2013/07/12 by Nilanjan Banik, N. Banik, P. Sikivie · 2 citations
Physics and Astronomy · #Angular momentum #Astrophysics #Axion #Baryon #Classical mechanics #Cold Atom Physics and Bose-Einstein Condensates #Cold dark matter #Cosmology and Gravitation Theories #Dark Matter and Cosmic Phenomena #Dark matter #Galaxy #Galaxy formation and evolution #Galaxy rotation curve #Milky Way #Particle physics #Physics #astro-ph.GA #hep-ph
paper · pdf · doi:10.1103/physrevd.88.123517
33 pages, 3 figures
arxiv created 2013/07/12 · openalex publication_date 2013/12/09 · arxiv updated 2015/06/16 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
We analyze the behavior of axion dark matter before it falls into a galactic gravitational potential well. The axions thermalize sufficiently fast by gravitational self-interactions that almost all go to their lowest-energy state consistent with the total angular momentum acquired from tidal torquing. That state is a state of rigid rotation on the turnaround sphere. It predicts the occurrence and detailed properties of the caustic rings of dark matter for which observational evidence had been found earlier. We show that the vortices in the axion Bose-Einstein condensate (BEC) are attractive, unlike those in superfluid 4He and dilute gases. We expect that a large fraction of the vortices in the axion BEC join into a single big vortex along the rotation axis of the galaxy. The resulting enhancement of caustic rings explains the typical size of the rises in the Milky Way rotation curve attributed to caustic rings. We show that baryons and ordinary cold dark matter particles are entrained by the axion BEC and acquire the same velocity distribution. The resulting baryonic angular momentum distribution gives a good qualitative fit to the distributions observed in dwarf galaxies. We give estimates of the minimum fraction of dark matter that is composed of axions.