Axions and the Galactic Angular Momentum Distribution

TL;DR

This paper investigates how axion dark matter thermalizes and forms a Bose-Einstein condensate with a rigid rotation, explaining observed galactic caustic rings and baryonic angular momentum distributions.

Contribution

It introduces a model where axion BECs form a rigidly rotating state, predicting caustic ring properties and baryonic angular momentum distributions consistent with observations.

Findings

Axion BECs form a single large vortex along the galaxy rotation axis.

The model explains the size and properties of galactic caustic rings.

Baryons acquire similar angular momentum distributions as axions, matching dwarf galaxy data.

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.