Herschel Spectroscopy of the Taffy Galaxies (UGC 12914/12915 = VV 254):
  Enhanced [C II] emission in the collisionally-formed bridge
  Using the PACS and SPIRE spectrometers on-board Herschel, we obtained
observations of the Taffy galaxies (UGC 12914/12915) and bridge. The Taffy
system is believed to be the result of a face-on collision between two gas-rich
galaxies, in which the stellar disks passed through each other, but the gas was
dispersed into a massive H I and molecular bridge between them. Emission is
detected and mapped in both galaxies and the bridge in the [C II]157.7 $\mu$m
and [O I]63.2 $\mu$m fine-structure lines. Additionally, SPIRE FTS spectroscopy
detects the [C I] $^3$P$_2$$\rightarrow$$^3$P$_1$(809.3 GHz) and [C I]
$^3$P$_1$$\rightarrow$$3$P$_0$(492.2 GHz) neutral carbon lines, and weakly
detects high-J CO transitions in the bridge. These results indicate that the
bridge is composed of a warm multi-phase medium consistent with shock and
turbulent heating. Despite low star formation rates in the bridge, the [C II]
emission appears to be enhanced, reaching [C II]/FIR ratios of 3.3% in parts of
the bridge. Both the [C II] and [O I] lines show broad intrinsic
multi-component profiles, similar to those seen in previous CO 1-0 and H I
observations. The [C II] emission shares similar line profiles with both the
double-peaked H I profiles and shares a high-velocity component with
single-peaked CO profiles in the bridge, suggesting that the [C II] emission
originates in both the neutral and molecular phases. We show that it is
feasible that a combination of turbulently heated H$_2$ and high column-density
H I, resulting from the galaxy collision, is responsible for the enhanced [C
II] emission.
