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To provide a little bit of context, it is known that globular clusters should produce a lot of black holes, but it has long been believed that they should retai
by antognini 10y ago
To provide a little bit of context, it is known that globular clusters should produce a lot of black holes, but it has long been believed that they should retain very few, if any of these black holes. One reason for this is that black holes should receive a kick upon their formation. The gravitational potential of a globular cluster is weak enough that this would generally be enough to kick the black hole out of the cluster.
For those that remain in the cluster, the black holes would be much more massive than the rest of the stars in the cluster. Globular clusters undergo a phenomenon called mass segregation, where the most massive stars settle to the center of the cluster and lighter stars occupy the outer regions. So over time the black holes would sink to the center of the cluster where they would interact and form binaries with each other. These binary-binary interactions would, over time, kick single black holes out of the cluster. Since these interactions happen on timescales that are much shorter than the lifetime of the cluster, the received wisdom about twenty years ago was that globular clusters should have only a couple of black holes left, if any at all.
In 2007, Thomas Maccarone (who is an author on this paper) discovered a black hole in a globular cluster in another galaxy. It wasn't until 2012 that any were discovered in a globular cluster around the Milky Way. Now that computers have gotten fast enough that it has started to become possible to simulate the dynamics of globular clusters, theorists have found that globular clusters retain a much higher fraction of black holes than they originally thought. The reason for this seems to be that the distribution in black hole masses leads the black hole population to remain better mixed with ordinary stars than was originally thought.
- frandroid 10y agoHow does this "kicking" of black holes happen? Who is this galactic Henrik Lundqvist who keeps the black discs out of the cluster?
- neikos 10y agoI'd guess that if a lower mass orbits a higher mass body in a very tight orbit it might get thrown out.
- antognini 10y agoThere are two ways that black holes could get kicked: 1. Natal kicks. The process of forming a black hole is fairly violent, and could involve a supernova in many cases. If the supernova is asymmetric, this will result in a kick on the black hole. Even if there is no supernova, an asymmetric infall process could produce an asymmetric burst of gravitational waves. Since gravitational waves carry momentum, this will also result in a kick on the new black hole. But it's generally thought that black hole natal kicks are weaker than neutron star natal kicks. The received wisdom seems to be that natal kicks will remove some black holes from a globular cluster, but probably not most. 2. Few-body interactions. A general principle of dynamical interactions (called Heggie's Law) is that if a single star interacts with a binary system, it is likely that the binary will become tighter if its orbital velocity is greater than the velocity of the incoming star, and wider if its orbital velocity is less than the velocity of the incoming star. The BH-BH binaries that would form in the cores of globular clusters would be relatively tight, so almost all interactions they have with other black holes will result in them getting tighter and giving more kinetic energy to the interloping black hole. (Or there may be an exchange and the interloping black hole will take the place of one of the black holes in the binary and kick out one of the original black holes.) Back when I was in grad school I made a visualization of a binary-binary interaction, which you can watch here: https://www.youtube.com/watch?v=Y3nPOQXl6co https://www.youtube.com/watch?v=Y3nPOQXl6co
- Florin_Andrei 10y agoI've done n-body simulations like that in college. It's amazing to watch how, in certain cases, some random star, previously well-behaved, just shoots out of the cluster at high velocity after some unlikely chain of close encounters. The deserters extract some energy from the cluster when they leave.
- frandroid 10y agoThanks for the explanation and the video!
- Florin_Andrei 10y ago> Globular clusters undergo a phenomenon called mass segregation, where the most massive stars settle to the center of the cluster and lighter stars occupy the outer regions. Intuitively, this seems simple, since "heavier objects" should sink further down, no? But thinking about this for a while, it doesn't seem so simple anymore. No matter what's the mass of the star, if it's already on a given orbit around the cluster, it will tend to stay on that orbit, unless there are close interactions with other stars of comparable mass. So what would be an intuitive explanation for this phenomenon?
- antognini 10y agoThis is one of those rare cases where one's immediate intuition isn't so far off. Globular clusters are very dense stellar environments. For context, the radius of a globular cluster is roughly the distance to the nearest star to our Sun (a few light-years). In this space there are packed roughly a million stars. So dynamical interactions are frequent. A consequence of this is that stars exchange energy with each other frequently, and when you have frequent energy exchanges, you end up with a system that is in energy equipartition --- i.e., every star has about the same amount of energy as every other. Since more massive stars have the same energy as lighter stars, they must move slower, and consequently sink to the center of the cluster. Ultimately energy equipartition is the same reason that there is gravitational settling in a gas or a liquid on Earth.
- Tloewald 10y agoWonderful explanation. Thank you!
- effie 10y ago> when you have frequent energy exchanges, you end up with a system that is in energy equipartition Could you support this claim with some explanation? This can be proven for the Maxwell-Boltzmann distribution of velocities, which is valid for gas in thermodynamic equilibrium, but probably not so valid for stars as these interact via long range forces, not through collisions like molecules do. EDIT Also, the stars that move too fast will escape the system so the equipartition cannot be sustained for stars whose mass is lower than some value corresponding to average quadratic speed being equal to escape speed.