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A question about the flatness of galaxies

  1. Sep 5, 2015 #1
    Good day to you all,

    I'm very sorry since there are countless of topics that have discussed this question before. Call me dumb, but I've read almost every single one of them and I'm still having a bit of a hard time understanding why most galaxies have their planets, stars, dust, gas etc. all rotating in one 2D plane. If someone could explain this to me in simple layman terms I'd appreciate it very much.

    I keep seeing people talking about conserving the angular momentum but I don't get HOW the angular momentum gets conserved that way. How does the angular momentum look like before and after the galaxy being a flat disc formula-wise?

    My theory what I think so far, is that a galaxy always starts with two relatively large particles orbiting each other. They got larger because smaller dust particles interacted and fused together. Since they're larger than other particles and thus having a stronger gravitational pull, that means all the other smaller particles get pulled towards them. The plane on which the 2 large particles orbit each other in the beginning, will be the very same 2D plane on which ALL of the other particles will eventually orbit. Is this correct?

    Looking forward to your explanations
     
  2. jcsd
  3. Sep 5, 2015 #2
    Not all galaxies are disk shaped, although many of them are.
    What it comes down to is that galaxies condense out of gigantic accumulations of primordial gases, (and very probably dark matter is involved too).
    The component parts (atoms etc) of the condensing material all have some individual momentum, but while the structure is huge and diffuse this doesn't have much effect on it as a whole.
    However,when the structure gets dense enough then tiny differences of angular momentum carried by the parts accumulate and it acquires an overall spin direction.
    The spin direction which eventually predominates is random, and sometimes no particular direction 'wins', so the resulting galaxy is not disk like, instead it is globular or eliptical.
    It comparable to a ballet dancer who can start with slow turning then speed it up to a rapid spin when she pulls her arms in.
    The same effect applies to solar systems which form with planets in roughly the same plane.
     
    Last edited: Sep 5, 2015
  4. Sep 5, 2015 #3
    So if I understand correctly, you're comparing it to a ballet dancer because before she pulls her arms in, she spins relatively slowly because of the many different angular momentums the galaxy particles have that gives a result of a small net angular momentum? Eventually, one large angular momentum starts to predominate that accumulates and cancels all the deviating momentums?

    Is this correct?
     
  5. Sep 5, 2015 #4

    Vanadium 50

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    Before understanding why something is true, it is important to understand if it is true. Not all galaxies are in a plane - ellipticals are not, dwarf spheroidals are not and irregulars are not. And not all planets are in the same plane - our own solar system is not: it's tilted by 63 degrees.

    We don't discuss personal theories on PF.
     
  6. Sep 5, 2015 #5
    That's why I said most galaxies ;). I'm aware that there are other types. Regarding planets not being in the same plane, what I meant is that they are more or less if you look at a disk galaxy from a sufficient distance.



    I was showing my personal understanding of how I think it works so that someone could enlighten me if my understanding is correct or wrong. Hence my question at the end: "Is this correct?"
     
  7. Sep 5, 2015 #6

    Vanadium 50

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    Still not true. Most galaxies are irregulars and dwarfs. Most known galaxies are spirals, because they are big and bright.

    Again, not true. The angle between the solar system's plane and the galactic plane is 63 degrees, no matter how far away from it you are.

    People before you posted personal theories, thinking a question mark at the end would make it OK. Many of them aren't here any more. I suggest you not go down this path.
     
  8. Sep 5, 2015 #7
    Which is exactly what I meant with "most" galaxies since they are known to us. Apologies if I wasn't clear enough on that.

    What I'm talking about is the fact that the development and shape of these galaxies are explained by using angular momentums to explain their more or less 2D plane. Ofcourse, there are always some irregularities but it's about the general concept to explain its shape from a larger point of view. That's why there are macroscopically other shapes of galaxies for which there are other explanations.


    My intention wasn't to give a theory at all and put a question mark behind it to make it look OK. It was just a matter of expression to explain how I understood it and make it subject to correction.
     
    Last edited: Sep 5, 2015
  9. Sep 6, 2015 #8

    Vanadium 50

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    63 degrees is not "more or less" in a 2-D plane. It's almost perpendicular to the plane.

    You're asking us to explain something that isn't true.
     
  10. Sep 6, 2015 #9
    I never specifically referred to our galaxy in particular, you brought it up as an example. As I said, I'm referring to galaxies that are more or less on a 2-D plane in general. And I see a lot of people acknowledge that there are galaxies that are more or less on a 2D plane for which they use angular momentum to explain them, just like rootone here did. That's why I'd like to understand how they actually explain it since for them, it is true that they are more or less disk shaped.
     
    Last edited: Sep 6, 2015
  11. Sep 6, 2015 #10

    marcus

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    I think your paraphrase of Rootone's explanation is, in fact, correct. And shows a good physical understanding. The above strikes me as a brief constructive exchange where you asked a question and quickly learned something---an example of PF working at its best. FWIW I liked both these posts very much.

    We could try to build constructively on these two posts and bring in some more detail. I think that large ELLIPTICAL galaxies can be formed by the merger of two or more SPIRAL ones
    where the spirals are tilted differently---have different planes of rotation. So they cancel and disrupt each other: the merger destroys their planar spiral structures and produces more of a simple ellipsoid blob.

