Imaging of a Complete Sample of Ir-excess Pg Qsos

Sanders et al. (1988) proposed an evolutionary connection between Ultraluminous Infrared Galaxies (ULIGs) and optically-selected QSOs. In this scenario, mergers of dust and gas-rich galaxies provide the fuel to create and/or fuel an AGN and circumnuclear starburst. This dust completely enshrouds the AGN, and subsequent reradiation of the shortwavelength AGN emission produces the high far-IR luminositiy that defines ULIGs as a class. Dust clearing by superwinds eventually begins to unveil the central AGN, which is then perceived as a QSO. Surace et al. (1998, 2000) carried out a comprehensive program of multi-wavelength high spatial resolution observations using HST and ground-based tip/tilt in order to characterize the morphology and colors of ULIGs. Particular emphasis was given to ULIGs with “warm” mid-IR colors, whose spectral energy distributions (SEDs) and emission line features suggested that they were the most evolved ULIGs in the process of becoming QSOs. These studies showed that ULIGs were the merger of two L galaxies, and that they had compact central sources whose luminosity and colors are similar to reddened AGN. They also possess “knots” of star formation distributed in the circumnuclear regions and along the merger-generated tidal debris.