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Published May 10, 2009 | Published
Journal Article Open

The COSMOS Active Galactic Nucleus Spectroscopic Survey. I. XMM-Newton Counterparts


We present optical spectroscopy for an X-ray and optical flux-limited sample of 677 XMM-Newton selected targets covering the 2 deg^2 Cosmic Evolution Survey field, with a yield of 485 high-confidence redshifts. The majority of the spectra were obtained over three seasons (2005-2007) with the Inamori Magellan Areal Camera and Spectrograph instrument on the Magellan (Baade) telescope. We also include in the sample previously published Sloan Digital Sky Survey spectra and supplemental observations with MMT/Hectospec. We detail the observations and classification analyses. The survey is 90% complete to flux limits of f_(0.5-10 keV) > 8 × 10^(–16) erg cm^(-2) s^(–1) and i^+_(AB) < 22, where over 90% of targets have high-confidence redshifts. Making simple corrections for incompleteness due to redshift and spectral type allows for a description of the complete population to i^+_(AB) < 23. The corrected sample includes a 57% broad emission line (Type 1, unobscured) active galactic nucleus (AGN) at 0.13 < z < 4.26, 25% narrow emission line (Type 2, obscured) AGN at 0.07 < z < 1.29, and 18% absorption line (host-dominated, obscured) AGN at 0 < z < 1.22 (excluding the stars that made up 4% of the X-ray targets). We show that the survey's limits in X-ray and optical fluxes include nearly all X-ray AGNs (defined by L_(0.5-10) keV > 3 × 10^(42) erg s^(–1)) to z < 1, of both optically obscured and unobscured types. We find statistically significant evidence that the obscured-to-unobscured AGN ratio at z < 1 increases with redshift and decreases with luminosity.

Additional Information

© 2009 The American Astronomical Society. Received 2008 June 9; accepted 2009 February 25; published 2009 April 22. We thank the anonymous referee for thorough and helpful comments. We additionally thank A. Dressler and the IMACS team for creating an excellent instrument, as well as telescope operators H. Nunez, F. Sanchez, G. Valladares, S. Vera, H. Rivera, and the Las Campanas Observatory staff for support while observing. We thank M. Westover, A. Coil, S. Burles, and the Carnegie queue mode scientists for help with some of the IMACS observations. We thank D. Fabricant for designing the excellent instrument Hectospec and thank MMT operators and Hectospec queue mode scientists for their help during observations. We also thank the COSMOS team for their work in creating the catalogs used for selecting our targets. We thank P. Shopbell, A. Alexov, and the NASA IPAC/IRSA team for assisting in uploading the data and catalogs to public IRSA archives. Observations reported here were obtained at the Magellan Telescopes, which are operated by the Carnegie Observatories, as well as the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. The HST COSMOS Treasury program was supported through NASA grant HST-GO-09822. J.R.T. acknowledges support from an ARCS foundation fellowship.

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