2M MPH galaxy smash-up seen in unprecedented detail

2M mph = 900 kps = three one-thousandths of light speed

Source paper: WEAVE First Light Observations: Origin and Dynamics of the Shock Front in Stephan’s Quintet https://academic.oup.com/mnras/article/535/3/2269/7904663

> As the shock moves through pockets of cold gas, it travels at hypersonic speeds—several times the speed of sound in the intergalactic medium of Stephan's Quintet

What's the speed of sound in space?

2M mph ~= 0.5 AU/day

Shouldn't the "sonic boom" here provide good data as to the existence of dark matter (akin to the Bullet cluster)? Anyone on hn with good background care to comment? Don't see anything in the article about it, but would think is one of the most significant experimental goals from detecting these sorts of collisions.

I wonder how well the local intelligence was ready for that event.

"Collision" is a bit of a misnomer when we talk about galaxies. As large as stars are, the matter in galaxies is still so sparse because stars are so far apart. I saw some maths about the coming collision between the Milky Way and Andromeda and the number of projected collisions is like less than 10 (IIRC).

Now I'm sure it would be highly disruptive and there'd be localized catastrophe but it's important to remember how large the distances in space really are.

This is unambiguous proof that speed limit[0]s don't work.

[0] https://www.amnh.org/exhibitions/einstein/light/cosmic-speed...