Optical/Near-Infrared Imaging of Infrared-Excess Palomar-Green QSOs

Ground-based high spatial-resolution (FWHM < 0.3–0.8) optical and nearinfrared imaging (0.4—2.2 μm) is presented for a complete sample of optically selected Palomar-Green QSOs with far-infrared excesses at least as great as those of “warm” AGN-like ultraluminous infrared galaxies (Lir/Lbig−blue−bump > 0.46). In all cases, the host galaxies of the QSOs were detected and most have discernable two-dimensional structure. The QSO host galaxies and the QSO nuclei are similar in magnitude at H-band. H-band luminosities of the hosts range from 0.5-7.5 L with a mean of 2.3 L, and are consistent with those found in ULIGs. Both the QSO nuclei and the host galaxies have near-infrared excesses, which may be the result of dust associated with the nucleus and of recent dusty star formation in the host. These results suggest that some, but not all, optically-selected QSOs may have evolved from an infrared-active state triggered by the merger of two similarly-sized L galaxies, in a manner similar to that of the ultraluminous infrared galaxies. Subject headings: quasars: general—galaxies: active—infrared: galaxies