Optically faint X-ray sources in the CDFN: Spitzer constraints

Abstract

We investigate the properties of the most optically faint sources in the GOODS-N area (R > 26.5 AB). Such extremely optically faint populations present an uncharted territory despite the fact that they represent an appreciable fraction of the X-ray sources in the GOODS-N field. They are believed to contain either red AGN at moderate redshifts or possibly QSO at very high redshift. We compile our sample by first finding the 3.6um IRAC counterparts of the X-ray sources and searching for the optical counterparts of the IRAC sources. 35 sources do not have counterparts in the R-band Subaru optical images. Of these, 18 have HST-ACS counterparts while the remaining have no optical counterparts. The vast majority of our 35 sources are classified as Extremely Red Objects (EROs) on the basis of their V-K lower limits. Their photometric redshifts show that these populate moderate redshifts (median z~2.8), being markedly different from the already spectroscopically identified population which peaks at z~0.7. The Spitzer-IRAC mid-IR colours of the sources which have no HST counterparts tend to lie within the mid-IR colour diagram AGN "wedge", suggesting either QSO, ULIRG (Mrk231), or early-type galaxy templates at z>3. A large fraction of our sources (17/35), regardless of whether they have HST counterparts, can be classified as mid-IR bright/optically faint sources (Dust Obscured Galaxies) a class which is believed to include many heavily absorbed AGN. The co-added X-ray spectrum of the optically faint sources is very flat having a spectral index of Gamma~0.87, significantly flatter than the spectrum of the X-ray background. The optically faint R>26.5 X-ray sources constitute more than 50% of the total X-ray population at redshifts z>2 bearing important implications for the luminosity function and its evolution; considering X-ray sources with 2<z<4 we find good agreement with a modified PLE model.

Accepted for publication in A&A

References in corpus (16)

• EAZY: A Fast, Public Photometric Redshift Code
• An Observational Determination of the Bolometric Quasar Luminosity Function
• Photometric redshift and classification for the XMM-COSMOS sources
• Multiwavelength study of massive galaxies at z~2. II. Widespread Compton thick AGN and the concurrent growth of black holes and bulges
• A Significant Population of Very Luminous Dust-Obscured Galaxies at Redshift z ~ 2
• The Luminosity Function of X-ray Selected Active Galactic Nuclei: Evolution of Supermassive Black Holes at High Redshift
• Unveiling obscured accretion in the Chandra Deep Field South
• Photometric redshifts in the SWIRE Survey
• The nature of faint Spitzer-selected dust-obscured galaxies
• The role of AGN in the colour transformation of galaxies at redshifts z~1
• A systematic search for very massive galaxies at z > 4
• The OPTX Project I: The Flux and Redshift Catalogs for the CLANS, CLASXS, and CDF-N fields
• Searching for mid-IR obscured AGN in the CDF-N
• High Redshift Dust Obscured Galaxies, A Morphology-SED Connection Revealed by Keck Adaptive Optics
• The X-ray luminosity function of AGN at z~3
• Optical colours of AGN in the Extended Chandra Deep Field South: Obscured black holes in early type galaxies

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