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Anton Koekemoer, L. Moustakas (STScI), D. Alexander, F. Bauer (IoA), J. Bergeron (IAP), W. N. Brandt (PSU), C. Conselice, R. Chary, H. Yan (Caltech), S. Cristiani (Trieste), M. Dickinson (NOAO), N. Grogin (JHU), E. Treister, C. M. Urry (Yale), V. Mainieri (MPE), D. Stern (JPL), GOODS Team
A significant population of supermassive black holes in the early universe has been recently revealed through the discovery of Extreme X-ray / Optical sources (`EXO's). These are active galactic nuclei (AGN) that are well detected in deep X-ray surveys with Chandra and XMM but completely undetected in deep optical imaging with HST and 8-10m ground-based telescopes, to limits that place them at the extreme end of the Fx/Fopt parameter space, with values about 100 times above those typical for AGN. The first mid-IR Spitzer observations of these sources with IRAC and MIPS in the deep GOODS imaging survey has provided detections of these sources, which are combined with the deep optical and near-IR data to investigate their physical nature by means of comprehensive SED modelling. Some of the sources are well fit by evolved stellar populations similar to Extremely Red Objects (EROs) but at higher redshifts, indicating that evolved systems at those redshifts already contain accreting supermassive black holes. The remainder of the sources are best fit by younger stellar populations with substantial dust reddening, at redshifts in the approximate range 3 - 7 or possibly above. If the reddening is associated with intense star-formation related to galaxy interactions, then these objects may provide a unique probe of the relationship between galaxy evolution and accretion-driven black hole growth in the early universe.