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We present [C II] and [O I] observations from Herschel and CO(1-0) maps fromthe Combined Array for{\dag} Research in Millimeter Astronomy (CARMA) of theHickson Compact Group HCG 57, focusing on the galaxies HCG 57a and HCG 57d. HCG57a has been previously shown to contain enhanced quantities of warm molecularhydrogen consistent with shock and/or turbulent heating. Our observations showthat HCG 57d has strong [C II] emission compared to L$_{\rm FIR}$ and weakCO(1-0), while in HCG 57a, both the [C II] and CO(1-0) are strong. HCG 57a liesat the upper end of the normal distribution of [C II]/CO and [C II]/FIR ratios,and its far-IR cooling supports a low density warm diffuse gas that falls closeto the boundary of acceptable PDR models. However, the power radiated in the [CII] and warm H$_2$ emission have similar magnitudes, as seen in othershock-dominated systems and predicted by recent models. We suggest thatshock-heating of the [C II] is a viable alternative to photoelectric heating inviolently disturbed diffuse gas. The existence of shocks is also consistentwith peculiar CO kinematics in the galaxy, indicating highly non-circularmotions are present. These kinematically disturbed CO regions also showevidence of suppressed star formation, falling a factor of 10-30 below normalgalaxies on the Kennicutt-Schmidt relation. We suggest that the peculiarproperties of both galaxies are consistent with a highly dissipative off-centercollisional encounter between HCG 57d and 57a, creating ring-like morphologiesin both systems. Highly dissipative gas-on-gas collisions may be more common indense groups because of the likelihood of repeated multiple encounters. Thepossibility of shock-induced SF suppression may explain why a subset of theseHCG galaxies have been found previously to fall in the mid-infrared greenvalley.

Strong Far-IR Cooling Lines, Peculiar CO Kinematics and Possible Star Formation Suppression in Hickson Compact Group 57