TheSpacewriter

What Happens in a Black Hole Stays in a Black Hole

But the Matter Surrounding It? That’s Another Issue…

Black holes, as the old bumper sticker said, suck. They also don’t have any hair, as Stephen Hawking once said. They gobble up stuff like stars and gas and dust, and they don’t give anything back. You can’t tell anything about them by simply looking at them, although you can infer their masses by the gravitational influence they have on material around them. And, you can tell that one is around by the heat and x-rays and other signals given off by the material that spirals into a black hole. And, if the supermassive black hole has a jet, you can detect THAT. But, all of the mass they take in stays there and the information about it stays secret forever. It’s a sort of cosmic version of “what happens in Vegas stays in Vegas.”

It turns out there are some other interesting things about black holes besides the fact that they suck. For one thing, for a while, astronomers thought that there was a correlation between size of a galaxy’s central bulge (if it has one) and the size/mass of its central supermassive black hole. The more massive the black hole, the larger the bulge of stars at the center of a galaxy would be. That makes sense, since supermassive black holes have to have a lot of matter to eat to keep them hefty and massive, and big galaxy bulges would have a lot of stars and gas and dust to feed them.

Well, this relationship seems to work for some galaxies, but not all of them. Some galaxies, like M33 in Triangulum, have massive black holes, but don’t have central bulges. So, maybe there’s something else influencing black hole growth. Something as mysterious as a black hole: like, dark matter.

Now, that’s not to say that there are dark-matter-munching black holes out there in skinny galaxies. The relationship is something far more complex and so far, astronomers are still figuring out what it is.

Dark matter exists, but you can’t see it. You CAN, however (if you have the right methods) measure its influence on regular matter. It has a gravitational influence. And, how much influence it has depends on how much of it there is. So, maybe there are galaxies out there with huge dark matter components; some of them with bulges and some without. And, maybe all that dark matter is having some influence on the growth of the black holes at the hearts of those galaxies, whether they have bulges or not. Maybe the dark matter is influencing the bulge. And, maybe the black hole’s growth rate and size is telling us something about the dark matter surrounding it. If so, it may be the only message we get about matter from a black hole!

Galaxy Munch

What Do You Feed Your Inner Supermassive Black Hole?

Remember that black hole at the center of our galaxy I talked about in the last entry? It feeds off of stars, gas and dust that happens to get too close. This happens at the center of any galaxy with a supermassive black hole. Where does all that black hole “food” material come from? And, if a supermassive black hole at the center of a galaxy has a supermassive appetite to match, won’t it clear out its neighborhood quickly? And if so, and it continues to grow, what’s feeding it?

In the case of some types of galaxies that are different from ours–such as those with extra-bright cores (called Seyfert galaxies), their central black holes may be getting the cosmic equivalent of a Big Mac with fries whenever two of these galaxies interact with each other. When such a close encounter takes place, the interaction stirs up gas, which brings more material within the reach of the central black hole of one of the galaxies.

Now, this isn’t something you can see easily with an optical telescope. But, if you scan the sky with a radio telescope and trace out the emissions from hydrogen gas in the interacting galaxies, you find something interesting: a transfer of material from one galaxy to the other’s supermassive black hole. That’s what a Cheng-Yo Kuo, a graduate student at University of Virginia did, using the Very Large Array (VLA) radio telescope in Socorro, New Mexico. He looked at several sets of nearby Seyfert galaxies that appear to be snacking on each other. As the galaxies interact, the gas and dust goes goes from one to the other, on a sort of death spiral into the hungry maw of the black hole. The material don’t go quietly; it’s heated (ionized) by friction and radiation, and slapped around by twisted magnetic fields. Those actions produce energy as the material is consumed. Depending on how rapidly the black hole is eating up its neighbor’s contribution to the cosmic picnic, the galaxy can show a wide range of activity. The most action is at the center, which is the brightest (and quite “loud” in radio wavelengths). Seyfert galaxies have the mildest version of this activity, while quasars and blazars are hundreds of times more powerful.

Astronomers can trace the flow of material between galaxies by looking for neutral hydrogen, which is pretty abundant in galaxies. It gives off emissions at a wavelength of 21 centimeters, which radio telescopes can detect. It also traces out the route that material is taking between the two galaxies into the heart of the black hole. That’s what this image from the NRAO press release illustrates, showing what a good tool neutral hydrogen can be when it comes to diagnosing the inevitable “heartburn” that follows when two galaxies interact and feed a supermassive black hole.

PSST!! Wanna See the Middle of the Milky Way?

Look Over Here…

It may come as a surprise to folks to learn that we on Earth don’t live in the middle of the Milky Way Galaxy. We actually live out in the suburbs, about 26,000 light-years away from all the action at the center of our stellar city. That’s actually a good thing, because from all accounts, the core of the Milky Way has a black hole or two, and a whole lot of starburst activity and other stuff going on, some of it not very healthy to be around. Those aren’t conditions conducive to a nice quiet life on a water-bearing world such as ours.

Nonetheless, like urban folk all over the world, sometimes we get an itch to see the “downtown” area with its bright lights and excitement. So, we try to look at the center of the galaxy, only to find that it’s hidden by dust clouds. In northern hemisphere summer, you can go out a couple of hours or so after sunset and look south toward the constellation Sagittarius (shown in the image above from Wikipedia). Just off the tip of the spout in the teapot shape of Sagittarius is where the center of the Milky Way is located. The bright clouds are stars that lie between us and the core of the galaxy, which is hidden behind dust clouds. For folks in the southern hemisphere, Sagittarius is going to be overhead or even north of overhead (depending on where you are). But, no matter where you live, if you can get outside and take a gander at Sagittarius, you’ll be looking toward the heart of our home galaxy.

Now, it turns out we can look through that dust if we use a telescope equipped with infrared detectors. Infrared light CAN get through the dust. The image at left is from the Spitzer Space Telescope, and it shows the core of the galaxy-the stuff we can’t see with our visible-light eyes. There are hundreds of millions of stars packed into that scene, along with dark dust clouds that even infrared light couldn’t pierce.

It’s kind of fascinating to go out and look up at that region of the sky, which seems rather placid in visible light. Yet, behind all those dust clouds are some fascinating events taking place. Think about it when you go out to check out the center of our galaxy when you’re stargazing over the next couple of months.