The "real" reason why some galaxies are flat

  • #1

Main Question or Discussion Point

I'm doing research on why some galaxies and other phenomenon like Saturn's rings are flat. There seems to be two answers. Here they are in shortened form:

1) When the galaxy is forming, there is some net angular momentum in some direction.Over time, all particles with an angular momentum pointing at some tilted direction will collide with other such particles, and will either be destroyed, ejected, or their tilts will be less extreme (Although I must admit I don't get who this will flatten it out, it seems to me this will only create a system where most particles are moving in the same direction). This appears to be the more popular explanation.

2) When spheres rotate, they are flattened along their axis of rotation due to the centrifugal force being stronger perpendicular to the axis of rotation. This is why the Earth is flattened a little bit. ICRAR in 2014 found that the faster a galaxy rotates, the thinner/flatter they are, which goes well with this explanation.

I'm confused which explanation is correct, or if both are - and if both, how they are related.
Thanks for any help.
 

Answers and Replies

  • #2
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I think it's a bit of both.
As the progenitor gas cloud contracts the more the angular momentum settles into some overall direction.
If the eventual rotation velocity of forming stars is not particularly fast you get an eliptical galaxy, an oblate spheroid.
At faster rates of spin though, your second point becomes more noticible, to the point where it has a dominant effect on the shape of the galaxy.
That is, faster spinning galaxies are more likely to assume a disc shape, and the fastest of all should be the flatest ones.
 
  • #3
DrSteve
Gold Member
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1) is much closer to reality, as the initial galactic cloud is nothing more than gas and dust, e.g., particles - certainly not a solid object like the Earth. The flattening occurs because though gravity is trying to collapse the cloud uniformly, the requirement that angular momentum be conserved prevents the collapse from fully occurring perpendicular to the axis of rotation.
 
  • #4
It's not a matter of being "closer to reality" or not. It's a matter of fact. Is 2 true, or is not true? Or is it perhaps true, but it doesn't contribute much to the overall effect?
 
  • #5
phyzguy
Science Advisor
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I think of it this way. An assembly of particles has two conserved quantities, the total energy and the total angular momentum. Friction between the particles allows kinetic energy to be converted to thermal energy, which can then be radiated away. So the assembly can lose total energy, but it has no effective way to lose total angular momentum. So it moves towards the state which has a minimum energy for a given amount of angular momentum, which is a rotating disk. So how 'flat' the system is is not simply a function of how fast it is rotating, meaning that your statement (2) doesn't really tell the whole story. It is determined by how far along the system is is this process of shedding energy. This is determined by a number of factors, including how much gas the system has and how old it is. The gas content is important because rotating gas clouds have friction which leads to energy dissipation, whereas systems containing only stars are nearly frictionless and so lose energy very slowly.
 
  • #6
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ICRAR in 2014 found that the faster a galaxy rotates, the thinner/flatter they are, which goes well with this explanation.
Don't faster rotation speeds also contribute to greater angular momentum? I would think over time, there would be more interactions and therefore conservation of angular momentum would produce a flatter disk more efficiently. Wouldn't this then also contribute to the first model?
 
  • #7
Don't faster rotation speeds also contribute to greater angular momentum? I would think over time, there would be more interactions and therefore conservation of angular momentum would produce a flatter disk more efficiently. Wouldn't this then also contribute to the first model?
Hmm, that escaped me but that does seem to be correct.

I would still consider my question to be open for further explanations.
 
  • #8
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Hmm, that escaped me but that does seem to be correct.

I would still consider my question to be open for further explanations.
Honestly I can't remember anything about centrifugal forces in my galactic courses. Although I don't know why 2 seems wrong (apart from the fact that galaxies are not rigid bodies), the popular view point is that galactic disks (at least 1D models) are formed through conservation of angular momentum, a rotationally distributed exponential profile and having a galaxy mass considerably less than the dark matter halo mass.
 

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