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A. M. Koekemoer, E. J. Schreier, N. A. Grogin (STScI), R. Giacconi (AUI/JHU), R. Gilli, L. Kewley, C. Norman (JHU), P. Tozzi (Trieste), P. Rosati, J. Bergeron (ESO), G. Hasinger (AIP), A. Marconi (Arcetri)
We discuss results from recent programs combining HST and deep Chandra imaging surveys, that have uncovered a number of new populations of objects. These include relatively low-luminosity X-ray emitting AGN at redshifts up to 1 - 2, together with more nearby AGN, starbursts and normal galaxies, that are now being revealed by their faint X-ray emission. By examining the optical morphologies and colors of these objects, together with their X-ray fluxes and hardness ratios, we show that the X-ray sources detected in ultra-deep Chandra surveys can be broadly divided into two classes: 1) optically bright, relatively nearby sources comprising a mix of star-forming and normal galaxies, together with obscured and unobscured AGN; and 2) a population of optically fainter sources that can be interpreted as consisting predominantly of more distant, obscured AGN. We present models based on luminosity functions and spectral distributions of the various classes of objects, and discuss the implications for their evolution and relationships with one another.