SUBARCSECOND NEAR-INFRARED IMAGING OF ULTRALUMINOUS IRAS GALAXIES

Near-infrared (NIR) K-band images with subarcsecond resolution are presented for the ultraluminous far-infrared (FIR) IRAS galaxies Mrk 273, Arp 220, and IRAS 15250+3609. Both Mrk 273 and Arp 220 are found to contain double nuclei with arcsecond-scale separations that yield merger lifetimes of only 15-28 Myr when simple dynamical time scales are adopted. The recognition of a double nucleus in Mrk 273 brings the number of ultraluminous galaxies with close double nuclei to six of the 10 galaxies in the original Sanders et al. sample. The apparently short dynamical lifetimes for the extreme cases of Arp 220 and Mrk 273 may mean that simple dynamical friction timescales are incorrect. More likely, they imply that the FIR luminosity during a merger event rises significantly during the final stages of the merger, so that systems in this brief but spectacular phase are disproportionately represented in FIR-selected samples. The peak intensities in the K-band and radio continuum images do not coincide in the cases of both Arp 220 and Mrk 273. This complicates the identification of the ''true'' nuclei in these systems. The spectra and energy of ultraluminous galaxies with close double nuclei systematically differ in a number of ways from the galaxies with single nuclei. The double-nucleus galaxies tend to have energetically insignificant NIR luminosities, NIR colors consistent with reddened starlight, relatively narrow optical emission lines whose flux ratios are typical of LINERs or H II regions, and relatively simple ''blackbody-like'' FIR spectral energy distributions (consistent with the dominance of the emission from warm dust with 60 < T < 80 K). In contrast, the single-nucleus galaxies tend to have energetically significant NIR emission with colors that suggest a contribution from a nonthermal source, broader emission lines of the Seyfert (I or 2) variety, and a FIR spectral energy distribution characterized by a pronounced 25 mum excess. These systematic differences strongly suggest that a powerful AGN is making a readily identifiable contribution to the optical and infrared emission in the single-nucleus galaxies, but not in the double-nucleus galaxies. Two of the four single-nucleus FIR galaxies are optical ring galaxies, with plausible identifications for the interloping dwarf galaxy which collided with the main, disk galaxy. One of these, IRAS 15250+3609 has optical-infrared properties more like the ultraluminous galaxies with double nuclei than those with single nuclei, while the spectral properties of the more complicated ring system UGC 05101 are more like the single-nucleus galaxies Mrk 231 and IRAS 05189-2524, which are likely recently completed mergers. These three galaxies also exhibit the clearest evidence at optical, infrared, and radio wavelengths for the presence of energetic dust-enshrouded AGNs and may represent the evolutionary descendents of the double-nucleus systems. In this case, the conspicuous presence of an AGN in these galaxies may imply that an AGN is either first activated or deshrouded only during the postmerger phase. Although this evolutionary picture is similar to that proposed by Sanders et al., we find no conclusive evidence for, nor compelling reason to include, a buried quasar at the heart of any galaxy in the ultraluminous sample. However, if buried quasars are there, they must be very deeply buried in order that they not contribute to the K-band flux, nor produce a significant amount of hard X-ray flux.