"This object is roughly three million times the mass of our sun inside a very tiny volume. A large mass in a small volume tells you that it has to be a supermassive black hole."
Andrea Ghez is an astronomer at UCLA and a member of UCLA's Galactic Center Group. A decade ago, her research helped prove the existence of a black hole at the center of our Milky Way galaxy. Earth & Sky's Jorge Salazar spoke to her in 2005 about her continuing investigations.
Salazar: Can you explain to our readers why astronomers have come to believe that a black hole exists in the center of the Milky Way?
Ghez: We've been doing research on the center of our galaxy to address this question, of whether or not there's a black hole there. And the best way to prove that it's there is to look for its pull of gravity.
The challenge with black holes is that you can't see them directly. The nature of these beasts is that they don't emit any light themselves. So to find a black hole, you look for stars near the hole that are moving extremely quickly because of the pull of the black hole's gravity.
An analogy would be to think about the pull of gravity of our sun. Our sun actually makes the planets orbit it. So, in fact we could tell that the sun was there, even if it weren't shining, by looking at the orbits of the planets. And, likewise, there's a gravitational attraction between a black hole and stars near it. The black hole tugs on the stars and forces them to orbit the black hole. So you can measure the orbits of stars, and that allows you to weigh the mass of the black hole.
The signature of a black hole is a lot of mass inside a very small volume. And the orbits of stars near the center of our Milky Way have told us just that: there's a huge amount of mass inside a very, very small volume in the galactic center.
So we have been looking for the motion of stars to tell us that there's something big and massive at the center of the galaxy. The challenge for doing this is then Earth's own atmosphere. Our atmosphere distorts the images of astronomical sources. For a long time, that's what prevented people from seeing evidence of a supermassive black hole in the center of the Milky Way.
In our research, we use techniques that allow us to correct for the atmosphere. We are using these techniques in conjunction with a telescope located in Hawaii, on Mauna Kea, and it's the largest telescope in the world. It allows us to see the finest detail possible. And with this telescope we've been able to watch the motion of stars that have revealed a very massive object at the center of the Milky Way.
This object is roughly three million times the mass of our sun inside a very tiny volume. A large mass in a small volume tells you that it has to be a supermassive black hole.
Salazar: Is there something special about our galaxy? Or does it seem that every galaxy has a black hole?
Ghez: The whole question of supermassive black holes started from observations of them in other galaxies. These black holes are basically feeding on a large amount of material in the centers of galaxies. As that material falls into the black hole, it's lit up. So, in fact, the idea that supermassive black holes existed in other galaxies came from these earlier observations of black holes whose surroundings were lit up.
Our galaxy is a completely ordinary, garden-variety galaxy. There's nothing special about it. In contrast, there are many other galaxies that have very energetic centers, and today we understand that these energetic centers are supermassive black holes that are "feeding" on material, or are being fed by material surrounding it. Now we've learned that our galaxy has a supermassive black hole. The implication is that almost every galaxy must contain a supermassive black hole, if our galaxy, which is so ordinary, contains one.
This is particularly important, because what we're learning now is that there's an intimate connection between the existence of the supermassive black hole and the properties of the galaxy that tells us that these things had to have been formed together. A couple of years ago we might have asked the question, which came first, the chicken or the egg, or rather, the black hole or the galaxy. And today we understand that they had to come together. It wasn't possible to form them separately.
What we're learning is that there's a correlation, or a link between the mass of the black hole and the mass of the center part of the galaxy. And that link, that connection, tells us that whatever formed the galaxy must have also formed the supermassive black hole at the center, at the same time. The observations of our own galaxy give us a laboratory for understanding the astrophysics of supermassive black holes at the center of other galaxies. Because it's so much closer than any other galaxy, we get to learn in detail how it's being fed, how it's growing, and how it affects its environment.
A number of unusual observations that we've made in the past few years suggest that there are many young stars close to the supermassive black hole at the center of the Milky Way. And it turns out that, to form the stars, you need an environment that's very different from the environment around a supermassive black hole. A supermassive black hole is not a conducive neighbor to star birth. So we have what I'm calling these days "the paradox of youth." We have a really hard time understanding why these young stars exist in such close proximity to the supermassive black hole.
|An artist’s impression of a supermassive black hole at the center of a galaxy. (NASA/MAXIM)|
To get stars forming near a supermassive black hole, one needs a huge amount of gas to overcome what are called the tidal forces from the hole. These tidal forces tend to want to rip things apart, as opposed to allowing them to collapse to form a star. There are many ideas that have been floated about, and none of them are very likely, which makes it a lot of fun to work on! Whenever you have an observation where the theorists are producing ideas from all sorts of directions, you know that you have an interesting topic. These ideas include forming stars at large, large distances away from the supermassive black hole where the tidal forces aren't a problem. Then, according to those theories, the stars migrate in very quickly. The problem there is that we don't know how to get them in quickly enough. Basically there's no method of transport that will get them in from a neighborhood which is conducive to star birth to one that is not conducive which is where we find them today.
Another idea is that these things were formed where they are located today, in the vicinity of the Milky Way's central supermassive black hole. But it's thought that the environment in the center of our galaxy is very different now from the past. And one idea is that perhaps our galaxy looks more like these active galaxies that we see at large distances where there's a tremendous amount of gas that's available not only feeding the black hole, but also maybe available for forming these new stars.
So the suggestion of a huge amount of gas would be a new idea. It would mean that the environment in the center of our galaxy was radically different in the not-so-distant past than what we see today. We understand that when our galaxy first formed, it must have looked different. But we didn't think it would look too different from the way it looks today.
