See that cool pattern around this edge-on spiral galaxy, NGC 5907? The one that sort of looks like two halos? Those are the only remains of another galaxy, which NGC 5907 slaughtered in cold blood. (Or maybe cold dark matter?) We've shown you galaxies colliding before, but here's an arresting look at the aftermath of one of those galactic hit-and-runs, courtesy of the Instituto de Astrofisica de Canarias. These "stellar fossils," or "ghost galaxies," result from the spiral galaxy's collision with a dwarf galaxy, which it mostly absorbed. Click through to see a bigger image, plus a look at NGC 4013's "ghost trail."
The cover story of the June issue of Astronomy magazine is a cool tale by a couple of astronomers who did a major, mondo supercomputer simulation of the impending collision of Andromeda and the Milky Way. They named the new, larger merged galaxy "Milkomeda." Nerd Nirvana!