We report the results from a deep HST NICMOS H-band imaging survey of a carefully selected sample of 33 luminous, late-stage galactic mergers at z 0.3. This program is part of QUEST (Quasar/ULIRG Evolutionary Study). Signs of a recent galactic interaction are seen in all of these objects, including all seven IR-excess Palomar-Green (PG) QSOs in the sample. Unsuspected double nuclei are detected in five ULIRGs. A detailed two-dimensional analysis of the surface brightness distributions in these objects indicates that the great majority (81%) of the single-nucleus systems show a prominent early-type morphology. However, low-surface-brightness exponential disks are detected on large scales in at least four of these sources. The hosts of "warm" (IRAS 25 to 60 μm flux ratio f25/f60 0.2), AGN-like systems are of early type and have less pronounced merger-induced morphological anomalies than the hosts of cool systems with LINER or H II region-like nuclear optical spectral types. The host sizes and luminosities of the seven PG QSOs in our sample are statistically indistinguishable from those of the ULIRG hosts. The hosts of ULIRGs and PG QSOs lie close to the locations of intermediate-size (~1L*-2L*) spheroids in the photometric projection of the fundamental plane of ellipticals, although there is a tendency for the ULIRGs with small hosts to be brighter than normal spheroids. The black hole masses derived from the galaxy host luminosities imply sub-Eddington accretion rates for all objects in the sample.
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