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Origin of Galaxies and Super Massive Black Holes

  1. Jul 2, 2015 #1

    lavinia

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    I just watched a popular science program entitled Super Massive Black holes that proposed the idea that galaxies were formed long ago from hydrogen gas clouds whose centers collapsed to become super massive black holes that then generated star formation and ultimately the formation of galaxies. Since this was a popular show I wonder what current research says. References would be appreciated.

    The program did not discuss current formation of galaxies and ask whether galaxies are currently being formed at all.
     
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  3. Jul 2, 2015 #2
    Supermassive black holes seem to exist and apparently are not uncommon.
    They can only have arisen from the collapse of clouds of primordial hydrogen and helium since nothing else was there in any significant amount.
    Whether they are an essential prerequisite for galaxy formation, I don't think anyone knows.
     
    Last edited: Jul 2, 2015
  4. Jul 2, 2015 #3

    phinds

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    Yeah, this is an interesting question. It seems that all galaxies (or perhaps it's just all mid-sized and up galaxies) have super-massive BH's at their center. Cosmologists are trying to understand that as part of the understanding of how galaxies form. I don't know if it's still an open question but I'm sure it was at one time as to which comes first the galaxy or the super-massive BH?

    Since the super-massive BH's are only a very small percentage of the mass of the galaxies, it seems to be unclear that one is needed in order for a galaxy to form. Perhaps their formation and the formation of the galaxies go hand in hand, not as cause and effect (either way) but rather with both as effects of the formation process.
     
  5. Jul 2, 2015 #4

    Chronos

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    It seems black holes provide a reasonable anchor around which primordial galaxies formed. The prospect that SMBH are likely to have at least been seeded by direct collapse in the early universe is generally conceded most cosmologists. It is otherwise awkward to explain the abundance of high z quasars, which are believed to be powered by SMBH. While Loeb suggested this route back in the 90's, the earliest paper I can recall seriously discussing the direct collapse model was by Begelman in; http://arxiv.org/abs/0709.0545, Did supermassive black holes form by direct collapse?
     
  6. Jul 4, 2015 #5

    lavinia

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    Thanks for the paper. It is over my head though.
     
  7. Jul 4, 2015 #6

    lavinia

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    I just rewatched the show and I think understand the reasoning that places super massive black holes at the origin of galaxies. Here is a creation scenario.

    - The center of a large cloud of hydrogen gas collapses into a black hole.
    - The black hole "feeds" on the surrounding gas cloud pulling more gas into it. The incredible energy released during the feeding process creates a quasar.
    - Inside the quasar, the stars of the galaxy form.
    - As the the black hole increases in mass during its feeding frenzy, the energy released in the quasar pushes the surrounding gasses outward away from it. This gas is pushed outside of the grip of the black hole and the black hole can not feed on it. The remaining gas is eventually depleted, the quasar fades, and the black hole is left isolated at the center of the galaxy that has formed.

    The support for this scenario is the correlation between the rotation velocity of the stars at the outside edge of the galaxies and the size of the central super massive black hole, the larger the black hole the greater the "sigma", the velocity of the peripheral stars. During the quasar's life the outward pressure of the energy it releases is counteracted by the rotation of the gas. The greater the rotation, the more the resistance, and the more time the black hole has to get larger. With greater rotation it takes longer for the quasar to push enough gas outside of it to end the quasar's life and arrest the accretion of the black hole.

    The astronomers interviewed in the program stated that most if not all "large" galaxies have black holes. One has found evidence of a black hole at the center of the Milky Way.
     
    Last edited: Jul 4, 2015
  8. Jul 4, 2015 #7
    Was that an explanation given in the program?
     
  9. Jul 4, 2015 #8

    lavinia

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    Yes.

    They interviewed the astronomer who derived the theory - but they left out all of his calculations.

    He points out that the velocity of rotation of the peripheral stars today is not affected by the black hole because the stars are too far away to feel its gravity. So this correlation must have come from a previous time.

    The astonomer who found the black hole at the center of the Milky Way has also observed new light coming from where she believes that black hole is located. She hypothesizes that a small amount of interstellar gas got close enough to get sucked in.

    Another astronomer interviewed on the program did a computer simulation of the collision of Andromeda and the Milky Way. As I understand it, the collision would reactivate the feeding process of both galaxies' black holes.

    The program can be downloaded on Netflix
     
    Last edited: Jul 4, 2015
  10. Jul 5, 2015 #9
    What about dark matter?
     
