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B Black holes in the centres of galaxies

  1. Sep 13, 2017 #1
    Why is it believed that almost every big galaxy has a black hole on its center? Does the explanation rely on General Relativity?
     
  2. jcsd
  3. Sep 13, 2017 #2

    kimbyd

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    See here:
    https://en.wikipedia.org/wiki/Supermassive_black_hole#Outside_the_Milky_Way

    Due to the difficulty of detecting most black holes, only a small number have been observed directly. However, the tight relationship between certain properties of the galaxy and the mass of the black holes seems to indicate that the formation of such a supermassive black hole is intimately tied to the formation of the galaxy. This would imply that essentially every galaxy has one.
     
  4. Sep 13, 2017 #3
  5. Sep 14, 2017 #4

    Vanadium 50

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    Before discussing why something is true we should figure out if it is true. M33 appears not to have a central black hole.
     
  6. Sep 14, 2017 #5

    jim mcnamara

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  7. Sep 14, 2017 #6

    kimbyd

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  8. Sep 14, 2017 #7

    Vanadium 50

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    That doesn't say they see something, or even maybe see something. If M33 has a central black hole, it weighs less than 0.1% of the Milky Way's, and probably less than 0.05%. That's like looking for an elephant and reporting that there's nothing bigger than a pug. Or maybe a chihuahua.
     
  9. Sep 15, 2017 #8
    I understand it's difficult to explain how SMBHs have grown so large in relation to the age of the observable universe, is there any correlation between the size of SMBHs and the expected dark matter in their galaxies?
     
  10. Sep 15, 2017 #9

    kimbyd

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    What they're saying is that the kinematics of the galaxy and a small black hole are consistent with the M-sigma relationship. The M-sigma relationship is the main reason why it's believed nearly all galaxies have supermassive black holes at their centers.
     
  11. Sep 15, 2017 #10

    kimbyd

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    I don't think there's believed to be any direct relationship between dark matter and the masses of the black holes, for the reason that dark matter doesn't collapse like normal matter does, because it doesn't experience much of any friction. There is likely a loose correlation because more massive galaxies tend to have more dark matter associated with them, and more massive galaxies also tend to have larger black holes at their centers. But it's likely not due to the black holes swallowing dark matter, as the dark matter isn't really able to lose its orbital energy efficiently enough to fall into the black holes (some will, naturally, but not much).
     
  12. Sep 20, 2017 #11
    I think that spiriform galaxies. such as ours, look like swirlings around a central drain.
     
  13. Sep 20, 2017 #12

    kimbyd

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    Perhaps, but that's an illusion. Black holes are only a tiny, tiny part of the masses of galaxies like our own (typically a fraction of a percent). The spiral arms themselves are likely a result of density waves. Here's one popular article that talks about them, and the science that surrounds them:
    https://www.scientificamerican.com/article/what-process-creates-and/

    The spiral arms are all about how stars interact with one another in the galaxy. The gravity from the black hole at the center doesn't play any significant role (the black hole may play a role if it's active, by blowing away the galaxy's dust that provides the friction which keeps it in a disk shape).
     
  14. Sep 20, 2017 #13
    That seems to me like saying a singularly identifiable impetus is probably not the only force affecting or effecting a result -- here's an obvious analogy: an open drain in a half-full bathtub doesn't entirely account for all the observable fluid dynamics of the swirling phenomena occurring therein -- it's just a part of an explanation of why the water doesn't present as pristinely still.
     
  15. Sep 20, 2017 #14

    kimbyd

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    No.

    The spiral arms form due to the gravitational dynamics of the stars within the arms.

    The black hole's gravity has basically nothing to do with it, because the black hole's gravity is minuscule compared to the galaxy as a whole.

    Where the black hole has an effect, it comes from the fact that the black hole at the center of the galaxy can get extremely bright when it's swallowing matter at a rapid pace. It can get so bright that it can blow nearly all of the gas and dust out of a galaxy, which removes the friction which keeps the galaxy in a disk shape, resulting in the galaxy becoming spheroidal and having no significant structure at all.
     
