IDENTIFYING LUMINOUS ACTIVE GALACTIC NUCLEI IN DEEP SURVEYS: REVISED IRAC SELECTION CRITERIA

Abstract

Spitzer IRAC selection is a powerful tool for identifying luminous AGN. For\ndeep IRAC data, however, the AGN selection wedges currently in use are heavily\ncontaminated by star-forming galaxies, especially at high redshift. Using the\nlarge samples of luminous AGN and high-redshift star-forming galaxies in\nCOSMOS, we redefine the AGN selection criteria for use in deep IRAC surveys.\nThe new IRAC criteria are designed to be both highly complete and reliable, and\nincorporate the best aspects of the current AGN selection wedges and of\ninfrared power-law selection while excluding high redshift star-forming\ngalaxies selected via the BzK, DRG, LBG, and SMG criteria. At QSO-luminosities\nof log L(2-10 keV) (ergs/s) > 44, the new IRAC criteria recover 75% of the hard\nX-ray and IRAC-detected XMM-COSMOS sample, yet only 38% of the IRAC AGN\ncandidates have X-ray counterparts, a fraction that rises to 52% in regions\nwith Chandra exposures of 50-160 ks. X-ray stacking of the individually X-ray\nnon-detected AGN candidates leads to a hard X-ray signal indicative of heavily\nobscured to mildly Compton-thick obscuration (log N_H (cm^-2) = 23.5 +/- 0.4).\nWhile IRAC selection recovers a substantial fraction of luminous unobscured and\nobscured AGN, it is incomplete to low-luminosity and host-dominated AGN.\n