Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Why Optically--Faint AGN Are 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 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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The categories of this paper are: cmp-lg
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The categories of this paper are: hep-lat
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The categories of this paper are: math
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The categories of this paper are: stat
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The categories of this paper are: solv-int
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The categories of this paper are: adap-org
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The categories of this paper are: supr-con
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The categories of this paper are: alg-geom
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The categories of this paper are: acc-phys
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The categories of this paper are: eess
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The categories of this paper are: q-bio
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The categories of this paper are: nlin
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The categories of this paper are: chem-ph
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The categories of this paper are: q-alg
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The categories of this paper are: cs
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The categories of this paper are: atom-ph