  11. Jul 5, 2015 #10
    Sure it's a possibility, but hard to say more than that because we still don't know what dark matter actually is.
    As far as I know, neither is there a model of how dark matter comes into being and how much of it was present in the earliest times.
     
  12. Jul 6, 2015 #11
    Our observations have also shown that stars with masses greater than 130 M will collapse into pair-instability supernovae when they die, leaving nothing behind. The prevailing theories seem to indicate that post-Hadron Epoch black holes must be formed by stars within the 10 to 130 M range. Pop. III stars with 130 M would have extremely short lives (~52,000 years), and there would be considerably more of these massive stars during they early universe (z > ~6). However, it would require so many mergers to achieve the size of a SMBH (> 109 M ), that it would seem to be an unlikely explanation, even when taking into consideration the super-Eddington accretion theory.

    I will not even begin to speculate the possibility of primordial SMBH, other than to say that the environment was conducive for their creation.

    Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. IV. Hβ Time Lags and Implications for Super-Eddington Accretion - arXiv : 1504.01844
    Direct collapse black hole formation via high-velocity collisions of protogalaxies - arXiv : 1504.00676
    Can very massive Population III stars produce a super-collapsar? - arXiv : 1504.01202
     
    Last edited: Jul 6, 2015
  13. Jul 6, 2015 #12

    lavinia

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    So how were the longer lived stars that populate galaxies formed?
     
  14. Jul 6, 2015 #13
    Most galaxies consist of Pop. II low mass stars (Type M), particularly in the halo of the galaxy. No Type M star ever created since the Big Bang has come close to dieing, and none will for tens of billions of years yet to come. A 0.5 M Type M star will have a life span of > ~56 billion years.

    With regard to SMBH, there must be another explanation for how they can bulk up so quickly, such as the high-velocity collisions of protogalaxies, for example. What they are calling a "direct collapse" SMBH is not really accurate, because once you exceed 130 M, and I am sure there were many Pop. III stars that exceeded this mass limit, they would die in a pair-instability supernovae, leaving nothing behind. For a star to leave behind a black hole when it dies, it must fall within the 10 to 130 M range. A 130 M star is extremely short lived (~52,000 years), but will still only produce a black hole of ~39 M. Which would mean that it would require the merger of more than 25.6 million of these ~39 M stellar black holes in order to produce a SMBH (> 109 M), and do all of this within the first ~600 million years after the Big Bang. It is possible, I suppose, but it just seems to be an unlikely explanation.
     
    Last edited: Jul 6, 2015
  15. Jul 6, 2015 #14

    lavinia

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    My sense was that the Supermassive black holes did not form by mergers but by accretion of a small seed black hole that did form from direct collapse but then accreted because there was a large amount of gas within the reach of its gravitational field.
     
  16. Jul 6, 2015 #15

    lavinia

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  17. Jul 6, 2015 #16
    Direct collapse black hole formation is not possible due to the Eddington limit. At some point the collapsing gas will achieve hydrostatic equilibrium and begin burning as a star. There is also a limit on how much mass a black hole can consume at any given time. Any excess mass would be flung out of reach of the black hole through its polar jets.
     
  18. Jul 6, 2015 #17

    lavinia

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    OK. I can see that I need to learn a lot more Physics. Chronos suggested reading this paper.

    http://arxiv.org/abs/0709.0545
     
  19. Jul 6, 2015 #18
    I read the paper, and agree with its premise, but you will note because the life and death of such massive stars are so short they are being "considered" as a direct collapse, when in fact they really are not. A life span of less than ~52,000 years is extremely short, cosmologically speaking, granted, but it is still a star's life span and should not be considered a direct collapse into a black hole. Mergers may also be logarithmic in nature. Meaning, for example, that two 39 M black holes may merge, creating a 78 M black hole, followed by the merger of another 78 M black hole creating a 156 M black hole, followed by the merger of another 156 M black hole forming a 312 M black hole, etc., etc. It would require very few mergers before you end up with an intermediary black hole, which would fit the "seed" black holes referred to by the paper.
     
  20. Jul 6, 2015 #19

    lavinia

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    Interesting. Thank you. Do you I understand you right to say that as the gas cloud collapses an enormous short lived star is born which then collapses into a black hole?
     
  21. Jul 6, 2015 #20
    Yes. The collapsing gas cloud will eventually achieve hydrostatic equilibrium, burn extremely briefly as a star, then continue collapsing into a black hole. It would not go directly from a gas cloud to a black hole in a single event, which is what a "direct collapse" black hole implies.
     
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