  Ground-based high spatial-resolution optical and near-IR imaging at BIHK' is
presented for a complete sample of optically selected Palomar-Green QSOs which
have infrared excesses at least as great as those of "warm" AGN-like
ultraluminous infrared galaxies. In all cases, the host galaxies of the QSOs
were detected and most have considerable two-dimensional structure. One-half of
the QSOs occur in spiral hosts while an additional one-quarter show extended
merger-induced tidal tails suggestive of mergers between similar-sized disk
galaxies. Eleven percent more are in elliptical hosts, and the remainder are
indeterminate. Slightly more than half of the spiral hosts have strong nuclear
bars or other nuclear disturbances consistent with (but not necessarily
implying) very advanced minor merger systems, bringing the possible merger
fraction to as high as 50%. Several of the host galaxies have knots of star
formation similar to those seen in other merger systems and in the ULIGs.
H-band luminosities of the hosts have a mean of 2.2 L*, and are also consistent
with those found in ultraluminous infrared galaxies. This result suggests that
some, but not all, optically-selected QSOs may have evolved from an
infrared-active state.
