Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Galaxy rotation

  1. Jan 17, 2008 #1


    User Avatar
    Gold Member

    What is it that makes a galaxy rotate, is the gas it was formed from rotating?
  2. jcsd
  3. Jan 17, 2008 #2
    If I would say: Yes, the gas it was formed from rotating...,
    You could ask again: That means the Big Bang was rotating as whole thing... but why?

    No, I would rather be silent... o:)
    Last edited: Jan 17, 2008
  4. Jan 17, 2008 #3
    The matter was rotating locally when the galaxy formed, like local turbulent swirls in otherwise expanding gas. The interesting question is why such a rotation of the system as a whole is observed on galaxy scales but not on galaxy cluster scales.

    The FRW metrics, describing the homogenous approximation to the Universe, are not rotating as a whole. I expect non-rotation on scales for which the Universe seems homogeneous, currently 100 Mpc. Galaxy cluster scale is way below that so I asked in another thread why clusters are not rotating with zero answers.
  5. Jan 17, 2008 #4
    A small tornado can make a hundred turns in a second. A huge cyclone can make only one turn in a week; otherwise it would be unstable.

    All galaxy clusters, which had significant rotation, lost their galaxies billions of years ago. Now you can see only extremely slowly rotating clusters. Because they are stable.
  6. Jan 17, 2008 #5
    Is that your personal opinion or a theory that can calculate the scale above which rotating system in the Universe is unstable?

    Rotation in astronomy is measured by measuring rotating speeds via redshift not angular velocity. On average according to wikipedia tornadoes have speeds only 2-3 times bigger than cyclones. There are cyclones with wind speeds of average tornado (~ 150 km/h). Of course the cyclones are much bigger so their angular velocity is way smaller. Of course the whole example of tornadoes-cyclones is just analogy, the air on earth is way more cocentrated than the gas from which galaxies formed so frictional effects will come into play, it is not expanding, and there are extra complications from Coriolis forces which actually dictate the direction of revolution of cyclones.
    Last edited: Jan 17, 2008
  7. Jan 17, 2008 #6


    User Avatar
    Gold Member

    As every thing was much closer in the distant past, i would expect if there was any rotation in the BB plasma every thing would have the same rotation, as this is not so
    rotation must occure in issolated systems, (seperate clouds of gas), i have read about the conservation of angular momentum but why would this have a prefered direction in
    a vast cloud of gas.
  8. Jan 17, 2008 #7
    The Stability of Clusters of Galaxies
    van den Bergh, S.
    Zeitschrift für Astrophysik, Vol. 55, p.21

    Problem of the stability of clusters of galaxies.
    Karpowicz, M.
    Postepy Astron. 20: No. 4, 297-305(Dec 1972).
  9. Jan 17, 2008 #8
    Any references written in english?
  10. Jan 18, 2008 #9
    I doubt anyone knows, the theory of galaxy formation is in its infancy, not to mention cluster formation.

    I can think of analogy with non-magnetized ferromagnet (iron for example). It consists of magnetic domains (analogue of galaxies) within which the magnetic moment is homogenous, but the domains are randomly oriented with respect to each other. As a result, any macroscopic volume (analogue of galactic cluster) that contains many domains has a high probability for the domains to cancel out and very low probability to have appreciable magnetic field. Some physics of the material determines the sizes of the domains (the rotation of some galaxies).
  11. Jan 18, 2008 #10


    User Avatar
    Gold Member

    I thought that the central black hole may cause the rotation but it seems its gravitational range is far to short.

    http://curious.astro.cornell.edu/question.php?number=150 [Broken]
    Last edited by a moderator: May 3, 2017
  12. Jan 19, 2008 #11
    Gravitation doesn't create rotation if there is no angular momentum present from somewhere. For example, most elliptical galaxies are not rotating as a whole - the orbits of the stars are ellipses with all possible elongations and orientations.
  13. Jan 19, 2008 #12
    The mainstream textbook answer to the original question is that a galaxy begins rotating in its formation stage due to the tidal gravitational forces exerted by its neighbor galaxies. According to Peebles (p. 542, 1993):

    "In hierarchical scenarios, where protogalaxies are assembled by gravity out of lesser systems, the usual assumption is that the rotational angular momenta of the galaxies came from tidal torques. As we noted ... a protogalaxy in the process of breaking away from the general expansion is unlikely to be spherically symmetric; a better picture would be a messy blob moving away from an irregular boundary separating it from other developing protogalaxies. The unequal pull of neighboring mass concentrations on the material within a blob produces a velocity shear that in general leaves the protogalaxy with angular momentum of rotation."

    Peebles examines the possibility that the angular momentum of rotation is caused by turbulent eddys inside the galaxy, but the mathematical model seems to rule this out because it would require galaxies to be much more dense than we observe them to be.

    I also understand it is the consensus view that disk structures in spiral galaxies formed as secondary effects relatively recently (i.e., long after the galaxy itself formed). The disk structure we observe really only affects the baryonic (normal) matter distribution. The dark matter which forms the great majority of a galaxy's mass is believed to have remained distributed in the form of a spherical halo, the radius of which is somewhat larger than the visible size of the radius of the luminous content of the galaxy.

    Last edited: Jan 19, 2008
  14. Jan 25, 2008 #13
    Just wanted to add that it looked like smallphi hijacked this thread
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook