Ultra-luminous IR galaxies at low and high redshift

In luminous infrared galaxies, nuclear starbursts and active nuclei are fueled by extraordinarily large masses of gas and dust concentrated at radii of a few hundred pc by galactic merging and viscous accretion. The nearby ULIRGS are probably excellent analogs of galaxies seen at high redshift during the epoch of galaxy formation and growth. We summarize results from the NICMOS-GTO survey of 24 ultra-luminous IR galaxies together with mm-interferometry of the molecular gas in the prototype, Arp 220. Eight of the 24 galaxies imaged with NICMOS have significant nuclear point sources and eleven have double nuclei. Most of the eight point source nuclei exhibit emission line spectra indicative of a hard ionization source, suggesting an AGN. For the remainder of the sample, most of the near-infrared flux clearly originates outside the central 50-200 pc and is thus stellar (probably starburst) in origin. Nine of the 24 systems are fit better by an r(1/4) law (rather than an exponential disk), suggesting that the young starburst population can relax rapidly in violent mergers. In the IR galaxies much of the enhancement in the star formation probably occurs via the collision of massive clouds since, often, large numbers of bright clusters may be found in the overlap regions of the colliding galaxies.