Lots of galaxies are spiral-shaped like our Milky Way. Some we see face-on and some edge-on, depending on their orientation towards the Earth. Those which look edge-on are generally pretty flat, sometimes with a pronounced bulge in the disk’s centre.
ESO 510-G13. Click for Hubblesight release.
But not this galaxy, the charmingly-named ESO 510-G13. Its disk, which we can see perfectly edge-on, has a definite warp to it as though someone had grabbed the edges and given it a twist. In fact this is evidence of a recent collision with another galaxy, whose gravitational effect has shifted millions of stars huge gas clouds off their orbits. The disk is being rippled through by titanically slow waves of density, like a pool’s surface after a bucket of water’s dumped into it. Although the shape of the starry and gassy components of the disk is hard to make out against the bright glow of the central bulge, the warp is easy to spot because of the dark dust lanes running through the disk. This dust, made up of tiny particles a fraction of a millimeter in diameter, absorbs and scatters the starlight from behind it.
You may also be able to pick out tiny blue smudges in the disk to the left and right edges. As I’ve said before, blue starlight indicates very young, hot stars - stars which, like last week’s Pistol Star, are destined to burn themselves out and explode shortly after forming. Their presence in the disk is probably due to a burst of star-formation a few million years ago caused by the gravitational effect of the collision, which slammed giant gas clouds together and collapsed some of them into new stars.
So what was the fate of the second galaxy? Almost certainly the same gravitational effects have torn it up, and ESO 510-G13 is in the process of consuming its fragments. Within a few million years more - no time at all in a galaxy’s lifetime - the merger will be complete, the wavy disk will have settled down into a more stable orbit again, and our distant descendants will see ESO 510-G13 as just another spiral galaxy.
Still, the effects of the collision and its attendant burst of new stars will probably have a profound effect on the structure of the galaxy. Astronomers believe that these collisions are fairly common in galaxies’ histories, and have a large role to play in determining their shape and properties. It’s an area which lends itself particularly well to computer simulation, as I wrote recently.
For a reasonable introduction to the role of mergers, you could start about half-way down Wikipedia’s article on galactic evolution.
