2 October 2003

Speaker: Anton Koekemoer (Space Telescope Science Institute)

Title: Extreme X-ray/Optical sources (EXOs): Probes of the High Redshift Universe

Abstract: Extreme X-ray / Optical ratio sources ("EXO"s) are a new class of source that are robustly detected in X-rays in the Chandra Deep Field Surveys, while being completely undetected in deep optical ground-based and HST/ACS imaging (including z-band), yet are detected in 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 (up to z ~ 3 - 5) then their host galaxies need to be exceedingly underluminous and/or excessively reddened compared with other AGN hosts; (2) if they lie above z ~ 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.

References for students:

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