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B Dark matter and SMBH orbital decay

  1. Oct 11, 2017 #1
    I have a curiosity. I saw recently how a team of researchers discovered two super massive black holes orbiting each other in a far off galaxy. (Google it if you didn’t hear, fascinating how they did it.).

    Anyway, I read a few things about unsolved mysteries of their formation and how SMBHs migrate onwards by kicking stars out, trading angular momentum. Then how they get stuck orbiting each other because there are no more stars to toss out and nobody seems to be sure how they progress from there. I read a few places how gas and dust may help a little but how that resource would be quickly gobbled up or blown out by the heat.

    So far I haven’t seen anything about dark matter though. So my question is why would dark matter not be a better candidate than gas and dust? Since it doesn’t interact with light, it wouldn’t be blown away and it seems like the supply of it at the galactic core would be for all practical purposes infinite.
  2. jcsd
  3. Oct 12, 2017 #2


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    The amount of dark matter falling into black holes is completely negligible. We had various calculations here in the past, the search function should find them.
    Yes, but there is also nothing that would capture it.
  4. Oct 12, 2017 #3
    But I’m not asking about it being captured. I’m asking about it being slingshotted away. The energy given to the accelerated particle is taken away from the orbital energy of the BH. Basically small thurst over a billion years.

    I’m under the impression that black holes move to the centers of galaxies by flinging things out, not eating them. Why wouldn’t it do the same with dark matter?
    Last edited: Oct 12, 2017
  5. Oct 12, 2017 #4


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    To get slingshotted away it has to get close to the black hole. That is easier if there is a mechanism that captures the matter in the vicinity of the black hole pair.
    Falling into the black hole and getting shot away are two possible outcomes that both bring (on average) the black holes closer together.

    Yes there is a thrust, but it is completely negligible.
  6. Oct 15, 2017 #5

    stefan r

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    A particle would get
    The black holes can fling both ways. From a distance a particle follows an elliptical orbit around the barycenter. If that orbit is a path "behind" one of the black holes then the particle flings out like a catapult round. If the orbit passes "in front" then the particle looses momentum and adds spin to the black hole pair. For example NASA has used Venus for gravity assist for missions to both Mercury and to Saturn.
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