Gas and Dust in the Taffy Galaxies: UGC 12914/15

We present a comprehensive study of the dust and gas properties in the after-head-on-collision UGC 12914/15 galaxy system using multitransition CO data and SCUBA submillimeter continuum images at both 450 and 850 μm. CO(3-2) line emission was detected in the disks of UGC 12914 and UGC 12915, as well as in a bridge connecting the two galaxies. Dust emission at 450 μm was detected for the first time in the two galactic disks and in the connecting bridge. Using a large velocity gradient excitation analysis model we have obtained good estimates of the physical parameters in different regions of this system, and the amount of molecular gas was found to be 3-4 times lower than that estimated by other investigators using the standard Galactic CO-to-H₂ conversion factor. Comparing with the dust mass derived from the SCUBA data, we found that the gas-to-dust ratio was comparable to the Galactic value in the two galaxy disks but a factor of ∼3 higher in the bridge. The physical condition of the molecular gas in the bridge is comparable to that in the diffuse clouds in our Galaxy. Our result is consistent with the scenario that the bridge molecular gas originated from the disk molecular clouds and has been drawn out of the galactic disks due to direct cloud-cloud collision. Our data indicate that the global star formation efficiency (SFE; L_IR/M) in UGC 12915 is comparable to that of normal spiral galaxies, and the SFE is 40% lower in UGC 12914 than in UGC 12915. Little star formation activity was found in the bridge except in an H II region adjacent to the disk of UGC 12915.