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This answer here says that black holes will "sink" to the centers of the galaxies they're in. How does this happen? It's not like there's a buoyant force in the vacuum of space. Shouldn't they orbit the center of the galaxy like everything else?

zucculent
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  • Mass segregation, but it hints that there hasn't been time for it to occur on a galactic scale. Maybe some of the references there will be useful in crafting an answer. – GrapefruitIsAwesome Feb 16 '22 at 01:34
  • I think the article answers it just fine. – zucculent Feb 16 '22 at 02:08
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    Not all black holes, only supermassive ones. See the propose duplicate. Supermassive black holes pull mass towards them as they move past, resulting in more mass behind them, slowing them down and causing them to fall. – James K Feb 16 '22 at 07:03
  • @JamesK Not a dupe, this question asks about black holes slowly tending to the center of the galaxy. Your proposed duplicate asks about SMBH mergers. – WarpPrime Feb 16 '22 at 13:44
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    @GrapefruitIsAwesome Mass segregation can only operate (on timescale less than the age of the universe) in very dense environments like globular clusters or nuclear star clusters. It doesn’t matter on galactic scales. – Peter Erwin Feb 17 '22 at 10:59
  • @PeterErwin yes, I know, my comment even states that the wikipedia article I linked to says as much. There are references to that statement in the article that might be useful in writing an answer that says that cannot be the reason for a SMBH to be at the center of a galactic size structure. – GrapefruitIsAwesome Feb 17 '22 at 11:37
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    Does this answer your question? Do all galaxies have a black hole in the center? or https://astronomy.stackexchange.com/q/7861/18054 – Rob Feb 19 '22 at 07:00

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There are two possible answers, depending on the spatial scale and what kind of black holes we're talking about.

The answer you link to appears to be talking about stellar-mass black holes in a nuclear star cluster, which means we're talking about the inner few parsecs/light years of a galaxy and an environment with a very high density of stars. In such a situation, black holes (and massive stars) will gradually lose orbital energy via close ("two-body") gravitational encounters with other, lower-mass objects (e.g., low-mass stars) and sink toward the center of the nuclear star cluster. (Lower-mass objects will, over time, gain energy and move onto larger orbits.) This is an example of mass segregation via two-body encounters.

If, instead, you want to know about massive (including supermassive) black holes sinking to the center of a galaxy on galactic spatial scales, then two-body mass segregation will take much too long (e.g., trillions of years). The relevant physical process then becomes dynamical friction, where the massive object creates a wake of lower-mass objects (stars and dark-matter particles) as it moves through the galaxy. Since this wake means there's a somewhat higher density of lower-mass objects behind the black hole, it feels a net force pulling back on it ("friction"), and so it loses orbital energy and moves to a smaller radius, closer to the center of the galaxy. Since the density of stars (and dark matter) is higher the closer to the center of the galaxy you go, the dynamical friction effect will actually increase. This can cause very massive objects to end up in the center of the galaxy on timescales of hundreds of millions of years.

Peter Erwin
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