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P.I. Choi, G. Helou, M. Lacy, L. Yan (Spitzer Science Center, Caltech, Pasadena)
We investigate the AGN sub-population of a purely 24\micron-selected galaxy sample (S24>200~\muJy) in the Spitzer First Look Survey. Using deep Keck optical spectroscopy and a series of emission-line diagnostics, we identify AGN-dominated systems over the redshift range 0 < z < 3.5. We find that down to the flux limits of our Spitzer MIPS sample (S24>200\muJy), \approx\!15-20% of sources exhibit strong AGN signatures in their optical spectra. This AGN population accounts for as much as \approx\!25-30% of the integrated 24\micron flux from the population as a whole. Looking at the IRAC colors of our spectroscopically-selected AGN sample, we estimate the completeness and reliability of various MIR AGN selection criteria. This comparison reveals that although there is considerable overlap between the various AGN samples, a significant fraction of genuinely active galaxies are not identifiable based on their IRAC MIR colors alone. Finally, our spectroscopic sample implies a 3\times higher AGN surface density than that of recent optical surveys, over comparable optical flux limits. This is suggestive of a large population of heavily obscured, optical/UV reddened AGN.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.