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A. M. Koekemoer (STScI), GOODS Team
Extreme X-ray / Optical ratio sources ("EXO"s) are a class of source that are robustly detected in X-rays in our GOODS Chandra Deep Fields, while being completely undetected in our deep optical ground-based and GOODS HST/ACS imaging (including z-band), yet are detected in our deep near-IR VLT imaging. The optical limits are sufficiently deep to place these sources at the extreme end of the Fx/Fopt parameter space for active galactic nuclei (AGN). Their Fx/Fopt values are at least 100 times higher than those for unobscured AGN, where the optical emission is dominated by the central engine, and at least an order of magnitude above those generally found for obscured AGN where the optical emission would be dominated by the host galaxy. Two scenarios are therfore discussed: (1) if these sources lie at low to moderate redshifts (redshifts between about 3 - 5) then their host galaxies need to be relatively underluminous and/or more reddened compared with other AGN hosts, including EROs; (2) if they lie above redshifts about 6 - 7, such that their Ly-alpha emission is redshifted out of the z-band filter, then their IR and X-ray properties can be accounted for in terms of relatively normal hosts and moderate-luminosity AGN. In this case, these objects serve as a valuable probe of black hole growth and accretion activity in the early universe.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.