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We present Hubble Space Telescope Cosmic Origin Spectrograph (COS) UV linespectroscopy and integral-field unit (IFU) observations of the intra-groupmedium in Stephan's Quintet (SQ). SQ hosts a 30 kpc long shocked ridgetriggered by a galaxy collision at a relative velocity of 1000 km/s, wherelarge amounts of molecular gas coexist with a hot, X-ray emitting, plasma. COSspectroscopy at five positions sampling the diverse environments of the SQintra-group medium reveals very broad (2000 km/s) Ly$\alpha$ line emission withcomplex line shapes. The Ly$\alpha$ line profiles are similar to or muchbroader than those of H$\beta$, [CII]$\lambda157.7\mu$m and CO~(1-0) emission.The extreme breadth of the Ly$\alpha$ emission, compared with H$\beta$, impliesresonance scattering within the observed structure. Scattering indicates thatthe neutral gas of the intra-group medium is clumpy, with a significant surfacecovering factor. We observe significant variations in the Ly$\alpha$/H$\beta$flux ratio between positions and velocity components. From the mean line ratioaveraged over positions and velocities, we estimate the effective escapefraction of Ly$\alpha$ photons to be 10-30%. Remarkably, over more than fourorders of magnitude in temperature, the powers radiated by X-rays, Ly$\alpha$,H$_2$, [CII] are comparable within a factor of a few, assuming that the ratioof the Ly$\alpha$ to H$_2$ fluxes over the whole shocked intra-group mediumstay in line with those observed at those five positions. Both shocks andmixing layers could contribute to the energy dissipation associated with aturbulent energy cascade. Our results may be relevant for the cooling of gas athigh redshifts, where the metal content is lower than in this local system, anda high amplitude of turbulence is more common.

astrophysical journal/the astrophysical journal

Extremely broad Lyman-alpha line emission from the molecular intra-group  medium in Stephan's Quintet: evidence for a turbulent cascade in a highly  clumpy multi-phase medium?