Shock-Enhanced C+ Emission and the Detection of H2O from Stephan's
  Quintet's Group-Wide Shock using Herschel
  We present the first Herschel spectroscopic detections of the [OI]63 and
[CII]158 micron fine-structure transitions, and a single para-H2O line from the
35 x 15 kpc^2 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 clumpy intergroup gas. Observations with the PACS spectrometer
provide evidence for broad (> 1000 km s^-1) luminous [CII] line profiles, as
well as fainter [OI]63micron 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 [CII]/PAH{tot) and [CII]/FIR ratios are too large to be
explained by normal photo-electric heating in PDRs. HII 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 [CII], [OI] and warm H2 line emission is powered by a
turbulent cascade in which kinetic energy from the galaxy collision with the
IGM is dissipated to small scales and low-velocities, via shocks and turbulent
eddies. Low-velocity magnetic shocks can help explain both the [CII]/[OI]
ratio, and the relatively high [CII]/H2 ratios observed. The discovery that
[CII] emission can be enhanced, in large-scale turbulent regions in collisional
environments has implications for the interpretation of [CII] emission in
high-z galaxies.
