Formation of relativistic axion stars

Abstract

Axions and axion-like particles are compelling candidates for the missing\ndark matter of the universe. As they undergo gravitational collapse, they can\nform compact objects such as axion stars or even black holes. In this paper, we\nstudy the formation and distribution of such objects. First, we simulate the\nformation of compact axion stars using numerical relativity with aspherical\ninitial conditions that could represent the final stages of axion dark matter\nstructure formation. We show that the final states of such collapse closely\nfollow the known relationship of initial mass and axion decay constant $f_a$.\nSecond, we demonstrate with a toy model how this information can be used to\nscan a model density field to predict the number densities and masses of such\ncompact objects. In addition to being detectable by the LIGO/VIRGO\ngravitational wave interferometer network for axion mass of $10^{-9} < m_a <\n10^{-11}$ eV, we show using peak statistics that for $f_a < 0.2M_{pl}$, there\nexists a "mass gap" between the masses of axion stars and black holes formed\nfrom collapse.\n