paper

Optical spectroscopy and X-ray detections of a sample of quasars and AGN selected in the mid-infrared from two Spitzer wide-area surveys

arXiv:astro-ph/0609594 · doi:10.1086/509617

Abstract

We present optical spectroscopy of a sample of 77 luminous AGN and quasars selected on the basis of their mid-infrared colors. Our objects are selected from the Spitzer Extragalactic First Look Survey and SWIRE XMM-LSS fields, with a typical 24mu flux density of 5mJy. The median redshift is 0.6, with a range of ~0.05-4. Only 33% (25/77) of these objects are normal type-1 quasars, with no obscuration. 44% (34/77) are type-2 objects, with high-ionization, narrow emission lines, 14% (11/77) are dust-reddened type-1 quasars, showing broad lines but a dust-reddened or unusually weak quasar continuum. 9% (7/77) show no sign of an AGN in the optical spectrum, having either starburst spectra or spectra which could be of either starburst or LINER types. These latter objects are analogous to the X-ray detected population of AGN with weak or non-existent optical AGN emission (the ``XBONGs''). 21 of our objects from the SWIRE field fall within moderately-deep XMM 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 AGN and quasars, and results from X-ray studies, confirms that obscured AGN 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.

30 pages, 10 figures (some color), accepted by AJ

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