A Deep<i>Hubble Space Telescope H</i>‐Band Imaging Survey of Massive Gas‐rich Mergers

Abstract

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 &amp;lt; 0.3. This program is part of QUEST, a comprehensive investigation of the most luminous mergers in the nearby universe, the ultraluminous infrared galaxies (ULIRGs) and the quasars (QUEST = Quasar / ULIRG Evolutionary Study). Signs of a recent galactic interaction are seen in all of the objects in the HST sample, including all 7 IR-excess Palomar-Green (PG) QSOs in the sample. Unsuspected double nuclei are detected in 5 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 scale in at least 4 of these sources. The hosts of ’warm ’ (IRAS 25-to-60 µm flux ratio, f25/f60&amp;gt; 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 HII region-like nuclear optical spectral types. The host sizes and luminosities of the