Open AccessA deep Chandra survey of the Groth Strip – II. Optical identification of the X‐ray sources
Author(s) -
Georgakakis A.,
Nandra K.,
Laird E. S.,
Gwyn S.,
Steidel C. C.,
Sarajedini V. L.,
Barmby P.,
Faber S. M.,
Coil A. L.,
Cooper M. C.,
Davis M.,
Newman J. A.
Publication year - 2006
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.10634.x
Subject(s) - physics , qsos , astrophysics , redshift , photometry (optics) , galaxy , active galactic nucleus , quasar , population , astronomy , photometric redshift , stellar population , star formation , stars , demography , sociology
In this paper, we discuss the optical and X‐ray spectral properties of the sources detected in a single 200‐ks Chandra pointing in the Groth‐Westphal Strip region. A wealth of optical photometric and spectroscopic data are available in this field providing optical identifications and redshift determinations for the X‐ray population. The optical photometry and spectroscopy used here are primarily from the Deep Extragalactic Evolutionary Probe 2 (DEEP2) survey with additional redshifts obtained from the literature. These are complemented with the deeper ( r ≈ 26 mag) multiwaveband data ( ugriz ) from the Canada–France–Hawaii Telescope Legacy Survey to estimate photometric redshifts and to optically identify sources fainter than the DEEP2 magnitude limit ( R AB ≈ 24.5 mag) . We focus our study on the 2–10 keV selected sample comprising 97 sources to the limit ≈ 8 × 10 −16 erg s −1 cm −2 , this being the most complete in terms of optical identification rate (86 per cent) and redshift determination fraction (63 per cent; both spectroscopic and photometric). We first construct the redshift distribution of the sample which shows a peak at z ≈ 1 . This is in broad agreement with models where less luminous active galactic nuclei (AGNs) evolve out to z ≈ 1 with powerful quasi‐stellar objects (QSOs) peaking at higher redshift, z ≈ 2 . Evolution similar to that of broad‐line QSOs applied to the entire AGN population (both types I and II) does not fit the data. We also explore the observed N H distribution of the sample and estimate a fraction of obscured AGN ( N H > 10 22 cm −2 ) of 48 ± 9 per cent. This is found to be consistent with both a luminosity‐dependent intrinsic N H distribution, where less luminous systems comprise a higher fraction of type II AGNs and models with a fixed ratio 2:1 between types I and II AGNs. We further compare our results with those obtained in deeper and shallower surveys. We argue that a luminosity‐dependent parametrization of the intrinsic N H distribution is required to account for the fraction of obscured AGN observed in different samples over a wide range of fluxes.