Herschel Spectroscopy of the Taffy Galaxies (UGC 12914/12915 = VV 254): Enhanced [C II] Emission in the Collisionally Formed Bridge

Abstract

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 μm and [O i]63.2 μm fine-structure lines. Additionally, SPIRE FTS spectroscopy detects the [C i] ^3P_2 → ^3p_1(809.3 GHz) and [C i] ^3P_1 → ^3p_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.