In Stephan's Quintet, the shock wave is due to the intruder galaxy traveling at speeds greater than Mach 100 as it plows into intergalactic gas within the cluster. |
It's a little bit analogous to dropping a stone into a pond, and watching a ripple and then sometimes a second ripple go out from the center. |
We know that these galaxies are involved in vast mergers and collisions. It's possible that some of the emission we see from them is created not by stars, but by vast shocks in the gas between colliding galaxies. |