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!
