Why optically faint AGNs are optically faint:: The Spitzer perspective

Optically faint X-ray sources ( those with f(X)/f(R) > 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 mu m photometry, relying heavily on the Spitzer Space Telescope. We estimate photometric redshifts for 17 of these 20 sources. We find that these AGNs are optically faint, both because they lie at significantly higher redshifts (median z similar to 1.6) than most X-ray-selectedAGNs, and because their spectra are much redder than standard AGNs. They have 2-8 keV X-ray luminosities in the Seyfert range, unlike the QSO luminosities of optically faint AGNs found in shallow wide-field surveys. Their contribution to the X-ray Seyfert luminosity function is comparable to that of z > 1 optically bright AGNs.