SHOCK-ENHANCED C+ EMISSION AND THE DETECTION OF H2O FROM THE STEPHAN'S QUINTET GROUP-WIDE SHOCK USING HERSCHEL

We present the first Herschel spectroscopic detections of the [O i] 63 μm and [C ii] 158 μm fine-structure transitions, and a single para-H2O line from the 35 × 15 kpc2 shocked intergalactic filament in Stephan's Quintet. The filament is believed to have been formed when a high-speed intruder to the group collided with a clumpy intergroup gas. Observations with the PACS spectrometer provide evidence for broad (>1000 km s−1) luminous [C ii] line profiles, as well as fainter [O i] 63 μm emission. SPIRE FTS observations reveal water emission from the p-H2O (111–000) transition at several positions in the filament, but no other molecular lines. The H2O line is narrow and may be associated with denser intermediate-velocity gas experiencing the strongest shock-heating. The [C ii]/PAHtot and [C ii]/FIR ratios are too large to be explained by normal photo-electric heating in photodissociation regions. H ii region excitation or X-ray/cosmic-ray heating can also be ruled out. The observations lead to the conclusion that a large fraction the molecular gas is diffuse and warm. We propose that the [C ii], [O i], and warm H2 line emission is powered by a turbulent cascade in which kinetic energy from the galaxy collision with the intergalactic medium is dissipated to small scales and low velocities, via shocks and turbulent eddies. Low-velocity magnetic shocks can help explain both the [C ii]/[O i] ratio, and the relatively high [C ii]/H2 ratios observed. The discovery that [C ii] emission can be enhanced, in large-scale turbulent regions in collisional environments, has implications for the interpretation of [C ii] emission in high-z galaxies.