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Mass: Accretion Disk vs. Black Hole

  1. Dec 5, 2013 #1
    I asked a question recently about orbiting black holes. Thanks for the answers.

    So if I'm correct in my thinking, long before the event horizons of orbiting black holes become close to each other, the two accretion disks get mightily disrupted and much of the mass of the two disks would fall into the two black holes.

    Which got me thinking: What is the relative mass of a black hole singularity and its accretion disk? Is the singularity extremely more massive than the disk?


    Edits:

    I seem to be making some basic assumptions here, and I don't know if they are true.

    I seem to assume that there is some sort of mean proportion between black holes and their accretion disks. Is that true? Or is the mass of the disk only loosely related to the mass of the singularity?

    I'm also assuming that the limits and the demarcation of the accretion disk is well defined. Is this true? One could consider an entire galaxy to be the accretion disk of a supermassive black hole if one were to define it that way - but I assume that there exists a more precise and constrained definition?

    I'm also assuming that the respective masses can be well estimated. Is that true? Or does dark matter mess up any attempts at estimating the mass of the singularity and the disk?
     
    Last edited: Dec 5, 2013
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  3. Dec 5, 2013 #2

    phinds

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    The mass of the accretion disk is dependant on the amount of matter that is available to make it up. I'm pretty sure it has to be some minimal amount, else you don't get the atomic collisions that cause it in the first place. In any case, I think it has to be trivial in mass compared to the BH.
     
  4. Dec 5, 2013 #3

    Chronos

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    The event horizon is much larger than the singularity, which is conventionally thought of as being of zero volume. Depending on how quantum effects are taken into account, it is believed it will be found the singularity has a non-zero volume.
     
  5. Dec 6, 2013 #4
    I wasn't really wondering about the size, but rather, the mass of the accretion disk compared to the mass of the singularity.


    Is it really true that supermassive black holes are significantly more massive than the rest of the galaxies which surround them, as was suggested above?
     
  6. Dec 6, 2013 #5

    phinds

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    No, even the largest super-massive black holes are on the order of billions of solar masses, but galaxies have hundreds of billions of star in them, and many are bigger than our sun so a typical super-massive black holes will be a very small fraction of the mass of the galaxy it's in.

    Also, it would not be correct to say that an entire galaxy is the accretion disk for a super massive black hole. Beyond a certain distance, orbits are stable and objects that far and farther away from the black hole will not spiral in to it and so cannot be thought of as part of the accretion disk. That distance is a tiny fraction of the radius of a galaxy.
     
  7. Dec 6, 2013 #6

    Chronos

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    No. Even the largest supermassive black hole is not nearly as massive as its host galaxy. The accretion disk is merely a skid mark of material that survives black hole feedings. It is far less massive than the black hole.
     
  8. Dec 15, 2013 #7
    Over extremely long time scales wont gravitational radiation eventually cause all orbits to decay? Or will galaxies meet some other demise before that happens?
     
  9. Dec 15, 2013 #8

    phinds

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    I don't know about gravitational decay. I am now aware that it should happen in galaxies, but I'm not really the right person to ask.

    Over long enough periods of time, all the stars in a galaxy will go out no matter how late in the life of the galaxy they are formed, so you can say that the galaxy will have died, in a very real sense but that doesn't mean all the matter will be in a black hole in what was once the center of the galaxy.
     
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