UGC8802: A Massive Disk Galaxy in Formation
  We report new observations of the galaxy UGC8802 obtained through GASS, the
GALEX Arecibo SDSS Survey, which show this galaxy to be in a remarkable
evolutionary state. UGC8802 (GASS35981) is a disk galaxy with stellar mass
M*=2x10^10 Msolar which appears to contain an additional 2.1x10^10 Msolar of HI
gas. New millimeter observations with the IRAM 30m telescope indicate a
molecular gas mass only a tenth this large. Using deep long-slit spectroscopy,
we examine the spatially resolved star formation rate and metallicity profiles
of GASS35981 for clues to its history. We find that the star formation surface
density in this galaxy is low (Sigma_SFR=0.003 Msolar/yr/kpc^2) and that the
star formation is spread remarkably evenly across the galaxy. The low molecular
gas masses measured in our three IRAM pointings are largely consistent with the
total star formation measured within the same apertures. Our MMT long-slit
spectrum reveals a sharp drop in metallicity in the outer disk of GASS35981.
The ratio of current star formation rate to existing stellar mass surface
density in the outer disk is extremely high, implying that all the stars must
have formed within the past ~1Gyr. At current star formation rates, however,
GASS35981 will not consume its HI reservoir for another 5-7 Gyr. Despite its
exceptionally large gas fraction for a galaxy this massive, GASS35981 has a
regular rotation curve and exhibits no sign of a recent interaction or merger.
We speculate that GASS35981 may have acquired its gas directly from the
inter-galactic medium, and that it and other similar galaxies identified in the
GASS survey may provide rare local glimpses of gas accretion processes that
were more common during the prime epoch of disk galaxy formation at z~1.
