ULTRALUMINOUS IRAS GALAXIES - FORMATION OF ELLIPTIC GALAXIES BY MERGER-INDUCED DISSIPATIVE COLLAPSE

We summarize evidence that ultraluminous IRAS galaxies are elliptical galaxies forming by merger-induced dissipative collapse. Tidal tails and shells show that they are recent merger remnants. Near the center, the mass in gas is comparable to the mass in stars within the same radius; since no progenitors are this gas-rich, gas must have collapsed toward the center. Typical central gas densities approximately 1O(2) M. pc-3 are as high as stellar mass densities in ellipticals; this solves the well-known problem of producing the observed high densities. Finally, an intense burst of star formation is in progress. These are precisely the properties expected in the dissipative collapse picture of galaxy formation. We consider whether IRAS starbursters could be local analogs of the formation of giant elliptical galaxies at redshifts z > 1. The idea is plausible because giant ellipticals are metal-rich: even the oldest must have formed from gas as enriched as that in IRAS merger progenitors. Therefore dust shrouding in protogalaxies may not be as negligible as is commonly assumed. Extrapolation of the spectral energy distribution of Arp 220 into the ultraviolet suggests that dust-shrouded protoellipticals should easily be detectable in K-band surveys and should become detectable in deeper V-band surveys. They could be recognized by their extremely red colors, V - K congruent-to 6 to 10.