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How do galaxies form and move?

  1. Mar 30, 2007 #1
    Just from general ideas gained over time I am under the impression that structure in our universe started with slight anisotropy in the CMBR. That slight fluctuation is gradually compounded and leads to structure, i.e. galaxy formation.

    Building on that idea I am concluding that black holes form inside galaxies and a balance is reached in the distribution of mass in the galaxy so the central mass usually represents about 1% or 2%, perhaps regulated by the jets near the center and accretion at the outer reaches?

    Having asked this question a year or so on a forum about whether galaxies form from the black hole or if the black hole forms inside the galaxy I have found that both views were supported.

    Is there now any prevailing view on this matter?
  2. jcsd
  3. Mar 30, 2007 #2


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    Unless there is a miswording there, a galaxy cannot form "from" a black hole, since matter can't come out of the black hole to form a galaxy.
  4. Mar 30, 2007 #3
    I think that view included the mass of a black hole attracting the matter that formed the galaxy, not the galaxy coming out of the black hole.
  5. Mar 30, 2007 #4


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    Since the total gravitational field strength of a giant cloud of "stuff" is the same for the same amount of mass regardless of whether there are black holes in the middle or not (and doesn't change before or after they form), I don't see how it could be said that the black holes pull in the matter to form the galaxy.

    That sounds a lot like the common "cosmic vacuum cleaner" misconception about black holes.
  6. Mar 30, 2007 #5
    Agreed. I'm trying to get to the current view of galaxy formation. Is my impression and conclusion in the OP anywhere near an acceptable description?
  7. Mar 30, 2007 #6


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    Galaxy formation is an extremely complex process of hierarchical structure formation with dissipative collapse of baryons, formation of dark matter halos, collisions and mergers, and feedback effects of star formation. A good pedagogical overview is given here:


    To my knowledge the role of supermassive black holes in that process is not fully understood.
    Last edited: Mar 30, 2007
  8. Mar 30, 2007 #7
    Thank you for the link. That is a very good explanation. I see the graphic at the beginning showing the Big Bang, then the Dark Ages, then reionization.

    Can you tell me where photon decoupling and the origin of the CMBR is on that time line? Is it at the start of the Dark Ages?

    Is there a connection between the anisotropy of the CMBR and the formation of galactic structure?
  9. Mar 30, 2007 #8


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    Yes, at the time indicated as ~300,000 years after the Big Bang

    Yes, tiny fluctuations seen in the CMBR are what develop into the structure we see in the universe. The precise mechanics of galaxy formation is not well understood as suggested above by others, but what that means is we can't perfectly model the way in which those fluctuations grew and turned into the structures we see. In the end, without those fluctuations having been there we would not have had galaxies form.
  10. Mar 30, 2007 #9
    Thank you.

    OK, so given that we don't yet know precisely, we can make the connection between galaxy formation and the fluctuations in the CMBR.

    So the formation of black holes in the center of galaxies is the next part as I mentioned in the OP. I often am wrong but logic would suggest that as galaxies form there is a natural process that enables them or requires them to form a black hole. Is my logic good or am I off on this?

    It appears that there is a balance in the distribution of the mass as I mentioned, with 1% or 2% positioned in the black hole. Is this the case or am I working with wrong or out dated ideas? Do we know anything about this process or do we have ideas of how such a balance could be maintained?
  11. Mar 30, 2007 #10


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    Remember that black holes also form because of the fluctuations seen in the CMBR, we are just not completely sure of all the details leading to the kinds of arrangements of galaxies and black holes that we see.

    I'm not quite sure what you mean by asking how 'such a balance could be maintained'. Are you asking how a black hole could sit in the middle of a galaxy and not gobble it up? If so then you may have been mislead by the "cosmic vacuum cleaner" misconception as suggested by Russ previously. There is nothing particularly special about black holes in the macroscopic sense, they are just a gravitating body so things can happy orbit them as easily as anything else. We don't worry about why the Sun dosn't gobble up the solar system and the same reasoning applies for galaxies and black holes. Apologies if this is not what you were thinking, I wasn't exactly sure what your question was asking.
  12. Mar 30, 2007 #11
    I understand. I'll check again in a year or so :).
    I am basing that question on my impression that there is a black hole in most galaxies and that it makes up about 1.5% of the mass of the galaxy. This could be completely wrong so let's clear that up first. Are you familiar with this 1.5% relationship?
  13. Mar 30, 2007 #12


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    Sorry I'm not familiar with any estimate of the standard black hole to galaxy mass ratio.

    Perhaps I and others have been too negative about what we know about galaxy formation. We know a hell of a lot, we just don't yet claim to have the full picture, so 'checking again in a year or so' might not be the best option. You could spend a year learning what we do know so far about galaxies and you still wouldn't have learned it all!

    When (if?) we do work out exactly how galaxies form it won't be in the form of a several sentence answer that you could post on a forum! In basic terms we do have a handle of galaxy formation, the unknown is in the details.
  14. Mar 30, 2007 #13
    Thank you. In this case just knowing that there is a relationship between anisotropy of the CMBR and galaxy formation, and that we know a lot but we don't know the exact details is good enough.

