Spectroscopically Selected Spitzer 24 μm Active Galactic Nuclei

Creators: Choi, P. I.; Yan, Lin; Helou, G.; Storrie-Lombardi, L. J.; Fadda, D.; Im, M.; Shim, H.

Abstract

We investigate the active galactic nucleus (AGN) sub-population of a 24 μm flux-limited galaxy sample in the Spitzer Extragalactic First Look Survey. Using deep Keck optical spectroscopy and a series of emission-line diagnostics, we identify AGN-dominated systems over broad redshift 0 < z < 3.5 and luminosity 9 < log (L_(TIR)) < 14 ranges, with sample means of = 0.85 and log <(L_(TIR))> = 11.5. We find that down to the flux limits of our Spitzer MIPS sample (f_(24) > 200 μJy), 15%-20% of sources exhibit strong AGN signatures in their optical spectra. At this flux limit, the AGN population accounts for as much as 25%-30% of the integrated 24 μm flux. This corresponds to an MIR AGN contribution ≈2-3 × greater than that found in ISOCAM 15 μm studies that used X-ray AGN identifications. Based on our spectroscopically selected AGN sample, we also investigate the merits of Infrared Array Camera (IRAC) color selection for AGN identification. Our comparison reveals that although there is considerable overlap, a significant fraction of spectroscopic AGNs are not identifiable based on their MIR colors alone. Both the measured completeness and reliability of the IRAC color selections are found to be strongly dependent on the MIR flux limit. Finally, our spectroscopic AGN sample implies as much as a 3 × higher AGN surface density at high redshift (z > 1.2) than that of recent optical surveys at comparable optical flux limits, suggestive of a population of heavily obscured, optical/UV reddened AGNs.

Additional Information

© 2011 American Astronomical Society. Received 2010 May 23; accepted 2011 February 17; published 2011 April 11. We thank the anonymous referee for the detailed and insightful comments that contributed greatly to the paper. This work is based (in part) on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a 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 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. Support for this work was provided by NASA through JPL/Caltech contract 1287611. M.I. acknowledges the support from the Creative Research Initiative program, No. 2010-0000712 of the Korea Science and Engineering Foundation (KOSEF) funded by the Korea government (MEST). The analysis pipeline used to reduce the DEIMOS data was developed by the DEEP2 group at UC Berkeley with support from NSF grant AST-0071048. Finally, we wish to recognize and acknowledge the significant cultural role and reverence that the summit of Mauna Kea has within the indigenous Hawaiian community. We are grateful for the opportunity to conduct observations from this mountain.

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