Star Formation Across the Taffy Bridge: UGC 12914/15

Abstract

We present Berkeley-Illinois-Maryland Association (BIMA) two-field mosaic CO (1–0) images of the Taffy galaxies (UGC 12914/15), which show the distinct taffy-like radio continuum emission bridging the two spiral disks. Large amounts of molecular gas (1.4 × 1010 M⊙, using the standard Galactic CO-to-H2 conversion applicable to Galactic disk giant molecular clouds) were clearly detected throughout the taffy bridge between the two galaxies, which, as in the more extreme case of H I, presumably results from a head-on collision between the two galaxies. The highest CO concentration between the two galaxies corresponds to the Hα source in the taffy bridge near the intruder galaxy UGC 12915. This H II region is also associated with the strongest source of radio continuum in the bridge and shows both morphological and kinematic connections to UGC 12915. The overall CO distribution of the entire system agrees well with that of the radio continuum emission, particularly in the taffy bridge. This argues for the star formation origin of a significant portion of the radio continuum emission. Compared with the H I morphology and kinematics, which are strongly distorted owing to the high-speed collision, CO better defines the orbital geometry and impact parameter of the interaction, as well as the disk properties (e.g., rotation, orientation) of the progenitor galaxies. Based on the 20 cm–to–CO ratio maps, we conclude that the starburst sites are primarily located in UGC 12915 and the Hα source in the bridge and show that the molecular gas in the taffy bridge is forming into stars with star formation efficiency comparable to that of the target galaxy UGC 12914 and similar to that in the Galactic disk.