The rings of Andromeda
The Andromeda galaxy, also known as M31, is the closest galaxy to Earth. It’s a spiral galaxy, like ours, but has an unusual property - a ring of stars surrounding the galaxy, and no-one was quite sure of its origin. There’s also strange warping of the the outer edges of the galaxy.
Now, in today’s edition of Nature magazine, new observations (subscription required, unfortunately!) have revealed a second, smaller ring of dust. According to the researchers’ analysis, the most likely explanation for all these strange properties is that a nearby galaxy M32 plunged through the centre of the Andromeda disk about 200 million years ago! Space.com has some cool pictures.
To back this up, they ran computer simulations on Andromeda - starting with basically a spinning clump of stars, they lett it run for a billion years (!) over which time the good old galaxy spiral arms formed. Then, they smashed a small galaxy (M32) into the centre of it. After letting everything settle for another couple of hundred million years, they got a galaxy which looks a lot like Andromeda - it had the two rings, and with the right brightness, and various other key points. The rings themselves are “density waves”, like ripples in a pond, caused by the gravity of the M32 galaxy. It’s not clear cut, but it’s pretty suggestive that at some time on our past, the two galaxies collided.
Beyond solving this particular mystery, while colliding galaxies may have been common in the past, it doesn’t happen so often today. To have a (comparatively) recent collision right next door to us would give us a great opportunity to understand more about galaxy formation and structure. Incidentally, it’s possible (though by no means certain) that Andromeda itself is on a collision course for the Milky Way - but don’t worry too much. Even if our paths do line up, it won’t be for about 3 billion years into the future! 