finoguenov, A. and Fruscione, A. and Gilli, R. and Hasinger, G. and Koekemoer, A. and Kartaltepe, J. and Ilbert, O. and Impey, C. and Le Floc'h, E. and Lilly, S. and Mainieri, V. and Martinez-Sansigre, A. and McCracken, H. J. and Menci, N. and Merloni, A. and Miyaji, T. and Sanders, D. B. and Sargent, M. and Schinnerer, E. and Scoville, N. and Silverman, J. and Smolcic, V. and Steffen, A. and Santini, P. and Taniguchi, Y. and Thompson, D. and Trump, J. R. and Vignali, C. and Urry, M. and Yan, L. (2009) Chasing highly obscured QSOs in the COSMOS field. Astrophysical Journal, 693 (1). pp. 447-462. ISSN 0004-637X.

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A large population of heavily obscured, Compton-thick active galactic nuclei (AGNs) is predicted by AGN synthesis models for the cosmic X-ray background and by the "relic" supermassive black hole mass function measured from local bulges. However, even the deepest X-ray surveys are inefficient to search for these elusive AGNs. Alternative selection criteria, combining mid-infrared with near-infrared, and optical photometry, have instead been successful in pinpointing a large population of Compton-thick AGNs. We take advantage of the deep Chandra and Spitzer coverage of a large area (more than 10 times the area covered by the Chandra deep fields, CDFs) in the Cosmic Evolution Survey (COSMOS) field to extend the search of highly obscured, Compton-thick active nuclei to higher luminosity. These sources have low surface density, and therefore large samples can be provided only through large area surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS sources with 24 μm fluxes higher than 550 μJy. For the MIPS sources not directly detected in the Chandra images, we produce stacked images in soft and hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS source population, we compare the observed stacked count rates and hardness ratios to those predicted by detailed Monte Carlo simulations, including both obscured AGN and star-forming galaxies. The volume density of Compton-thick QSOs (log L(2-10 keV) = 44-45 erg s^(–1), or logλL_λ(5.8 μm) = 44.79-46.18 erg s^(–1) for a typical infrared to X-ray luminosity ratio) evaluated in this way is (4.8 ± 1.1) × 10^(–6) Mpc^(–3) in the redshift bin 1.2-2.2. This density is ~44% of all X-ray-selected QSOs in the same redshift and luminosity bin, and it is consistent with the expectation of the most up-to-date AGN synthesis models for the cosmic X-ray background (Gilli et al. 2007). The density of lower luminosity Compton-thick AGNs (log L(2-10 keV) = 43.5-44) at z = 0.7-1.2 is (3.7 ± 1.1) × 10^(–5) Mpc^(–3), corresponding to ~67% of X-ray-selected AGNs. The comparison between the fraction of infrared-selected, Compton-thick AGNs to the X-ray selected, unobscured, and moderately obscured AGNs at high and low luminosity suggests that Compton-thick AGNs follow a luminosity dependence similar to that discovered for Compton-thin AGNs, becoming relatively rarer at high luminosities. We estimate that the fraction of AGNs (unobscured, moderately obscured, and Compton thick) to the total MIPS source population is 49 ± 10%, a value significantly higher than that previously estimated at similar 24 μm fluxes. We discuss how our findings can constrain AGN feedback models.

Item Type:Article
Related URLs:
URLURL TypeDescription
McCracken, H. J.0000-0002-9489-7765
Schinnerer, E.0000-0002-3933-7677
Scoville, N.0000-0002-0438-3323
Urry, M.0000-0002-0745-9792
Yan, L.0000-0003-1710-9339
Additional Information:© 2009 American Astronomical Society. Print publication: Issue 1 (2009 March 1); received 2008 May 25; accepted for publication 2008 October 20; published 2009 March 2. We acknowledge support from ASI/INAF contracts I/023/05/0 and I/024/05/0 and by PRIN/MUR grant 2006-02-5203. F.F. thanks Mari Polletta for useful comments. The zCOSMOSESO Large Program Number 175.A-0839 is also acknowledged.
Group:COSMOS, Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Agenzia Spaziale Italiana (ASI)I/023/05/0
Agenzia Spaziale Italiana (ASI)I/024/05/0
Ministero dell'Istruzione, dell'Universita e della Ricerca (MIUR)2006-02-5203
Istituto Nazionale di Astrofisica (INAF)UNSPECIFIED
Subject Keywords:galaxies: active; galaxies: high-redshift; X-rays: diffuse background
Record Number:CaltechAUTHORS:20090604-115049653
Persistent URL:
Official Citation:Chasing Highly Obscured QSOs in the COSMOS Field F. Fiore, S. Puccetti, M. Brusa, M. Salvato, G. Zamorani, T. Aldcroft, H. Aussel, H. Brunner, P. Capak, N. Cappelluti, F. Civano, A. Comastri, M. Elvis, C. Feruglio, A. Finoguenov, A. Fruscione, R. Gilli, G. Hasinger, A. Koekemoer, J. Kartaltepe, O. Ilbert, C. Impey, E. Le Floc'h, S. Lilly, V. Mainieri, A. Martinez-Sansigre, H. J. McCracken, N. Menci, A. Merloni, T. Miyaji, D. B. Sanders, M. Sargent, E. Schinnerer, N. Scoville, J. Silverman, V. Smolcic, A. Steffen, P. Santini, Y. Taniguchi, D. Thompson, J. R. Trump, C. Vignali, M. Urry, and L. Yan 2009 ApJ 693 447-462 doi: 10.1088/0004-637X/693/1/447
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14364
Deposited By: Jason Perez
Deposited On:28 Aug 2009 23:56
Last Modified:28 Jun 2017 21:39

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