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I have been told that galaxies will never stop rotating because conservation of angular momentum

But, there are planets inside of it can travel through dense nebulae and bodies of gas that would cause friction (and probably brake their travel speed through the Galaxy). Wouldn't that brake the galaxy's rotation?

Also, gravitational waves can be emitted by rotating massive objects that have uneven mass distributions by so called quadrupoles or higher poles in their angular momentum. Also, in this study (https://www.space.com/dark-matter-slows-milky-way-rotation) they showed evidence that the galaxy rotational speed is decreasing due to dark matter. Then, would galaxies (and all their contents: nebulae, stars, planets...) ever stop rotating? Or they would only decrease their velocity but it would stabilize at some point?

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Conservation of angular momentum applies to a closed system that suffers no external torques.

Obviously if torques are applied or angular momentum can be lost then rotation will slow down. Alternatively, angular momentum can be transferred between components in a system resulting in one component rotating faster and another slower.

In the work you refer to in your question (Chiba & Schonrich 2021), the authors have found evidence that some of the angular momentum of the bar in the Milky Way has been transferred outwards to the dark matter halo. The mechanism that applies a torque is dynamical friction. As a result, the bar is claimed to rotate about 24% slower than when it was formed.

Whilst the visible matter in a galaxy may slow its rotation by these means, the angular momentum is just being transferred to the dark halo. The process would become slower and slower as the two components matched rotation rates.

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  • $\begingroup$ Thanks. So, the other two cases that I mentioned (planets passing through very dense nebulae losing speed by friction and emission of gravitational waves of certain asymmetric bodies with uneven mass distributions) would not affect the roation of galaxies? @ProfRob $\endgroup$
    – vengaq
    Jul 17, 2023 at 11:18

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