Spectroscopically selected Spitzer 24 μm active galactic nuclei

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 〈z〉 = 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.