We investigate the properties of the most optically faint sources in the GOODS-N area ( R _ { AB } > 26.5 ) .
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 .
The optically faint sources 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 3.6 \mu m IRAC counterparts of the X-ray sources and in turn by 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 _ { 606 } - K _ { S } lower limits .
Their derived photometric redshifts show that these populate moderate redshifts ( median z \sim 2.8 ) , being at markedly different redshifts from the already spectroscopically identified population which peaks at z \sim 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 ) templates 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 of sources 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 \approx 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 per cent 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 Pure Luminosity Evolution ( PLE ) model .