Star Formation Across the Taffy Bridge: UGC 12914/15

We present BIMA two-field mosaic CO(1-0) images of the Taffy galaxies (UGC12914/15), which show the distinct taffy-like radio continuum emission bridgingthe two spiral disks. Large amounts of molecular gas (1.4 x 10^{10} Msun, usingthe standard Galactic CO-to-H$_2$ conversion applicable to Galactic disk giantmolecular clouds [GMCs]) were clearly detected throughout the taffy bridgebetween the two galaxies, which, as in the more extreme case of HI, presumablyresults from a head-on collision between the two galaxies. The highest COconcentration between the two galaxies corresponds to the H_alpha source in thetaffy bridge near the intruder galaxy UGC 12915. This HII region is alsoassociated with the strongest source of radio continuum in the bridge, andshows both morphological and kinematic connections to UGC 12915. The overall COdistribution of the entire system agrees well with that of the radio continuumemission, particularly in the taffy bridge. This argues for the star formationorigin of a significant portion of the radio continuum emission. Compared tothe HI morphology and kinematics, which are strongly distorted owing to thehigh-speed collision, CO better defines the orbital geometry and impactparameter of the interaction, as well as the disk properties (e.g., rotation,orientation) of the progenitor galaxies. Based on the 20cm-to-CO ratio maps, weconclude that the starburst sites are primarily located in UGC 12915 and theH_alpha source in the bridge and show that the molecular gas in the taffybridge is forming into stars with star formation efficiency comparable to thatof the target galaxy UGC 12914 and similar to that in the Galactic disk.