Most hard X-ray selected quasars in Chandra Deep Fields are obscured

Abstract

Measuring the population of obscured quasars is one of the key issues to understand the evolution of active galactic nuclei (AGNs). With a redshift completeness of 99%, the X-ray sources detected in Chandra Deep Field South (CDF-S) provide the best sample for this issue. In this letter we study the population of obscured quasars in CDF-S by choosing the 4 -- 7 keV selected sample, which is less biased by the intrinsic X-ray absorption. The 4 -- 7 keV band selected samples also filter out most of the X-ray faint sources with too few counts, for which the measurements of N_H and L_X have very large uncertainties. Simply adopting the best-fit L_2-10keV and N_H, we find 71% (20 out of 28) of the quasars (with intrinsic L_2-10keV > 10^44 erg/s) are obscured with N_H > 10^22 cm^-2. Taking account of the uncertainties in the measurements of both N_H and L_X, conservative lower and upper limits of the fraction are 54% (13 out 24) and 84% (31 out 37). In Chandra Deep Field North, the number is 29%, however, this is mainly due to the redshift incompleteness. We estimate a fraction of ~ 50% - 63% after correcting the redshift incompleteness with a straightforward approach. Our results robustly confirm the existence of a large population of obscured quasars.

18 pages, including 6 figures and 1 table, ApJ accepted (submitted on Apr. 10)

References in corpus (31)

• First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters
• Cosmological Evolution of the Hard X-ray AGN Luminosity Function and the Origin of the Hard X-ray Background
• Observational constraints on growth of massive black holes
• The Chandra Deep Field-North Survey. XIII. 2 Ms Point-Source Catalogs
• The Chandra Deep Field South: the 1 Million Second
• The Chandra Deep Field South: Optical Spectroscopy I
• A Wavelet-Based Algorithm for the Spatial Analysis of Poisson Data
• First Results from the X-ray and Optical Survey of the Chandra Deep Field South
• The Cosmic Evolution of Hard X-ray Selected Active Galactic Nuclei
• The HELLAS2XMM survey. VII. The hard X-ray luminosity function of AGN up to z=4: more absorbed AGN at low luminosities and high redshifts
• X-ray spectral properties of AGN in the Chandra Deep Field South
• The Chandra Deep Field North Survey. V. 1 Ms Source Catalogs
• Optical and Infrared Properties of the 2 Ms Chandra Deep Field-North X-ray Sources
• The HELLAS2XMM survey: IV. Optical identifications and the evolution of the accretion luminosity in the Universe
• A Classic Type 2 QSO
• XMM-Newton observation of the Lockman Hole II. Spectral analysis
• New Results from the X-ray and Optical Survey of the Chandra Deep Field South: The 300ks Exposure
• The obscuration by dust of most of the growth of supermassive black holes
• The Changing AGN Population
• Chandra Detection of a TypeII Quasar at z=3.288
• Photometric Redshift of X-Ray Sources in the Chandra Deep Field South
• The HELLAS2XMM survey: VI. X-ray absorption in the 1dfAGN sample through a spectral analysis
• The contribution of faint AGN to the hard X-ray background
• The XMM-Newton HBS28 sample: studying the obscuration in hard X-ray selected AGNs
• The BeppoSAX High Energy Large Area Survey HELLAS, III: testing synthesis models for the X-ray background
• The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program. III. Optical Spectroscopy
• The distribution of absorption in AGN detected in the XMM-Newton observations of the CDFS
• An XMM-Newton Study of the Hard X-ray Sky
• Exploring the spectral properties of faint hard X-ray sources with XMM-Newton
• Constraints on the distribution of absorption in the X-ray selected AGN population found in the 13H XMM-Newton/Chandra deep field
• On the fraction of X-ray obscured quasars in the local universe

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• X-ray Absorption and Optical Extinction in the Partially Obscured Seyfert Nucleus in Mrk 1393
• The Shape of the Cosmic X-ray Background: Nuclear Starburst Discs and the Redshift Evolution of AGN Obscuration