  16. Sep 21, 2017 #15
    What specifically in my post are you saying "no" to ?
    That seems plausible to me -- even so, I still think that the central anchor point being a super-massive black hole is a determinative factor in the overall phenomenon set.
    The mass of the central SMBH, despite being small compared to that of the entire galaxy, has extreme density compared to other intra-galactic aggregates of mass, and is vastly more massive than other mass aggregates (stars) in its vicinity. That, I suppose, is signifintly factorial regarding why smaller-mass objects swirl around it, rather than it around them, just as the moon orbits the earth, and the earth-moon system orbits the sun. I understand that the mass of the sun is much greater than that of all the planets combined, and that the mass of the SMBH near the mass-center of the galaxy is much less than that of the entire galaxy; however, the centrallly-located SMBH appears to be the tipping-point around which the balance of the galaxy revolves -- not especially unlike water swirling around and toward a drain.
    Please, given that you apparently disagree with me regarding the matter, explain your theory regarding why the "center of the galaxy" is occupied by a super-massive black hole, instead of by empty space, or by a less massive star..
    Are you sure that most of the gas and dust so impacted escapes the galaxy, and does not in passing accrete unto intra-galactic objects, such as stars?
    Isn't alignment of angular momenta involved? Don't we already employ our understandings of fluid dynamics in ideational modelling of formation and evolution of galaxies? I still think that spiriform galaxies that have a central SMBH (as far as I know, all spiriform galaxies that have been observed systematically have appeared to have such an object at or near center of mass) look like swirlings around a drain.
     
  17. Sep 21, 2017 #16

    Vanadium 50

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    If you are arguing that it looks like "swirlings around a drain" like Ursa Major looks like a big bear, I think nobody is arguing with you. If you are arguing that the trajectories of stars follow trajectories like "swirlings around a drain", that is not what is observed.
     
  18. Sep 21, 2017 #17
    My view is somewhere in between those -- I think that the macro-scale rotatory centrality and apparent angular momenta of the masses within spiriform galaxies that have a SMBH at or near center of mass is analogous to water swirling about a drain, and that eventually all the matter in the galaxy that does not attain galactic escape velocity will at some point become within the accretion range of the SMBH, and so become part thereof, just as, analogously, all the water in a vessel will, in time, as inexorably as gravity directs, flow down an open drain therein -- in the case of the galaxy we inhabit, it is predicted that the nearest other galaxy (Andromeda), also spiriform, also with a central SMBH, will likely catastrophically collide with ours long before our or its central SMBH accretes anything anywhere near what is now in our or its outermore region ...
     
  19. Sep 21, 2017 #18

    Bandersnatch

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    Galaxies are not giant accretion discs for their central black holes. They are not feeding their stars to their BHs.
    The stars are on stable orbits, with no global galactocentric velocity component. Once the material in the immediate proximity of the black hole is accreted, i.e. once a galaxy passes its active nucleus phase early in its life, there is no reason for any star to fall into the black hole, apart from being very unfortunately slingshot into what is a rather tiny target.
    A black hole of 1 million Sun's mass from 1000 ly has as much gravitational influence on orbiting stars as the Sun has on its environment from 1 ly away. Which is pretty meh, and way too low to keep the stars at that distance in orbit considering their velocities. At 4000 ly from the black hole, a star has as much reason to orbit it or fall into it as the Alpha Centauri system has to do the same w/r to the Sun.
    Which is to say, that stars behave in their orbits like they do due to the collective gravitational influence of other stars and gas, not of the black hole.
    You'd get density waves forming spiral patterns regardless of whether there is or isn't a black hole in the centre, because that's not what is causing them. They just look like they're spiralling down, but the individual orbits are stable.
     
  20. Sep 21, 2017 #19
    A set of stably orbiting objects around a galactic center is not a perpetual motion system. The fact that their, albeit variously elliptical, and inter-influenced, orbits are, in sum, approximately centered on or near the SMBH at or near the galactic center of mass, indicates that when the orbits eventually decay, they will, in individual and group order, do so in the direction of the SMBH around which they are orbiting.
     
  21. Sep 21, 2017 #20

    Vanadium 50

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    I'm sorry, but Bandersnatch is right. Galaxies are not giant accretion discs for their central black holes. They are not feeding their stars to their BHs. Some spirals, like M33, appear not even to have a supermassive black hole in their center.
     
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