Multiple Ultralight Axionic Wave Dark Matter and Astronomical Structures

Abstract

An ultralight scalar boson with mass $m_1 \simeq 10^{-22}$ eV is gaining credence as a Dark Matter (DM) candidate that explains the dark cores of dwarf galaxies as soliton waves. Such a boson is naturally interpreted as an axion generic in String Theory, with multiple light axions predicted in this context. We examine the possibility of soliton structures over a wide range of scales, accounting for galaxy core masses and the common presence of nuclear star clusters. We present a diagnostic soliton core mass-radius plot that provides a global view, indicating the existence of an additional axion with mass $m_2\simeq 10^{-20}$ eV, with the possibility of a third axion with mass $m_3 \gtrsim 0.5 \times 10^{-18}$ eV. We also argue that the relative mass densities measured for these axions are consistent with their cosmological production via the mis-alignment mechanism.