Question: How to identify that two galaxy are currently collide to each other?

Jillian Scudder answered on 18 Jun 2015:

The easiest thing to do is just to look for galaxies that appear close on the sky! Once you find all of those galaxies, you can find the ones that are moving slowly relative to each other, since those are most likely to actually be colliding. If you want to be really sure that you’re looking at things that have definitely already gone past each other once, you can look at the images of the galaxies, and see if they look distorted!