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Leaving on a Jet's Plane

Week of May 17, 1999

Last week's Snack talked about how black holes can affect the environment around them in such a way as to be very bright. I didn't give details, though. Let's take a minute to see what it looks like near a supermassive black hole.

Mind you, it need not look like anything. It's possible to have a tremendously massive black hole, one with a million or more times the Sun's mass, and have not much stuff around it. It's a common misconception that black holes always have matter swirling into them. That really only happens under certain circumstances. For example, take a spiral galaxy like our own. It has stars and gas and dust orbiting the center, some close in, some far out. These orbits are relatively stable and can continue as they are more or less for millions or billions of years. But not everything stays the same. Sometimes gas clouds collide, or sometimes a nearby small galaxy can gravitationally disrupt its bigger parent. When that happens, huge amounts of matter can be sent plummeting down towards the center of the galaxy-- in this case, towards the giant black hole.

Since spiral galaxies are pretty flat, that gas and dust tends to come in from a plane: the plane of the galaxy. Moreover, it tends to try to keep its circular motion as it comes in, so it spirals in. As this matter gets closer and closer to the black hole it piles up and a large, fairly flat disk forms. This is called the accretion disk, because matter from this disk accretes (that is, falls in) on the black hole. Because of friction (once again, check last week's Snack), the center of the accretion disk can get hot, millions of degrees hot. This energy gets pumped out into the outer parts of the disk and can puff them up, so that the outer parts of the disk aren't flat anymore.

More interesting, though, is what happens towards the center, near the black hole. Heat is pouring into the matter there, and heat makes a gas expand (that's how hot air balloons work). The matter in the disk tries to expand, but cannot move out into the plane of the disk because there is already matter there. So it goes the only way it can: up and down. There is so much energy pumped into the gas that it really gets a big kick; velocities have been measured in fractions of the speed of light, thousands of kilometers per second. So now we have a second seeming paradox: not only do black holes create tremendously bright displays, but their huge gravity actually can cause matter to escape at huge velocities.

image of galaxy M87 These jets of matter can be very large, thousands or even tens of thousands of light years long. Amazingly, and for reasons not completely understood, the jets can also be very narrow, as if they are beamed or focused. An excellent example of this is the nearby active galaxy M87, in the Virgo cluster. M87 is fairly bright as galaxies go; it can be seen with a pair of binoculars. The image on the left shows M87 as seen in a relatively short exposure, where it looks like a typical elliptical galaxy.

image of galaxy M87's jet
What takes a telescope (and longer exposures) to see, however, is that coming from the center of M87 is a very tightly collimated jet. The picture on the right shows a Hubble image of the jet. This jet is being sprayed out from M87's central black hole, which is estimated to be 2-3 billion solar masses. For a long time the jet is narrow and tight, but as it slams into the gas outside the center of the galaxy, it slows down and gets clumpy. It is thought that very complex magnetic fields keep the jet focused as well as make it glow. As it happens, I am involved with a project that is taking ultraviolet spectra of the jet, and we are finding it to be a weird place. I don't have anything concrete to say here yet, but in the coming weeks our observations should yield some interesting facts about the jet and the galaxy hosting it. I'll write more about it as we find out more!

There are numerous sites on the web about black holes and M87. The best place to start for images and info about M87 is The SEDS Messier object page. Info about black holes can be found at the Relativity FAQ (which also has lots of links).

©2008 Phil Plait. All Rights Reserved.

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