UGC8802: A massive disk galaxy in formation

We report new observations of the galaxy UGC8802 obtained through the GALEX Arecibo SDSS Survey (GASS), which show this galaxy to be in a remarkable evolutionary state. UGC8802 (GASS35981) is a disk galaxy with stellar mass M* = 2 × 1010M⊙, which appears to contain an additional 2.1 × 1010M⊙ of Hi gas. New millimeter observations with the IRAM 30 m telescope indicate a molecular gas mass only a tenth this large. Using deep long-slit spectroscopy, we examine the spatially resolved star formation rate (SFR) and metallicity profiles of GASS35981 for clues to its history. We find that the star formation surface density in this galaxy is low (Σsfr = 0.003 M ⊙ yr-1 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 SFR to existing stellar mass surface density in the outer disk is extremely high, implying that all the stars must have formed within the past ∼1 Gyr. At current SFRs, 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 intergalactic medium, and that GASS35981 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. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.