Optical spectroscopy and X-ray detections of a sample of quasars and active galactic nuclei selected in the mid-infrared from two Spitzer space telescope wide-area surveys

Creators: Lacy, M.; Petric, A. O.; Sajina, A.; Canalizo, G.; Storrie-Lombardi, L. J.; Armus, L.; Fadda, D.; Marleau, F. R.

Abstract

We present optical spectroscopy of a sample of 77 luminous active galactic nuclei (AGNs) and quasars selected on the basis of their mid-infrared colors. Our objects are selected from the Spitzer Extragalactic First Look Survey and the SWIRE XMM-Newton Large-Scale Structure Survey (XMM-LSS) fields, with a typical 24 μm flux density of 5 mJy. The median redshift is 0.6, with a range of ~0.05-4. Only 33% (25 out of 77) of these objects are normal type 1 quasars, with no obscuration. Forty-four percent (34 out of 77) are type 2 objects, with high-ionization, narrow emission lines, and 14% (11 out of 77) are dust-reddened type 1 quasars, showing broad lines but a dust-reddened or unusually weak quasar continuum. Nine percent (7 out of 77) show no sign of an AGN in the optical spectrum, having either starburst spectra or spectra that could be of either starburst or LINER type. These latter objects are analogous to the X-ray-detected population of AGNs with weak or nonexistent optical AGN emission (the X-ray-bright, optically normal galaxies). Of our objects from the SWIRE field, 21 fall within moderately deep XMM-Newton exposures. All the unobscured quasars and about half the obscured quasars are detected in these exposures. This sample, when taken together with other samples of Spitzer-selected AGNs and quasars and results from X-ray studies, confirms that obscured AGNs dominate the AGN and quasar number counts of all rapidly accreting supermassive black hole systems, at least for z ≾ 4. This implies a high radiative efficiency for the black hole accretion process.

Additional Information

© 2007 The American Astronomical Society. Received 2006 April 24; accepted 2006 September 19; published 2006 December 11. We would like to thank Alejo Martínez-Sansigre and Steve Rawlings for obtaining the WHT spectra, Michael Gregg for obtaining the Lick spectra, and the anonymous referee for a helpful report. The SWIRE team is thanked for producing and making available their Spitzer catalogs. Most of the optical data were obtained at the Hale Telescope, Palomar Observatory, as part of a continuing collaboration between the California Institute of Technology (CIT), the Jet Propulsion Laboratory (JPL; operated by CIT for the National Aeronautics and Space Administration [NASA]), and Cornell University. M. L. and A. P. were visiting astronomers at the IRTF, which is operated by the University of Hawaii under Cooperative Agreement NCC 5-538 with NASA, Science Mission Directorate, Planetary Astronomy Program. This paper is based on observations made with the Spitzer Space Telescope, which is operated by JPL, CIT, under a NASA contract. Support for this work was provided by NASA through JPL. Funding for the creation and distribution of the SDSS Archive has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, NASA, the National Science Foundation, the US Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society. This research has made extensive use of the NASA/IPAC Extragalactic Database, which is operated by JPL, CIT, under contract with NASA. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among CIT, the University of California, and NASA. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community.

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