Salazar: Can you tell me more about the techniques you're using to study the central regions of the Milky Way?
Ghez: As I said before, the challenge is Earth's atmosphere. But there are a couple of ways of beating the atmosphere. There's the solution of the Hubble Space Telescope, which is the technique of getting above the atmosphere. But the Hubble Space Telescope is a relatively modestly-sized telescope in contrast to telescopes that we have on the ground. And, in principle, the larger the telescope, the finer the detail that you can see. So we've developed some techniques that allow you to beat the atmosphere without having to get above it.
The most current technique that's being used is called adaptive optics. That's a technique where you have a mirror in your telescope system that's deformable. So as opposed to having a fixed shape, it can actually flex, and it flexes on short, short time scales. Something like every 1/10 of a second it changes its shape to reflect the distortions introduced by the Earth's atmosphere. This is sort of like a fun house mirror that changes on very short time scales. And with this technology, we're able to produce images that are as good as images taken above the atmosphere.
The wavelengths we observe are not wavelengths that we observe with our own eyes. We can't use optical wavelengths because, between us and the center of our galaxy, there's a tremendous amount of dust. This dust acts like smog. If you live in a place like Los Angeles or Dallas, you're very familiar with the phenomenon. Dust particles in our atmosphere make it very difficult to see locally. Likewise, within our galaxy, dust along our line or sight absorbs the optical light. Only one out of every 10 billion light packets from the galactic center make it to us in the optical. But if we go to the near-infrared, where we're working, 1 out of every 10 light packets makes it to us, and we can actually see the stars that are located at the center of the galaxy.
Salazar: Can you tell me the context of this discovery, with respect to the rest of what we know about astronomy? Why is it important to know about a black hole at the center of our galaxy?
Ghez: The reason why this experiment was important was to demonstrate that supermassive black holes really do exist.
We have theories that predicted that little black holes should exist. In other words, stars that start their life off with maybe 30 to 40 times the mass of the sun are expected to end their lives as little black holes, with maybe three times the mass of the sun.
But there's been observational evidence that maybe there is a second class of black holes which are more like a million or a billion times the mass of the sun. And while these observations were tantalizing, they weren't definitive. And there was no a priori theory that predicted the existence of supermassive black holes.
So our observation is in the center of the galaxy. Since we're so close, we can come up with a very clean experiment showing that this supermassive black hole does indeed exist, which means that we now need to understand how such things are formed. Since it's relatively nearly, it also gives us a great opportunity to understand in detail how these things are formed, how they affect their environment. And then from a more physical end, we get to probe gravity on scales that we've never probed before.
There are a number of other aspects that we're trying to pursue now. One of the key questions that we'd like to know is whether or not there's any other form of dark matter surrounding the supermassive black hole at the center of our galaxy. And the way we can see that is by looking at the orbits of the stars, and if they don't quite close, that is if they don't quite end up where they start, and, rather make a spirograph pattern, that's a key signature that there's something else out there. And if it's not emitting light, that's a signature of dark matter.
There are two forms of dark matter that might exist. One is the relic dark matter particles from the birth of our galaxy. And the second form, which is the more likely form that we'll detect, are these little black hole remnants that might be clustered in an entourage around our central supermassive black hole.
A black hole is a form of dark matter. Dark matter in its most basic definition, is any kind of matter that doesn't emit light. So that could be in the form of a black hole. But it could also be in the form of an elementary particle which would be on a much smaller scale than the black hole.
So one of the things that our group is investigating right now is understanding how the supermassive black hole at the center of our galaxy gains weight. Basically, it gathers matter from its surroundings and adds that matter to itself. We can look for this dark matter falling into the black hole, because, when it does, it emits light. So we're looking for light in the vicinity of the black hole. It's not actually coming from the black hole itself, but rather the material falling in.
And one of the big mysteries of this is that there's so little light emitted from this environment in spite of the fact that there is quite a bit of material available there to fall into the black hole.
One of the key questions is whether or not we can learn about a black hole from the materials falling onto it, and in particular, if we can see any variations in the light that's emitted from the material that's falling in. The variations in the light might actually give you some clues as to the properties of the central supermassive black hole.
Salazar: You said earlier that the mass of the black hole seems to be roughly three million times the mass of our sun. Does the mass of the black hole relate to the mass of our entire galaxy?
Ghez: You're right on. There seems to be a connection between the size of the black hole and the mass of the central part of the galaxy. While the mass of the black hole is much, much, much smaller than the mass of the galaxy, this connection always seems to work for many galaxies, no matter what the absolute weight is.
And that tells us that when the galaxy was forming from the primordial "goo", one outcome from the formation of the galaxy is the formation of a central supermassive black hole. That means that the central supermassive black hole was there from the very get-go. It wasn't something that formed as the galaxy evolved.
One of the goals in astrophysics is to always see a chronology, from the birth, until the death, to see the whole lifetime. And what we need to do in astronomy is not look at a single object, because there's no way that we as humans are going to live long enough. Rather, we look back in time, which means to look further and further away from Earth. This gives us a history lesson simply because light takes a finite amount of time to travel to us.
So by looking at very far distances, we're looking at very early stages of the universe where we would hope to catch galaxies in the act of formation, and where again, to see if this connection between the mass of the black hole and the mass of the galaxy still holds up at the very earliest moments in the history of the universe. This is, right now, at the very forefront of what people are trying to do.
Salazar: Thank you, Dr. Ghez. We appreciate your time.