As many as 300 rogue black holes may roam the outer regions of the Milky Way galaxy, according to a new theory by Ryan M. O’Leary and Abraham Loeb of Harvard’s Department of Astronomy, Cambridge, Mass.
When our galaxy was forming, acquiring smaller galaxies, these smaller versions would collide and their central black holes would interact gravitationally. Binary black holes, destined to merge, would circle each other, their gravitational reactions tossing them to distant parts of our larger galaxy. They would have enough momentum to fly out there but not enough to leave the Milky Way.
The holes’ closest stars would remain bound to them while the rest of the galaxies would be absorbed into the larger Milky Way. — That’s the theory of O’Leary and Loeb, as mentioned in the previous blog entry, May 13.
O’Leary agreed to an interview by email. The following, lightly edited, are this blog’s questions and his responses on the fascinating subject.
Nightly News: How did you and your colleague come up with the theory?
O’Leary: For over three decades, people have realized that if two black holes merge, they may recoil because of how they radiate away gravitational waves. More specifically, the merge may send more gravitational waves in one direction than another, effectively acting like a rocket. Unfortunately, it is an incredibly difficult problem to calculate how fast this ‘kick’ is when they merge. Finally, after many decades of work, the first computer simulations of the inspiral and merger of two black holes has been successfully completed just a few years ago.
With this new data in hand, my adviser and I wanted to think of some ways we might find a sign of these unique phenomena. During one of our many discussions, we recognized that the black holes in the earliest galaxies should have been kicked out when they merged, and brought along some stars around them. The key is that these stars can help us find these black holes. What is most interesting, however, is that we can then use these black holes to see what the first galaxies may have been like, as well as the first black holes.
NN: Do you think black holes are a product of the Big Bang? That is, did galaxies form around them because black holes were present, or did galaxies form first and then black holes develop?
O’Leary: This has been one of the biggest questions for decades. Stephen Hawking was one of the first people to discuss the formation of black holes during or immediately after the big bang. Although that idea is exciting, observations suggest that most black holes formed through some more mundane astrophysical process, although we still are not exactly sure how. One thing that astronomers have shown quite conclusively, however, is that it seems galaxies and black holes grow hand in hand. That is, when astronomers look at a galaxy, the size of the black hole in the center seems to depend on the size of the galaxy — the bigger the galaxy the bigger the black hole.
NN: Are most globular star clusters free of black holes? If so, does that mean they aren’t the result of interactions among the kind of galactic building blocks that you mentioned?
O’Leary: Globular clusters are inherently far and small. This makes it very difficult to detect if there is a black hole in the center of them. In addition, the favored way of detecting a black hole is by finding those that are accreting gas, and globular clusters are notoriously devoid of gas. However, some globular clusters may be the remnant of galaxies, which makes many think that some of them should have a black hole in their centers. Unfortunately, some of the signatures of a black hole in the center of a globular cluster are difficult to detect.
NN: How long do you think it might be before there is observational confirmation of rogue black holes?
O’Leary: I have been trying to find these objects myself using the Sloan Digital Sky Survey. However, there are so many uncertainties in what they may look like, that even I’m unsure of what is the best thing to look for. This survey has millions upon millions of stars and galaxies in it, and it is truly like finding a needle in a haystack, but worse. Someone more clever than I might be able to find it in only a year or so, however.
NN: What were your personal feelings when it all began to come together?
O’Leary: I was incredibly excited about trying to find these black holes. Finding one would truly be an amazing discovery. As a theorist, I don’t look at — real — data or observations very often, but it would be remarkable to see one of these objects.