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

March 2018 • 2018ApJ...855..141P

Authors • Peterson, B. W. • Appleton, P. N. • Bitsakis, T. • Guillard, P. • Alatalo, K. • Boulanger, F. • Cluver, M. • Duc, P. -A. • Falgarone, E. • Gallagher, S. • Gao, Y. • Helou, G. • Jarrett, T. H. • Joshi, B. • Lisenfeld, U. • Lu, N. • Ogle, P. • Pineau des Forêts, G. • van der Werf, P. • Xu, C. K.

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] {}3{{{P}}}2\to {}3{{{P}}}1(809.3 {GHz}) and [C I] {}3{{{P}}}1\to 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 H2 and high column-density H I, resulting from the galaxy collision, is responsible for the enhanced [C II] emission.


IPAC Authors


Phil Appleton

Senior Scientist

George Helou

IPAC Executive Director