Axions and the Galactic Angular Momentum Distribution

Abstract

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 ${}^4$He 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 axions.

33 pages, 3 figures

References in corpus (5)

• Bose-Einstein Condensation of Dark Matter Axions
• Ultra-Light Scalar Fields and the Growth of Structure in the Universe
• The Caustic Ring Model of the Milky Way Halo
• Empirical distributions of galactic $\lambda$ spin parameters from the SDSS
• Evolution and thermalization of dark matter axions in the condensed regime

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