Galaxies get fat and weird along with their black holes

Contributed by
Jun 2, 2008

I mentioned earlier that all big galaxies have big black holes in their hearts. I also mentioned that the size of the black hole is related to various features of the galaxies, but that might be a little weird because, after all, galaxies don't live in a vacuum*. There are other galaxies out there, and they sometimes collide, forming bigger galaxies. So why should there still be a relationship between the black hole and the galaxy it sits in?

Well, the idea is that when two galaxies collide and merge, so do their black holes. Complicated physical processes tend to favor the two black holes getting ever closer, until they eat each other and become one somewhat fatter black hole. At the same time, the galaxy formed by the merger around the two black holes also grows.

That's the idea, at least. But there's a way to see if that's what really happens.

For example, when two galaxies collide and merge, their gas clouds collide. This can collapse them, forming stars. But since the event happens over a short period of time -- compared to the age of the galaxies, at least -- it's called a star burst, or a burst of star formation. Massive, luminous stars form this way and stay bright for a few hundred million years.

Also, at the same time, it's expected that a vast amount of gas will fall to the center of the new galaxy. As this junk falls into the black hole, it forms a flat disk which gets incredibly hot and bright. The inner part of the disk can outshine the entire rest of the galaxy combined. This kind of galaxy is called a quasar.

So if this idea is right, you might expect to see some galaxies that are quasars, and show signs of having had a star burst a few hundred million years in the past. Not only that, you'd expect them to look a little funny, distorted and goofy-shaped after having suffered such a cosmic collision.

Enter Mike Brotherton, astronomer, science fiction writer, blogger and, may I add, BABloggee. He and his team figured that they could use Hubble to look at a bunch of these galaxies and see what's what. They went through a sky survey that had 15,000 quasars (!) and culled them down to 600 that looked promising. From those they picked the best 29, and when they got the Hubble observations what they found was pretty convincing...

Those are three of the 29. Look at them! Weird, twisted, distorted; just as expected. In fact, all 29 were pretty weird (check 'em out yourself). They all bear the scars of recent mergers, and support very strongly the idea that "post-starburst quasars" are the results of violent collisions between galaxies, and, furthermore, black holes and their galaxies grow together.

Take a close look at those images. Right now, even as you read this (assuming you've read this far), the Andromeda Galaxy is barreling toward us at more than 100 kilometers per second. In a billion years, plus or minus, we'll plunge together in an event of epic violence. If there is any gas left in our two galaxies -- I haven't been able to confirm whether it would all be used up by then or not, but at least one paper I've read (and quoted in my book, Death from the Skies!, about this) said it'll all be gone -- then we too will become a starburst galaxy, and if any gas gets dumped into our merging black holes a vast amount of energy will pour out. It's potentially enough to cause problems, should that energy be aimed our way.

However, we do still have quite a bit of time to work out any potential problems. I'm not too concerned about it. But what a view it'll be...

*Well, OK, they don't live at all, and space is a vacuum. But I'm being all metaphorical here, so try to play nicely and follow along.

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