WEAVE First Light Observations: Origin and Dynamics of the Shock Front in Stephan's Quintet

We present a detailed study of the large-scale shock front in Stephan's Quintet, a by-product of past and ongoing interactions. Using integral-field spectroscopy from the new William Herschel Telescope Enhanced Area Velocity Explorer (WEAVE), recent 144 MHz observations from the LOFAR Two-metre Sky Survey, and archival data from the Very Large Array and JWST, we obtain new measurements of key shock properties and determine its impact on the system. Harnessing the WEAVE large integral field unit's field of view (90 <inline-formula><tex-math id="TM0001" notation="LaTeX">$\times$</tex-math></inline-formula> 78 arcsec<inline-formula><tex-math id="TM0002" notation="LaTeX">$^{2}$</tex-math></inline-formula>), spectral resolution (<inline-formula><tex-math id="TM0003" notation="LaTeX">$R\sim 2500$</tex-math></inline-formula>), and continuous wavelength coverage across the optical band, we perform robust emission-line modelling and dynamically locate the shock within the multiphase intergalactic medium with higher precision than previously possible. The shocking of the cold gas phase is hypersonic, and comparisons with shock models show that it can readily account for the observed emission-line ratios. In contrast, we demonstrate that the shock is relatively weak in the hot plasma visible in X-rays (with Mach number of <inline-formula><tex-math id="TM0004" notation="LaTeX">$\mathcal {M}\sim 2\!-\!4$</tex-math></inline-formula>), making it inefficient at producing the relativistic particles needed to explain the observed synchrotron emission. Instead, we propose that it has led to an adiabatic compression of the medium, which has increased the radio luminosity 10-fold. Comparison of the Balmer line-derived extinction map with the molecular gas and hot dust observed with JWST suggests that pre-existing dust may have survived the collision, allowing the condensation of H<inline-formula><tex-math id="TM0005" notation="LaTeX">$_2$</tex-math></inline-formula> - a key channel for dissipating the shock energy.