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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents



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ID: 448123.0, MPI für Astronomie / Publikationen_mpia
Chasing highly obscured QSOs in the COSMOS field
Authors:Fiore, F.; Puccetti, S.; Brusa, M.; Salvato, M.; Zamorani, G.; Aldcroft, T.; Aussel, H.; Brunner, H.; Capak, P.; Cappelluti, N.; Civano, F.; Comastri, A.; Elvis, M.; Feruglio, C.; Finoguenov, A.; Fruscione, A.; Gilli, R.; Hasinger, G.; Koekemoer, A.; Kartaltepe, J.; Ilbert, O.; Impey, C.; LeFloc'h, E.; Lilly, S.; Mainieri, V.; Martinez-Sansigre, A.; McCracken, H. J.; Menci, N.; Merloni, A.; Miyaji, T.; Sanders, D. B.; Sargent, M.; Schinnerer, E.; Scoville, N.; Silverman, J.; Smolcic, V.; Steffen, A.; Santini, P.; Taniguchi, Y.; Thompson, D.; Trump, J. R.; Vignali, C.; Urry, M.; Yan, L.
Language:English
Date of Publication (YYYY-MM-DD):2009
Title of Journal:The Astrophysical Journal
Journal Abbrev.:ApJ
Volume:693
Start Page:447
End Page:462
Review Status:Peer-review
Audience:Experts Only
Abstract / Description: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 mum fluxes higher than 550 muJy. 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 loglambdaL lambda(5.8 mum) = 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 mum fluxes. We discuss how our findings can constrain AGN feedback models.
Free Keywords:galaxies: active; galaxies: high-redshift; X-rays: diffuse background
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
Identifiers:URL:http://adsabs.harvard.edu/abs/2009ApJ...693..447F [ID No:1]
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