Hubble Space Telescope Imaging in the Chandra Deep Field-South. I. Multiple Active Galactic Nucleus Populations

We present preliminary results from imaging three Hubble Space Telescope (HST) Wide Field Planetary Camera 2 fields in V and I within the Chandra Deep Field-South (CDF-S). HST's sensitivity and resolution are sufficient to reveal optical counterparts for 24 of the 26 CDF-S X-ray sources detected in the 300 ks X-ray catalog and determine the morphologies of most of these. We find that the X-ray sources comprise two apparently distinct populations of optical candidates: one optically faint (I>~24), with V-I colors consistent with the I>24 field population, and the other significantly brighter (I<~22), with colors redder than the I<22 field population. More than two-thirds of the X-ray source counterparts are resolved galaxies. The brighter sources are mostly active galactic nuclei (AGNs) based on their high X-ray luminosity. The optically resolved sources in the brighter population have a very narrow range of V-I color and appear to be a mix of both late- and early-type morphologies at low to moderate redshift. We show that the second population, with fainter optical counterparts, can be explained as higher redshift type 2 AGNs. Based on observations made with the NASA/ESA Hubble Space Telescope, operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.