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

Abstract

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.

Accepted for publication in the Astrophysical Journal

References in corpus (5)

• The Infrared Array Camera (IRAC) for the Spitzer Space Telescope
• The Spitzer Space Telescope Mission
• Reduction Algorithms for the Multiband Imaging Photometer for Spitzer
• Photometric Redshift of X-Ray Sources in the Chandra Deep Field South
• XMM-Newton observations of Extremely Red Objects and the link with luminous, X-ray obscured Quasars

Cited by in corpus (19)

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• Deep ATLAS radio observations of the CDFS-SWIRE field
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• Black hole growth and starburst activity at z = 0.6-4 in the Chandra Deep Field South. Host galaxies properties of obscured AGN
• Connecting Galaxy Evolution, Star Formation and the X-ray Background
• Why X-ray--Selected AGN Appear Optically Dull
• Star Formation in Distant Red Galaxies: Spitzer observations in the Hubble Deep Field South
• X-ray Constraints on the AGN Properties in Spitzer-IRS identified z~2 Ultraluminous Infrared Galaxies
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• Hard X-ray selected AGNs in low-mass galaxies from the NuSTAR serendipitous survey
• AEGIS: A Multi-wavelength Study of Spitzer Power-law Galaxies
• The HELLAS2XMM survey. XIII. Multi-component analysis of the spectral energy distribution of obscured AGN
• Chandra and Spitzer observations of CDFS X-ray obscured QSOs
• The HELLAS2XMM survey. IX. Spectroscopic identification of super-EROs hosting AGNs
• EROs found behind lensing clusters. II. Stellar populations and dust properties of optical dropout EROs and comparison with related objects
• X-Ray bright optically faint active galactic nuclei in the Subaru Hyper Suprime-Cam wide survey