What caused galaxies to start spinning?

[fbs7]

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?

 

[A.T.]

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.

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.

 

[fbs7]

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!

 

[A.T.]

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.

 

[PJC01]

The most likely explanation for why galaxies spin is due to the conservation of angular momentum. As stated in the content, a big, uniform blob of gas will not start spinning by itself, and an external force is needed to add angular momentum to the system. This external force could come from a number of sources, such as the initial rotation of the gas cloud before it formed into a galaxy, interactions with nearby galaxies, or the gravitational pull of dark matter. These external forces can cause the gas cloud to start spinning, and as the gas collapses and forms into a galaxy, the conservation of angular momentum causes the galaxy to continue spinning. This is similar to how a figure skater spins faster when they bring their arms in closer to their body, due to the conservation of angular momentum. Therefore, it is likely that some external force played a role in initiating the spinning motion of galaxies.