Axion BEC Dark Matter

TL;DR

This paper discusses how axion dark matter can form a Bose-Einstein condensate through gravitational interactions, leading to observable differences in galactic halo structures compared to other dark matter models.

Contribution

It introduces the concept that axion BEC formation results in distinctive galactic halo features, providing a potential observational test to distinguish axions from other dark matter candidates.

Findings

Axion BEC forms when photon temperature reaches ~500 eV.

Rethermalization causes axions to develop net rotation.

Galactic halo caustics differ between axion BEC and standard CDM.

Abstract

Cold dark matter axions thermalize through gravitational self-interactions and form a Bose-Einstein condensate when the photon temperature reaches approximately 500 eV. Axion Bose-Einstein condensation provides an opportunity to distinguish axions from the other dark matter candidates on the basis of observation. The rethermalization of axions that are about to fall in a galactic potential well causes them to acquire net overall rotation, whereas ordinary cold dark matter falls in with an irrotational velocity field. The inner caustics of galactic halos are different in the two cases.