Shocks and Star Formation in Stephan’s Quintet. I. Gemini Spectroscopy of Hα-bright knots.

We present a Gemini-GMOS spectroscopic study of HST-selected Hα-emitting regions in Stephan’s Quintet (HCG 92), a nearby compact galaxy group, with the aim of disentangling the processes of shock-induced heating and star formation in its intra-group medium. The ≈40 sources are distributed across the system, but most densely concentrated in the ∼kpc-long shock region. Their spectra neatly divide them into narrow- and and broad-line emitters, and we de-compose the latter into three or more emission peaks corresponding to spatial elements discernible in HST imaging. The emission line ratios of the two populations of Hα-emitters confirm their nature as H ii regions (90 % of the sample) or molecular gas heated by a shock-front propagating at.300 km s−1. Their redshift distribution reveals interesting three-dimensional structure with respect to gas-phase baryons, with no H ii regions associated with shocked gas, no shocked regions in the intruder galaxy NGC 7318B, and a sharp boundary between shocks and star formation. We conclude that star formation is inhibited substantially, if not entirely, in the shock region. Attributing those H ii regions projected against the shock to the intruder, we find a lopsided dis-tribution of star formation in this galaxy, reminiscent of pile-up regions in models of interactin