    Of course planetary systems like the solar system are a different topic from galaxies! the word "galaxy" is reserved for large collections of stars--e.g. billions. But there is a similar mechanism (involving the conservation of angular momentum) that tends to make both the small and large systems develop a plane of rotation.

    One can imagine both large and small systems condensing from roughly spherical clouds of gas and dust.
     
    Last edited: Sep 6, 2015
  12. Sep 6, 2015 #11
    Hey Marcus! Thank you so much for verifying my explanation, I'm happy I got right. That's what I love about PF; I don't hesitate to post questions and I always get very clear answers.

    I'd love to go into more detail about this. I'd really like to know first exactly HOW eventually one particular angular momentum predominates. Is it because of gas or dust particles eventually interact and get fused together? If that's the case, how does the angular momentum formula (L = m x v x r) change so that v gets larger and thus the spin gets faster? I'm seeing that m would get larger, but r would get smaller (2 particles get closer and fuse together) so that there is no need for v to change to conserve the angular momentum. I'm probably looking at this the wrong way.
     
  13. Sep 6, 2015 #12

    marcus

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    There are quite a few members who can answer that kind of question better than I, hopefully some will jump in. For me the first thing that comes to mind is the CANCELLATION of both ordinary linear momentum and angular momentum, when stuff is going in opposite directions and crashes and heats up and radiates the energy as heat radiation.

    there can be a lot of individual angular momentum to start with, as I picture it, but a lot of it gets canceled. The conservation law is only for the TOTAL. Rotation in opposite directions can cancel and only a net residue remain.

    Actually it is 8:20 AM here and I am not fully awake. I may have to go get coffee and discuss this later in the day. There is a beautiful calculation device called the CROSS PRODUCT which represents rotational momentum as a VECTOR. where the vector points along the axis of rotation and its length represents the angular momentum. And two rotations which are in opposite directions have vectors pointing in the opposite direction. So you can add vector-wise and get cancelation. L = mv x r. In this context the "x" is reserved for the cross product of two vectors. I bolded the vectors.

    But that's a technical issue about notation. Intuitively the cloud wants to collapse, and after all the cancelation of all that random internal motion there is some small amount of NET RESIDUAL rotation which prevents it from completely collapsing. A net residual L vector, which defines the axis, and defines the plane of rotation. So the roughly spherical cloud wants to collapse to a point but because of that residual rotation it cannot manage to do that. The best it can do is collapse to a plane. Its own remaining rotation (after cancellation) interferes with complete collapse. Anyway that's how I think of it, which may or may not be a good way to picture it.

    And that could happen on a small scale of one star and some planets forming---or on the "galaxy" scale which means millions and even billions of stars. There are some computer simulations of galaxy formation which are nice to watch if you can find them on google.
     
  14. Sep 6, 2015 #13
    Imagine a galaxy with two planes of rotation. The planes must intersect. There would be collisions at the intersection that would disrupt the two planes. I don't know what would happen, but would guess that either the whole thing would become a mess or the thing would devolve to one average plane of rotation.

    Don't elliptical galaxies have a plane of rotation?
     
  15. Sep 6, 2015 #14

    marcus

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    That would be atypical, I think
    ==quote https://en.wikipedia.org/wiki/Elliptical_galaxy ==
    ... Unlike flat spiral galaxies with organization and structure, they are more three-dimensional, without much structure, and their stars are in somewhat random orbits around the center. ...
    ...
    ....The motion of stars in elliptical galaxies is predominantly radial, unlike the disks of spiral galaxies, which are dominated by rotation.
    ==endquote==
    There are flattened-looking ellipticals, suggesting rotation, but according to this article the predominant type of orbital motion in most is radial, that is stars falling in towards center passing thru central region and going out radially then turning around and falling back in. Not organized into mainly circular orbits like our galaxy. More random and incoherent, more skinny elliptical orbits rather than rounded type..
     
    Last edited: Sep 6, 2015
  16. Sep 6, 2015 #15
    Amazing! I had no idea. The radial motion is particularly astounding to me.
     
  17. Sep 7, 2015 #16

    Chronos

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    Galaxies rotate, so what is difficult about pictuing them flattening out? Big galaxies like ellipticals flatten less because they are massive.
     
  18. Sep 7, 2015 #17
    If a N-body system has significant interactions between its constituent bodies, the orbital angular momentum will average out and the system will end up with most bodies rotating close to its "equatorial" plane.
     
  19. Sep 7, 2015 #18
    I'll definitely try out that cross product calculation device. Your explanation in your post is the same as how I thought of it, which is relieving.

    Can I say that the velocity of the planets, stars, etc in the 2D plane is (almost) the same as the escape velocity from the quasar (or any other center of the galaxy for that matter)? If so, could one think that canceling out the smaller deviating angular momentums of particles causes these particles to have a smaller velocity than the escape velocity and thus crash or collapse, resulting into only one plane which has just the right escape velocity for not collapsing into the center?

    @Hornbein : Thanks for your clear explanation. Elliptical galaxies seem to not have a plane of rotation. I think they have multiple angular momentums which are equally strong but have different directions/vectors. I could be wrong though, I should read more about them.
     
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