I'm probably missing something obvious, but in what way is the actual shock wave generated? How is something as large as a galactic cluster coherent enough in a

I'm probably missing something obvious, but in what way is the actual shock wave generated? How is something as large as a galactic cluster coherent enough in any way other than gravitational attraction enough to generate a cluster wide shock wave? Is it just the gas clouds colliding at extreme speeds as the clusters come in at each other at 'relatively' high speeds?