A deep Chandra survey of the Groth Strip – II. Optical identification of the X-ray sources
Abstract
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^(−1)6 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 observedNH 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 NH distribution is required to account for the fraction of obscured AGN observed in different samples over a wide range of fluxes.
Additional Information
© 2006 The Authors. Journal compilation © 2006 Royal Astronomical Society. Accepted 2006 May 25. Received 2006 May 18; in original form 2005 November 7. Article first published online: 24 Jul. 2006. We thank the anonymous referee for valuable comments and suggestions. AG acknowledges funding from PPARC and the Marie-Curie Fellowship grant MEIF-CT-2005-025108. This work uses data obtained with support of the National Science Foundation grants AST 95-29028 and AST 00-71198. Funding for the DEEP2 survey has been provided by NSF grant AST-0071048 and AST-0071198. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The DEEP2 team and Keck Observatory acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community and appreciate the opportunity to conduct observations from this mountain. Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the CFHT which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France and the University of Hawaii. This work is based in part on data products produced at the Canadian Astronomy Data Centre as part of the CFHTLS, a collaborative project of NRC and CNRS.Attached Files
Published - GEOmnras06.pdf
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Additional details
- Eprint ID
- 23664
- Resolver ID
- CaltechAUTHORS:20110513-112624137
- Particle Physics and Astronomy Research Council (PPARC)
- MEIF-CT-2005-025108
- Marie Curie Fellowship
- AST 95-29028
- NSF
- AST 00-71198
- NSF
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2011-05-17Created from EPrint's datestamp field
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2021-11-09Created from EPrint's last_modified field