Why Optically--Faint AGN Are Faint: The Spitzer Perspective

Details Why Optically--Faint AGN Are Faint: The Spitzer Perspective Why Optically--Faint AGN Are Faint: The Spitzer Perspective

2005-03-24

arxiv.org/abs/astro-ph/0503524v1

Astrophys.J. 627 (2005) 134-139

Astronomy and Astrophysics

Astrophysics

Accepted for publication in the Astrophysical Journal

Scientific paper

10.1086/430398

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 um 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.

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