WHY OPTICALLY–FAINT AGN ARE OPTICALLY–FAINT: THE Spitzer PERSPECTIVE

Optically–faint X-ray sources (those with fX/fR> 10) constitute about 20% of X-ray sources in deep surveys, and are potentially highly obscured and/or at high redshift. Their faint optical fluxes are generally beyond the reach of spectroscopy. For a sample of 20 optically–faint sources in CDFS, we compile 0.4–24 μm photometry, relying heavily on Spitzer. We estimate photometric redshifts for 17 of these 20 sources. We find that these AGN are optically–faint both because they lie at significantly higher redshifts (median z ∼ 1.6) than most X-ray–selected AGN, and because their spectra are much redder than standard AGN. They have 2–8 keV X-ray luminosities in the Seyfert range, unlike the QSO–luminosities of optically–faint AGN found in shallow, wide–field surveys. Their contribution to the X-ray Seyfert luminosity function is comparable to that of z > 1 optically–bright AGN. Subject headings: galaxies: active—X-rays: galaxies—infrared: galaxies