    One more thing while I have a thread going about galaxies. I think it is safe to say that galaxies are in motion relative to each other. I think it is safe to make a relationship between the concept of expansion and the relative motion of galaxies; in general they are all moving away from each other.

    I think it safe to say the if accelerating expansion is the case, that galaxies would appear to be moving away from each other at an accelerating rate.

    My question is, if accelerating expansion is the case, can we conclude that in terms of closed, flat or open, the universe would be open? In other words, there is no indication in theory that deceleration would ever occur given the case of acceleration?
  15. Mar 30, 2007 #14


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    This is a reasonable statement, with some caveats about the 'moving away' part, since thinking entirely kinematically about receding galaxies implies that they can move apart at greater than the speed of light, which can't be explained kinematically. This question however is a can-o-worms that has been opened in many other threads, so it's probably best not to confuse this present discussion with it! So yes, in basic terms your statement that the galaxies appear to be moving away from each other at an increasing rate is reasonable.

    The simple closed (universe collapses in big crunch), open (expansion rate goes to a linear relationship with t) and flat (the balance line between the two) picture you may have seen is only valid for matter only universes. In this case the universe model depends only on the total amount of mass relativity to a critical value.

    You cannot get acceleration in this picture at all, since matter will not accelerate the expansion, you need dark energy to do this (in the standard picture anyway) Once you add dark energy into the picture the universe models become more complex, since it depends not only on the amount of dark energy but also its properties. So you could have a flat universe that has a big crunch or a big rip or a closed universe that never re-collapses etc etc. It all depends on the properties of the dark energy.

    This attached image may help to guide your understanding of our current best guess model of the universe. What it shows is that initially the universe was in fact deccelerating as matter was the dominant energy component. At a certain time (roughly 5-6 Billion years ago I think, somewhere around there) dark energy became the dominant component in the universe, and began to cause the expansion to accelerate. So the theory does in fact have a deccelerating and accelerating phase. Once you add dark energy the flat, closed and open models are not a very useful tool to understanding the dynamics of the expansion.

    The reason that matter dominates early on and dark energy later is that the density of dark energy stays roughly the same for all time (if dark energy is a cosmological constant it stays exactly the same for all time). By contrast the density of matter drops with the inverse cube of the scale factor of the universe, since if you double the volume of the universe, but still only have the same amount of matter then the density will halve. Therefore the greater the total expansion since t=0 the more dominant dark energy will be over matter. Whichever component has the greater density at a given time will dominate the dynamics of the universe, early on matter, later dark energy.

    Another way to think about this might be that if dark energy is vacuum energy, i.e. some inherent energy associated with space itself (which is a possible candidate for dark energy, and almost equivalent to a cosmological constant) then as the universe expands there is more space in the universe hence more dark energy in total. By contrast the total amount of matter stays the same, reducing the matter density as the expansion proceeds.

    Attached Files:

    Last edited: Mar 30, 2007
  16. Apr 1, 2007 #15
    Wallace, thank you. I am very impressed with your post.

    I asked the moderator to change the name of the thread from "How galaxies form" to 'How galaxies form and move" in hopes I could carry on more of this discussion.

    Before I venture to pose some additional questions though, I am waiting to hear back from the moderator on guidelines that describe the extent that I can bring up speculative ideas about the thread topic. I'm not promoting any over speculative ideas but in the normal course of thinking through what you have said and thinking about the thread topic, thoughts enter my mind. I'm am sure that others have had these same thoughts and thoughts lead to questions, some of which display various degrees of speculation.
  17. Apr 6, 2007 #16

    “The greatest concentrations of star formation are found in the so-called starburst regions near the ends of the galaxy’s strong galactic bar.”

    This article and the NGC 1672 galaxy seem to lead to the conclusion that the galaxy may be growing by accumulating “dust”.

    That idea supports my thinking about galaxy formation and the sequence of events starting from the slight anisotropy that existed in the CMBR and the future structure of our universe, i.e. galaxy formation.

    Any thoughts on this observation?
  18. Apr 6, 2007 #17


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    How did you so conclude?
    Sounds rather speculative - have you written a paper on this yet? Or worked - or sketched - out the mechanisms by which this might happen?
  19. Apr 6, 2007 #18
    Thanks for the warning

    Hello Nereid.

    Did you read the article? Paragraphs two and three lead to the conclusion I made about the article. Maybe you have an different opinion? Please contribute (or did you just want to let me know that you are watching).
  20. Apr 7, 2007 #19


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    I read it twice the first time, and once again just now ... I still can't see how you came to that conclusion!

    That barred spirals somehow funnel gas into the nucleus is hardly news - I recall an ESO PR on this several years ago, for example, and you'll find lots of papers on the topic if you do some searching on http://adswww.harvard.edu/" [Broken].

    This PR, and the topic in general, is about how (spiral) galaxies are evolving "today"; there's little, if anything, from this research that points directly to mechanisms of galaxy formation.
    Last edited by a moderator: May 2, 2017
  21. Apr 7, 2007 #20
    You're right. I like that ADS site. I've added it to my favorites.
    All we have to go on is what we see happening today, right? What research do you look at for mechanisms of galaxy formation. What scenario is there to link it to anisotropy of the CMBR?
    Last edited by a moderator: May 2, 2017
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