Vortex in axion condensate as a dark matter halo

Abstract

We study the possibility of the vortices formation in axion condensate on the galactic scale. Such vortices can occur as a result of global rotation of the early universe. We study analytical models of vortices and calculate exemplary galaxy rotation curves. Depending on the setup it is possible to obtain a variety of shapes which give a good qualitative agreement with observational results. However, as we show, the extremely low velocity dispersions of the axions velocity are required to form the single vortex on the galactic scales. We find that the required velocity dispersion is of the order of $σ\approx 10^{-12} \frac{m}{s}$. This is much smaller that predicted within the present understanding of the axion physics. The vortices in axion condensate can be however formed on the much smaller scales and give seeds to the galaxy formation and to their angular momenta. On the other hand, the vortices can be formed on the galactic scales, but only if the mass of the axion-like particles is of the order of $10^{-30}$eV. In this case, the particle de Broglie wavelength is comparable with the galactic diameter. This condition must be fulfilled in order to keep the coherence of the quantum condensate on the galactic scales.