2022ApJ...925...63G

We present Hubble Space Telescope Cosmic Origin Spectrograph (COS) UV line spectroscopy and integral-field unit (IFU) observations of the intragroup medium in Stephan's Quintet (SQ). SQ hosts a 30 kpc long shocked ridge triggered by a galaxy collision at a relative velocity of 1000 km s^–1, where large amounts of molecular gas coexist with a hot, X-ray-emitting, plasma. COS spectroscopy at five positions sampling the diverse environments of the SQ intragroup medium reveals very broad (≃2000 km s^–1) Lyα line emission with complex line shapes. The Lyα line profiles are similar to or much broader than those of Hβ, [C II]157.7 µm, and CO (1-0) emission. The extreme breadth of the Lyα emission, compared with Hβ, implies resonance scattering within the observed structure. Scattering indicates that the neutral gas of the intragroup medium is clumpy, with a significant surface covering factor. We observe significant variations in the Lyα/Hβ flux ratio between positions and velocity components. From the mean line ratio averaged over positions and velocities, we estimate the effective escape fraction of Lyα photons to be ≃10%-30%. Remarkably, over more than four orders of magnitude in temperature, the powers radiated by X-rays, Lyα, H₂, and [C II] are comparable within a factor of a few, assuming that the ratio of the Lyα to H₂ fluxes over the whole shocked intragroup medium stay in line with those observed at those five positions. Both shocks and mixing layers could contribute to the energy dissipation associated with a turbulent energy cascade. Our results may be relevant for the cooling of gas at high redshifts, where the metal content is lower than in this local system, and a high amplitude of turbulence is more common.