I Why Do Galaxies Spin and What Drives Their Motion?

[berkeman]

Thread is reopened after cleanup of a post with misinformation in it.

 

[ohwilleke]

I found this link which discusses spiral, lenticular and elliptical galaxies in terms of their angular momentum and 'bulge factor'. It has references from the 1990s and later. Well done if you can read every word!

Imo, it's important to understand that spiral arms are not like spokes on a wheel and they don't rotate 'in step' with the individual stars' orbits. The stars in other galaxies also orbit at similar rates - they would have to or collapse into the centre PDQ.

Incidentally, while it is very easy to piece together an argument that most galaxies should have angular momentum, the scaling relations discussed in the linked paper, which are very regular, are harder to explain and require a much more involved model than the bare bones ones offered up in this thread.

 

[Mendrys]

This is new and describes other processes that contribute to the spin of a galaxy.

https://phys.org/news/2022-10-flips-galaxies-cosmic-web.html

This is from the article and sums it up a bit:

"Galaxies which are mostly disk, with a low-mass bulge, tend to have their spin axis parallel to the nearest filament. This is because they form mainly from gas falling onto the filament and 'rolling it up.' Galaxy bulges grow when galaxies merge, generally as they move along the filament. So, mergers also tend to 'flip' the alignment between the galaxy spin and the filament from parallel to perpendicular."

 

[sophiecentaur]

This is new and describes other processes that contribute to the spin of a galaxy.

That sums up what we would be after. The 'why' question is never very fruitful. All we can say we know is that there is unlikely, ever, for the component parts of a nebula to have net zero angular momentum. So you have your input spin from the very beginning and it is all about the statistics of many body interactions.