How do you weigh a supermassive black hole?

Contributed by
Jul 16, 2008
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Black holes are cool. They're also scary, and weird, and exciting. But if there's one word I'd use to describe them (besides, well, "black"), it's mysterious.

They twist our minds, and they push our math and science to the limit. After all, they're like punctures in the fabric of space and time, and that has some inherent effects on our common sense.

But maybe maddening would be a good word, too. Astronomers love black holes for the same reasons you do, but we also hate them, because they're so frackin' resistant to study. They're black. They don't give off any light, so that makes them pretty difficult to observe.

However, they do affect their environment, and that can be measured. And, as you'd expect, the more massive the black hole, the bigger its effect. In a very real sense, they throw their weight around.

Chandra X-ray image of NGC 4649

At the center of this image is a very big black hole. Don't bother squinting: on this scale, even as big as the hole is, it appears microscopic. But the picture itself tells the tale of a galaxy, a black hole, and the gas its slowly consuming.

In the center of every big galaxy, we think, lies a supermassive black hole. And I mean supermassive: they can be millions or even billions of times the mass of the Sun. We think that the very formation of the galaxy itself is affected by the black hole in its heart. Not because the black hole has a large fraction of the galaxy's mass: in fact, these black holes only have a fraction of a percent of the galaxy's mass (our own in the core of the Milky Way has about 4 million solar masses, compared to the 200 billion solar masses in the entire galaxy: a ratio of just 0.002%). We think that matter falling into the black hole creates huge winds that blow out, curtailing star formation in the galaxy at large.

We'd love to know more about this, and in the end it all ties into one thing: the black hole's mass. The thing is, that's hard to measure. One way is to observe how fast stars are orbiting the center of the galaxy very close in to the black hole (the more massive the black hole, the faster the stars move). On average -- that is, if you do this for a lot of galaxies -- this method ain't bad. But for specific galaxies it has uncomfortably large error bars. So what we need are more methods to weigh these black holes, and use them as a check on the other methods. That makes us more confident we know what we're doing.

So astronomers came up with a clever idea: galaxies have lots of gas floating around, and this gas will tend to fall to the center of the galaxy. As it does, two things happen: it piles up in the center, like water swirling in a drain, and it also heats up, getting to a pretty high temperature. A hot gas tries to expand, and that expansion is balanced by the gas falling in. The whole shebang reaches a kind of balance called hydrostatic equilibrium.

Slowly, the gas in the center cools, shrinks, and falls into the black hole. It cools by radiating away heat in the form of X-rays -- the higher the temperature the gas, the more energetic the X-rays it emits. By measuring these X-rays, astronomers take the temperature of the gas. And, it turns out, the bigger (more massive) the black hole, the hotter the gas gets as it falls to the galaxy's center.

So if you measure the temperature of the gas, you can infer the mass of the black hole using relatively simple gas physics. Cool!

Well, hot, really. The gas gets pretty agitated as it falls to the its inevitable death, heating up to millions of degrees. The means it emits X-rays, and that's what the Chandra Observatory sees. So astronomers used Chandra to observe NGC 4649, an elliptical galaxy about 50 million light years away (pretty close, as galaxies go). In the picture above, which is a combination of Hubble and Chandra images, the smooth purple stuff is the gas, and foreground stars and background galaxies are bluish-white. The gas is extremely smooth and featureless, which is good: that makes it easier to measure.

The astronomers took the temperature of the gas, and cranked through the equations. They found that the black hole in the center of NGC 4649 is a whopping 3.4 billion times the mass of the Sun. That's the biggest black hole I have heard of for which we have a good measure of its mass. Wow. Supermassive indeed.

That number agrees pretty well with estimates made be measuring the way stars orbit the galactic center, which is reassuring. What's also nifty is that this is totally independent of that method, which means that we can use it on bunches of different galaxies, and get good statistics on these monsters. That in turn means we have better numbers to wield when trying to figure out how black holes and galaxies form, and what happens as they grow old together.

And its just in time, too: with the launch of the gamma-ray observatory GLAST in June, we'll be finding thousands of previously unseen black holes in the centers of galaxies. This new method probably won't work for those galaxies -- we need quiet black holes for it to work, and the ones GLAST finds will be anything but -- but the more black holes we see, the better our understanding gets.

And given how scary, weird, and exciting -- I mean mysterious -- black holes are, the more we know, and the cooler they get.