Early Spitzer Detections of Extreme X-ray/Optical Sources (EXOs)

Abstract

The first Spitzer images from the GOODS survey have revealed detections of all the Extreme X-ray / Optical sources (‘EXO’s) in the CDF-South. These X-ray sources are completely undetected in our deep optical GOODS HST/ACS imaging, to limits that place them at the extreme end of the Fx/F_opt parameter space, with values about 100 to 1000 times higher than generally found for Active Galactic Nuclei (AGN). We use their Spitzer detections to investigate two possible scenarios for their nature: (1) their hosts could be extremely faint, obscured high-redshift Extremely Red Objects (EROs), at redshifts about 2 - 5, thus higher redshift than previously studied EROs; (2) some of them could lie at redshifts above about 6 - 7, such that their Lyman-alpha emission is completely redshifted into the IR. In either case, these objects serve as a valuable probe of black hole growth and accretion activity in the early universe.