VISUAL AND NEAR-INFRARED IMAGING OF ULTRALUMINOUS INFRARED GALAXIES -THE IRAS 2-JY SAMPLE

New near-infrared and visual images at 2.2 mu m and 6550 Angstrom are presented for 46 galaxies having infrared luminosities of L(IR)>8.5x10 (11) L(.), 60 mu m flux densities greater than 1.94 Jy, and declinatio ns greater than -35 degrees. These galaxies make up a significant frac tion of a complete, northern hemisphere sample of ultraluminous infrar ed galaxies. Visual and/or near-infrared imaging data now exist for 56 ultraluminous infrared galaxies out to nearly 50 000 km s(-1). Of the se 56 galaxies, 53 (95%) show evidence for current or past interaction s. Among these systems, there are a large variety of visual morphologi es, including strongly interacting pairs with apparent tidal tails, as well as single, distorted galaxies with close double nuclei. There ar e three galaxies which, to the limits of the imaging data, do not appe ar to have suffered a recent interaction or merger. Approximately 47% (25/53) of the interacting systems have double nuclei, with projected nuclear separations ranging from 0.3 to 48 kpc. Seven systems have nuc lear separations larger than 10 kpc. If the 53 interacting galaxies ar e viewed as stages in the evolution of pairs of interacting spiral gal axies to a single, luminous AGN or starburst, the present imaging data can be used to estimate the lifetime of the bright infrared phase. In cluding only those sample galaxies with morphological evidence for int eractions, we calculate a lower and an upper limit to the lifetime of the ultraluminous infrared phase of the sample as a whole to be 2x10(8 ) and 2x10(9) yr, respectively. Comparison of these dynamical estimate s to models of the mergers of gas-rich galaxies and the rates at which fuel is exhausted by starbursts or AGN suggests the lifetime of the u ltraluminous phase lies much closer to the smaller of these two values . Selecting galaxies based upon luminous infrared activity clearly bia ses the sample towards merging galaxies with small physical separation s. However, the existence of pairs with large separations indicates th at the ultraluminous phase may in some cases start early during the me rger process. Alternatively, these systems may contain unresolved thir d nuclei responsible for triggering the ultraluminous activity. We bri efly compare our results to recent models of merging spiral galaxies. (C) 1996 American Astronomical Society.