What caused galaxies to start spinning?

AI Thread Summary
Galaxies require an external force or pre-existing angular momentum to begin spinning, as gravity alone cannot induce rotation in a uniform gas blob. The discussion highlights that a gas cloud with zero total angular momentum is unlikely to exist in reality, as even minor velocity variations can create non-zero angular momentum. When such a gas cloud coalesces, it can form multiple spinning galaxies while maintaining overall zero angular momentum. The key takeaway is that the initial conditions of the gas, including any inherent angular momentum, play a crucial role in the eventual spin of galaxies. Ultimately, the smallest disturbances during contraction can initiate rotation, making non-rotating galaxies practically nonexistent.
fbs7
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If I take a big, uniform blob of gas that is not spinning, and mark its limit as the limit of a system, then even if that big blob of gas coalesces to make a galaxy, it will not start spinning by itself: its own gravity is an internal force, and conservation of angular momentum says I need an external force to add angular momentum to the system.

As galaxies spin, either:
(a) my big blob of gas was already spinning before it started to become a galaxy, or
(b) there was some external torque applied to my system

So, what's the best explanation as to why galaxies spin?
 
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fbs7 said:
If I take a big, uniform blob of gas that is not spinning, and mark its limit as the limit of a system, then even if that big blob of gas coalesces to make a galaxy, it will not start spinning by itself: its own gravity is an internal force, and conservation of angular momentum says I need an external force to add angular momentum to the system.
A quasi uniform gas cloud with zero total angular momentum, can form many spinning galaxies while preserving zero total angular momentum. They spin around different axes in different directions.
fbs7 said:
As galaxies spin, either:
(a) my big blob of gas was already spinning before it started to become a galaxy, or
(b) there was some external torque applied to my system
It's (a). The gas that later forms a single galaxy already has non zero total angular momentum. The closer it gets together, the higher the angular velocity becomes.
 
A.T. said:
It's (a). The gas that later forms a single galaxy already has non zero total angular momentum. The closer it gets together, the higher the angular velocity becomes.
Ah, I see. So there's no such thing in reality as a uniform blob of gas with perfectly zero angular momentum relative to its center of mass. It will have some variations in velocity, and these variations create a non-zero angular momentum. The bigger the blob, the bigger the angular momentum generated by these irregularities.

Now that you explained, it seems obvious to me - the smallest disturbance will get the thing to spin as it contracts.

Thanks for the explanation! :smile:
 
fbs7 said:
Ah, I see. So there's no such thing in reality as a uniform blob of gas with perfectly zero angular momentum relative to its center of mass.
It is statistically unlikely. And, event there was one, then it would not form a Galaxy, just collapse radially into a BH. So you never see those non-rotating Galaxies.
 
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