Comment by WazWaz on 23/09/2021 at 13:24 UTC

47 upvotes, 1 direct replies (showing 1)

View submission: Why is the dark matter halo spherical?

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True, and interesting! Are there any large scale phenomena that involve a significant amount of elastic collision?

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Comment by Aseyhe at 23/09/2021 at 13:41 UTC*

53 upvotes, 1 direct replies

One hypothetical example is self-interacting dark matter. In that model, dark matter halos actually become more spherical than halos of non-self-interacting dark matter. This paper[1] has some example pictures (Figure 7).

1: https://academic.oup.com/mnras/article/472/3/2945/4104642

Some background: one major motivator for self-interacting dark matter is that it can produce halos whose central density does not go to infinity. This could potentially match observations better. It is not yet clear that there is actually a discrepancy between non-self-interacting dark matter and observations, though, for two reasons:

• The gravitational influence of the ordinary matter can smooth out the central halo density even without any dark matter collisions.

• The dark matter distribution within halos has to be inferred indirectly by, for example, the motions of stars. The precise dark matter distribution that you infer depends on assumptions about the three-dimensional stellar distribution (which you only observe in the line-of-sight projection). Here's[2] an interesting relatively recent paper on that.

2: https://academic.oup.com/mnras/article/474/1/1398/4590054