The Spitzer Mid-infrared Active Galactic Nucleus Survey. I. Optical and Near-infrared Spectroscopy of Obscured Candidates and Normal Active Galactic Nuclei Selected in the Mid-infrared

Creators: Lacy, M.; Ridgway, S. E.; Gates, E. L.; Nielsen, D. M.; Petric, A. O.; Sajina, A.; Urrutia, T.; Drews, S. Cox; Harrison, C.; Seymour, N.; Storrie-Lombardi, L. J.

Abstract

We present the results of a program of optical and near-infrared spectroscopic follow-up of candidate active galactic nuclei (AGNs) selected in the mid-infrared. This survey selects both normal and obscured AGNs closely matched in luminosity across a wide range, from Seyfert galaxies with bolometric luminosities L_(bol) ~ 10^(10) L_☉ to highly luminous quasars (L_(bol) ~ 10^(14) L_☉), all with redshifts ranging from 0 to 4.3. Samples of candidate AGNs were selected with mid-infrared color cuts at several different 24 μm flux density limits to ensure a range of luminosities at a given redshift. The survey consists of 786 candidate AGNs and quasars, of which 672 have spectroscopic redshifts and classifications. Of these, 137 (20%) are type 1 AGNs with blue continua, 294 (44%) are type 2 objects with extinctions A_V ≳5 toward their AGNs, 96 (14%) are AGNs with lower extinctions (A_V ~ 1), and 145 (22%) have redshifts, but no clear signs of AGN activity in their spectra. Of the survey objects 50% have L_(bol) > 10^(12) L_☉, in the quasar regime. We present composite spectra for type 2 quasars and objects with no signs of AGN activity in their spectra. We also discuss the mid-infrared—emission-line luminosity correlation and present the results of cross correlations with serendipitous X-ray and radio sources. The results show that: (1) obscured objects dominate the overall AGN population, (2) mid-infrared selected AGN candidates exist which lack AGN signatures in their optical spectra but have AGN-like X-ray or radio counterparts, and (3) X-ray and optical classifications of obscured and unobscured AGNs often differ.

Additional Information

© 2013 American Astronomical Society. Received 2013 April 23; accepted 2013 August 15; published 2013 September 27. N.S. is the recipient of an Australian Research Council Future Fellowship. M.L., A.P., and T.U. were visiting astronomers at the Infrared Telescope Facility, which is operated by the University of Hawaii under cooperative agreement no.NNX-08AE38A with the National Aeronautics and Space Administration (NASA), Science Mission Directorate, Planetary Astronomy Program. The Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc, under a cooperative agreement with the National Science Foundation on behalf of the Gemini Partnership. Partly based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). We thank the Telescope System Instrumentation Program (TSIP), administered by NOAO, for the opportunity to observe on the Multiple Mirror Telescope (MMT). This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of data obtained from the Chandra Source Catalog, provided by the Chandra X-Ray Center (CXC) as part of the Chandra Science Archive. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This paper is dedicated to the late Steve Rawlings, whose work on radio galaxy samples directly inspired the approach taken to this project.

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