Detection of Powerful Mid-IR H2 Emission in the Bridge between the Taffy Galaxies

TL;DR

This study reports the detection of strong warm H2 emission in the bridge connecting the Taffy galaxies, revealing shock-heated molecular gas with significant mass and temperature, likely driven by collision-induced turbulence.

Contribution

First detection of prominent warm H2 emission in the Taffy galaxies' bridge, demonstrating shock heating as the primary excitation mechanism in a galaxy collision context.

Findings

Warm H2 mass in the bridge is approximately 4.2 x 10^8 solar masses.

H2 emission in the bridge is more intense than in Stephan's Quintet shock.

Shock heating from the galaxy collision sustains the warm H2 excitation.

Abstract

We report the detection of strong, resolved emission from warm H2 in the Taffy galaxies and bridge. Relative to the continuum and faint PAH emission, the H2 emission is the strongest in the connecting bridge, approaching L(H2)/L(PAH8{\mu}m) = 0.1 between the two galaxies, where the purely rotational lines of H2 dominate the mid-infrared spectrum in a way very reminiscent of the group-wide shock in the interacting group Stephan's Quintet. The surface brightness in the 0-0 S(0) and S(1) H2 lines in the bridge is more than twice that observed at the center of the Stephan's Quintet shock. We observe a warm H2 mass of 4.2 \times 108 M\odot in the bridge, but taking into account the unobserved bridge area, the total warm mass is likely to be twice this value. We use excitation diagrams to characterize the warm molecular gas, finding an average surface mass of 5 \times 106 M\odot kpc-2 and typical excitation temperatures of 150-175 K. H2 emission is also seen in the galaxy disks, although there the emission is more consistent with normal star forming galaxies. We investigate several possible heating mechanisms for the bridge gas, but favor the conversion of kinetic energy from the head-on collision via turbulence and shocks as the main heating source. Since the cooling time for the warm H2 is short (5000 yr), shocks must be permeating the molecular gas in bridge region in order to continue heating the H2.