On the fate of gas in ultraluminous galaxies at low and high redshift

It is often suggested that the distant galaxies recently identified in 850 micron surveys with the SCUBA bolometer array on the JCMT Telescope are high-redshift analogues to local ultraluminous infrared galaxies, based on their similar spectral energy distributions and luminosities. We show that these two populations of objects must differ in at least one fundamental way from each other. This assertion is based on a consideration of the possible fates of gas in the high redshift SCUBA galaxies, given the requirement that they most evolve into some subset of the low-redshift galaxy population with a comoving density of about 10^{-4} Mpc^{-3}. One possibility is that the SCUBA galaxies have similar gas density profiles to local ultraluminous galaxies. If this is the case, then they must derive almost all their power from AGNs, which appears not to be the case for local ultraluminous galaxies, which are predominantly star-formation powered. Another possibility is that the SCUBA galaxies have more extended gas density profiles than local ultraluminous galaxies. In this case they must be almost all star-formation powered, and much of the star formation in the Universe can happen in these objects. Either way there is a significant difference between the low- and high- redshift populations.