I've noticed that, when we astronomers are attacking a problem or question, we tend to attack from many different angles. Some work, some don't, and eventually the picture becomes clearer. But we have a memory of the mess that existed during the highest frenzy of research, and we tend not to see how nicely all the pieces fit together into an overwhelmingly convincing argument. It is only when someone comes along and organizes all the evidence in a linear fashion that we can admire the result.
This week on our preprint server (the webpage where astronomers can post their research before the journal containing the paper arrives in the mail), Mark Reid of the Harvard-Smithsonian Center for Astrophysics published this article, which is a scientific review of the search for a black hole at the center of the Milky Way galaxy over the past five decades. The review starts with some of the early evidence for something big at the center of the Milky Way and other galaxies. But it quickly moves to a discussion of the strongest current evidence for a black hole in the center of the Milky Way:
- The orbits of stars at the center of the galaxy: Over the past decade, two groups of astronomers (one led by Andrea Ghez at UCLS, and the other by Rheinhard Genzel in Garching, Germany) have used infrared cameras on large telescopes to take pictures of the center of our galaxy. Over this time, they have been able to see stars completing orbits around the same point in space. One star even comes within 100 Astronomical Units (100 times the Earth-Sun distance) of this point, and moves at 6000 miles per second at closest approach, about 3% of the speed of light. Using the laws of gravity and orbits laid out by Johannes Kepler and Isaac Newton 300-400 years ago, we know that the object at the center of these orbits must have a mass about 4 million times that of the sun! So, we have four million times the mass of the sun in an area no larger than our Solar System. That's a lot of stuff! (You can see animations of the stars' movements here (UCLA) and here (Germany).
- The focus of these orbits is at the same point as a strong radio source, but this source has to emit less infrared light than a single star. The pictures used to track the stars' orbits were taken in infrared light, but there was no light coming from the center of their orbits. So, we need an object that needs to be 4 million times the mass of the sun, is smaller than our Solar System, can emit radio signals, yet emits almost no infrared light. (We don't know about visible light, because dust between us and the center of the Milky Way blocks visible light.)
- The size of the radio source is less than 1 Astronomical Unit across. In other words, unless only part of the object is making radio waves (which would be a very improbably feat), all of that matter, four million times the mass of the Sun, is constrained to be in an area smaller than the Earth-Sun distance. A black hole with a mass of 4 million times the mass of the sun would be about 1/8 of an astronomical unit in size, This means that the object we are looking is smaller than eight times bigger than a black hole.
- The object is sitting still at the exact center of the Milky Way. Those stars we see in orbit around the dark object at the center of the Milky Way are pretty big, and there are a lot of them, adding up to almost 4 million times the mass of the sun themselves . If the thing at the center of their orbits is not a single object at the exact center of the galaxy, then it should be whipping through space at speeds similar to those of the stars around it. But it is sitting almost perfectly still, moving at a speed of less than a quarter of a mile a second, or "just" 900 miles per hour. That may sound fast, but remember those nearby, massive stars are zipping through at 6000 miles per second. The sun is moving at about 150 miles per second around our galaxy. So the fact that this thing isn't moving in such a harsh environment means that it is not only pretty honking massive, but it's right at the dead center of our galaxy.
- Ordinary matter can't explain all the data and survive for very long. It is possible to make very dense clusters of faint stars, white dwarfs, or neutron stars that would be visible only in radio waves and have this large amount of material in a very small space. But if you somehow make a tight ball of fairly ordinary stars, the stars will collide with each other, merging to make a single black hole, or slingshot each other out of the middle of the galaxy. In either case, the cluster of stars would only live for a million years. This sounds like a long time, but it is the blink of an eye in the 13-billion year lifetime of our galaxy. How could you make such a dense ball of stars, and why would it be at the exact center of our galaxy right now? And when we look at other big galaxies, they all have something big and massive at their center. How could all big galaxies have a short-lived cluster of faint stars at their middles at this instant in time? There's no reasonable answer.
When you add all these things up, there is only one object in all of currently-understood physics that can be this massive and this small, and that is a black hole. Other explanations, like the very dense cluster of faint stars or balls of weird subatomic particles, are just too contrived to seem reasonable. In the parlance of a criminal court, the presence of a black hole at the center of the Milky Way is beyond a reasonable doubt.
The best part is, there are still more tests that we can do to continue to test and probe this black hole. Every theory should be tested and re-tested as often as novel techniques arise. But, I think we can move on with confidence and stop asking "Is there a big black hole in the center of the Milky Way?" Instead, we can start asking the even harder question of "Where did the big black hole come from?" Hopefully, in another review article 50 years from now, the evidence answering that question will be just as overwhelming.