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B Black holes in the centres of galaxies

  1. Sep 22, 2017 #26
    This is not how orbits work in galaxies. Every star is attracted to every other star in the galaxy. (Plus the dark matter halo, which we will ignore for now.) A SMBH can put 10,000,000 solar masses into the volume of our solar system, which is a much larger average density than the rest of galaxy. However, if pick a star at random from a galaxy of 1 trillion stars and then draw a line from my star to the central black hole and then center an arc of width 3.6 arcseconds (1/1000th the way around a full circle) on that line the arc will enclose at the very least 1/1000th of the stars in the galaxy, which for a galaxy of 1 trillion stars is 1 billion stars. The 10 million solar mass black hole then constitutes (1/100th) of the mass in the smallest possible slice which is itself only 1 of 1,000. The SMBH is such a small portion of the total mass that you could likely remove it and see minimal change in the large scale structure of the galaxy.

    Assumptions in my argument: Spiral galaxies can be treated as 2 dimensional disks. The average mass of a star is 1 solar mass. A spiral galaxy cut into "slices" like a pizza would have equal amounts of stars per slice.
  2. Sep 24, 2017 #27

    Vanadium 50

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    I disagree with that. Or rather, I believe it may be true, but that the evidence is not strong enough to declare this a fact.
    What is known?

    1. Out of the few dozen SMBH's discovered in spirals, they all occur in galaxies with central bulges.
    2. There is an empirical relationship between the size of the central bulge and, if it has been detected, the size of the SMBH. This is purely empirical: there is no generl agreement that this is caused by known galaxy dynamics, although there are conjectures.

    I don't believe from 1 and 2 that the conclusion "All galaxies with central bulges have SMBH's" follows. It may be true, but it does not logically follow. I'd like to see one of two lines of evidence - either a dynamic explanation of M-sigma that shows SMBH's are inevitable, or a statement that given a known and calibrated SMBH-finding efficiency, the number of observed SMBH's is consistent with 100% of the bulgy spirals having them.

    Then there is the other fact
    3. Not all spirals have central bulges. M33, for example. The number without is maybe 20%? 25%? To me, that would mean most spirals have bulges, but not almost every spiral has a bulge.
  3. Sep 28, 2017 #28


    Staff: Mentor

    Here's an article from the Universe Today online magazine with Fraser Crain answering the question:


    Short answer is Astronomers just don't know why but have a couple of scenarios to consider:
    - the black hole was there and the galaxy formed around it
    - a portion of the galaxy collapsed to form the black hole
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