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Why do large elliptical galaxies not collapse due to their gravity?

  1. Oct 18, 2014 #1
    I have wondered this for a while now, what stops large elliptical galaxies from collapsing due to their gravity?
    Their rotational speed is relatively small, at the rotational axis perhaps zero, so what is preventing the stars from being pulled towards the center? Do spherical ellipticals flatten with time?

  2. jcsd
  3. Oct 18, 2014 #2


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    A galaxy's overall shape cannot change without friction. If a galaxy has enough dust in it, the friction between the dust and the stars will pull the stars into a disk (a spiral galaxy). If there isn't enough dust, the stars just continue in their individual orbits and the overall shape of the galaxy remains relatively unchanged (until the galaxy collides with another).
  4. Oct 18, 2014 #3


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    The article explains that the velocities refer to the bulk rotation. It's what you get when you average velocities of all stars. In elliptical galaxies stars go every which way, each individual one having large enough velocity to remain in orbit around the centre of galactic mass. In other words, if you had two stars going in opposite directions, their average velocity would be 0km/s.

    The discovery the article mentions reveals that the bulk rotation is not actually 0 as previously thought.
  5. Oct 18, 2014 #4
    Thanks for confirming Chalnoth, I think I worked out that one independently, because I had read spherical ellipticals are later stage galaxies following collisions of spirals, and I don't ever recall reading that spherical ellipticals could become spirals again.

    Thanks for confirming bandersnatch, so the individual star velocities are high enough to maintain relative orbital positions.
  6. Oct 18, 2014 #5


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    I think usually they remain elliptical because they just don't have any significant amount of dust to cause friction any longer. This is an active area of research, but I think it's generally expected that the behavior of the supermassive black hole at the galaxy's center is critical in determining the shape of the galaxy (when the supermassive black hole gets too bright, it strips the galaxy of dust).
  7. Oct 18, 2014 #6
    That was conventional wisdom, but it seems it has been challenged the last decade and now seems iffy with new observations targeted at the process:

    "Conventional wisdom (based on computer simulations from the 1970s) said that when two big disk-shaped galaxies merged, they’d create a big elliptical, a fairly featureless spheroid of stars.

    But about a decade ago, new-and-improved simulations by several teams suggested that, if the disk galaxies have a lot of gas, the object their merger creates will also be a disk galaxy, with spiral arms or maybe even a bar in the center.

    The reason is angular momentum. In a galaxy’s disk, stars and gas rotate together around the galactic center. When the disk galaxies merge and their material mixes, the stars can steal rotational energy from the gas via gravitational interactions, begetting a bulbous shape — but gas doesn’t steal from other gas. Instead, gas from both galaxies “shares” rotational energy. When there aren’t enough stars to steal the gas’s angular momentum, the material will inevitably settle down and create a disk in the newly formed galaxy."

    "[Detailed description of observations. Then:] In short, roughly half (46%) of the galaxies created by mergers are disk galaxies. The team can’t actually confirm that the galaxies that created the 37 they observed were also disk-shaped — but given that the long-gone progenitors had a whole lot of gas and the right rotational properties to create gas disks, it’s not crazy to connect the result with predictions for disks’ mergers.

    Regardless, the observations confirm that, indeed, disks are a common byproduct of galaxy mergers. That makes sense: big spirals are the most common type of galaxy in today’s universe, and so the fact that mergers can create them helps explain why."

    [ http://www.skyandtelescope.com/astronomy-news/mergers-create-disk-galaxies-10022014/ ]

    There is also a nice simulation that shows the merger-into-disk process.

    TL;DR: What Chalnoth said.
  8. Oct 19, 2014 #7
    Thanks Torbjorn, so would ellipticals happen after several mergers when the galaxies have become very large, or perhaps when two equal sized galaxies collide which are rotating opposite to each other, canceling out their angular momentums?
  9. Oct 19, 2014 #8
    You are welcome!

    If the conventional wisdom is incorrect, they would tend to happen because they are running out of gas over time (so could have time to participate in several mergers and grow large) and because their supermassive central black holes would tend to be disturbed, tend to interact, tend to grow and tend to scatter gas as Chalnoth describes.

    Mostly I thought the article was timely supporting what Chalnoth had said.

    It could perhaps happen, but the collision in itself would result in some excess angular momentum. (See their animation.)

    Cancellation would be a very low likelihood result.
  10. Oct 19, 2014 #9


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    Differential angular velocities of different parts are sufficient for shape changes. Or the dynamics of mass concentrations in spiral galaxies.
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