  We present Hubble Space Telescope Cosmic Origin Spectrograph (COS) UV line
spectroscopy and integral-field unit observations of the intergalactic medium
(IGM) in the Stephan's Quintet (SQ) galaxy group. SQ hosts a 30 kpc long
shocked ridge triggered by a galaxy collision at a relative velocity of 1000
km/s, where large amounts of cold (10-100 K) and warm (100-5000 K) molecular
gas coexist with a hot plasma. COS spectroscopy along five lines-of-sight,
probing 1 kpc-diameter regions in the IGM, reveals very broad (~2000 km/s) and
powerful Ly$\alpha$ line emission with complex line shapes. These Lyman-alpha
line profiles are often similar to, or sometimes much broader than line
profiles obtained in H$\beta$, [CII], and CO (1-0) emission along the same
lines-of-sight. In these cases, the breadth of the Ly$\alpha$ emission,
compared with H$\beta$, implies resonance scattering. Line ratios of
Ly$\alpha$/H$\beta$ for the two COS pointings closest to the center of the
shocked ridge are close to the Case B recombination value, suggesting that at
these positions Ly$\alpha$ photons escape through scattering in a low density
medium free of dust. Some Ly$\alpha$ spectra show suppressed velocity
components compared with [CII] and H$\beta$, implying that some of the
Ly$\alpha$ photons are absorbed. Scattering indicates that the neutral gas of
the IGM is clumpy, with multiple clumps along a given line of sight.
Remarkably, over more than four orders of magnitude in temperature, the powers
radiated by the multi-phase IGM in X-rays, Ly$\alpha$, H$_2$, [CII] are
comparable within a factor of a few. We suggest that both shocks and mixing
layers co-exist and contribute to the energy dissipation associated with a
turbulent energy cascade. This may be important for the cooling of gas at
higher redshifts, where the metal content is lower than in this local system,
and a high amplitude of turbulence more common.
