A possible new population of sources with extreme X-ray/optical ratios

We describe a possible new class of X-ray sources that have robust detections in ultradeep Chandra data yet have no detections at all in our deep multiband Great Observatories Origins Deep Survey Hubble Space Telescope Advanced Camera for Surveys (ACS) images, which represent the highest quality optical imaging obtained to date on these fields. These extreme X-ray/optical ratio sources (EXOs) have values of F-X/F-opt at least an order of magnitude above those generally found for other active galactic nuclei (AGNs), even those that are harbored by reddened hosts. We thus infer two possible scenarios: (1) if these sources lie at redshifts zless than or similar to6, then their hosts need to be exceedingly underluminous or more reddened, compared with other known sources, or (2) if these sources lie above zsimilar to6-7, such that even their Lyalpha emission is redshifted out of the bandpass of our ACS z(850) filter, then their optical and X-ray fluxes can be accounted for in terms of relatively normal similar toL(*) hosts and moderate-luminosity AGNs.