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Non-spherical structures.

  1. May 12, 2005 #1

    ohwilleke

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    Most of the stuff in astronomy is spherically symmetrical, or very nearly so. Stars, galactic cores, and planets all seem to be.

    I can think of exceptions of such the arrangement of matter in disk like solar systems or the outer portions of spiral galaxies, small irregular asteroids, nebula, and "superstrands" that impact large scale structure.

    Can anyone think of other exceptions?
     
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  3. May 12, 2005 #2
    most merging systems are no longer spherically symmetric. Not many galaxies are spherically symmetric.

    I think you should rephrase your first line to read "A lot of the stuff in astronomy is assumed to be spherically symmetric for ease of calculation"
     
  4. May 13, 2005 #3

    SpaceTiger

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    Asphericity is more common in astronomy than one might think. matt.o is absolutely right that it's often just assumed for simplicity and that the visible part of galaxies generally doesn't take a spherical form (though the dark matter halo is assumed to be spherical in most cases).

    Next to the sphere, probably the most common shape in astronomy is the disk. Not only do some galaxies show this form, but we see disks around compact objects (like neutron stars), young stars, and even a planetary disk around our own star. Disks are common because gravity has a tendency to cause clouds of matter to collapse, and with conservation of angular momentum, small initial rotation rates will be amplified. When the rotation rate is large, there is a new symmetry along the axis of rotation that cannot be neglected in the dynamics. In the absence of this rotation, there is no reason the system should prefer any direction and a sphere is the natural shape to take on.

    A shape that's often considered when talking about galaxies is the triaxial ellipsoid. The stars of elliptical galaxies are often modelled this way. We're still trying to determine whether or not our own galaxy has a triaxial component.

    Although often modelled as spheres, galaxy clusters are actually highly aspherical. Here's the weak lensing map of one such cluster:

    http://astro.uchicago.edu/home/web/joffre/comb3266.gif

    The reason the mass distribution isn't as spherical as, say, stars, is that it hasn't had nearly enough time to relax into a lowest energy configuration. Larger objects take longer to do so and the universe is still young relative to the relaxation time of a galaxy cluster.

    Finally, as you mentioned, there are filamentary structures in the large-scale distribution of matter. One expects this from the Zeldovich approximation of an object collapsing under gravity with negligible rotation. Basically, an object with some random shape initially will tend to collapse along its smallest principal axis first, forming a pancake. Then, it will collapse along its second shortest axis, forming a filament. Part of my thesis project is actually to determine whether the large-scale mass distribution is dominated by filamentary structures or "pancake" structures (also called "walls").
     
  5. May 13, 2005 #4

    tony873004

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    asteroids...
     
  6. May 15, 2005 #5
    I guess asteriods are aspherical but if it is big enough then it's gravity will pull it into a sphere. That's why planets and stars are spheres. A galaxy can be disk shaped just because that is the way the planets and stars aligned themselves...
     
  7. May 19, 2005 #6

    Nereid

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    Really cool ST! And will there also be a collapse along the 'third axis' (to form roughly spherical blobs)? (BTW, what cluster is this? and where can we read more?)

    Welcome to Physics Forums, Mike Centore!

    To an OOM or three, you're right; however, we must not forget rotation (Jupiter is certainly NOT a sphere!), nor material strength (e.g. Vesta, also certainly NOT a sphere).

    To add to what Space Tiger wrote, when the detailed studies of galaxy clusters are finally done, we find that they are not (generally) spherical; and the (very few) detailed studies of the DM distribution in rich clusters also shows an 'as-yet-not-relaxed' picture; for example.
     
  8. May 19, 2005 #7

    SpaceTiger

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    Yup. That's basically what we expect, the infall of matter into clusters and along filaments.


    Abell 3266. Can you be a bit more specific about what you'd like references for? I don't know of anything that specifically addresses this question, but I could give references on many of the details covered above.
     
  9. May 19, 2005 #8

    Nereid

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    Thanks ST.

    Now I know the cluster, I can (most likely) use Google Scholar to find any relevant papers.

    I was asking mainly about where one could read up on the weak lensing analysis of this cluster, and see if there were more (e.g. SZE, analysis of X-rays from the IGM, estimates of cluster mass from the Virial Theorem, ...), and also to see where - in the spectrum of richness and compactness - this particular cluster fit.

    Will you be doing analyses related to rich clusters? quasar clusters??
     
  10. May 20, 2005 #9

    SpaceTiger

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    Try here


    I'll be focusing on filamentary structures, though I have a colleague doing cluster-finding in SDSS.
     
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