For visible matter, due to spin dominance over gravity while formation of galaxy, planar symmetry of a galaxy is achieved and the galaxy will be planar (spiral). But dark matter's distribution is spherically symmetric around the galaxy; why is this?

I don't much know about dark matter interactions so please give a physical explanation. You can also give a mathematical explanation.

  • $\begingroup$ Dark matter interacts gravitationally and gravity is isotropic which leads to a spherically symmetrical distribution. $\endgroup$ – Photon Sep 16 '18 at 16:39
  • $\begingroup$ @Photon your comment do not answer my question. Spin effects are also present, so if i rephrase my question, why gravity dominant over spin for dark matter? $\endgroup$ – Aman pawar Sep 16 '18 at 16:42
  • $\begingroup$ @Photon Brief answers should still be posted as answers, rather than as comments. $\endgroup$ – rob Sep 16 '18 at 16:51
  • $\begingroup$ Comment to the question (v2): elliptical galaxies outnumber "planar" (spiral) galaxies. $\endgroup$ – rob Sep 16 '18 at 16:55
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    $\begingroup$ @PeterErwin I appear to have misremembered: elliptical galaxies are a minority, except possibly at extremely large masses. One source. $\endgroup$ – rob Sep 16 '18 at 23:14

The spin dominance is established for ordinary matter due to the gravitational collapse to the plane of rotation. The collapse is based on losing the kinetic energy through electromagnetic interactions. Dark matter does not interact electromagnetically, does not lose the kinetic energy, and therefore does not collapse to the plane of rotation. The halo remains elliptical when it rotates, but does not collapse to a disc.

EDIT: As @rob points out in the comments, there exist a great number of elliptical galaxies.

The reason for this is the same. For the spin dominance to be established, sufficient electromagnetic interactions should be present, such as collisions (of gas, stars, dust, particles, etc.). In an ideal elliptical galaxy with only stars and vacuum, the stars do not interact electromagnetically with each other (disregarding the radiation and direct collisions), but only gravitationally similar to the dark matter particles. For this reason the stars would not lose the kinetic energy, the spin dominance would not be established, and the galaxy would remain elliptical for a long time.

  • $\begingroup$ "The halo remains elliptical when it rotates, but does not collapse to a disc" It should be ellipsoidal not elliptical. $\endgroup$ – Aman pawar Mar 9 at 8:10

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