Cosmological simulations of two-component wave dark matter

Abstract

ABSTRACT Wave (fuzzy) dark matter ($\psi \rm {DM}$) consists of ultralight bosons, featuring a solitonic core within a granular halo. Here we extend $\psi \rm {DM}$ to two components, with distinct particle masses m and coupled only through gravity, and investigate the resulting soliton–halo structure via cosmological simulations. Specifically, we assume $\psi \rm {DM}$ contains 75 per cent major component and 25 per cent minor component, fix the major-component particle mass to $m_{\rm major}=1\times 10^{-22}\, \rm eV$, and explore two different minor-component particle masses with mmajor: mminor = 3: 1 and 1: 3, respectively. For mmajor: mminor = 3: 1, we find that (i) the major- and minor-component solitons coexist, have comparable masses, and are roughly concentric. (ii) The soliton peak density is significantly lower than the single-component counterpart, leading to a smoother soliton-to-halo transition and rotation curve. (iii) The combined soliton mass of both components follows the same single-component core–halo mass relation. In dramatic contrast, for mmajor: mminor = 1: 3, a minor-component soliton cannot form with the presence of a stable major-component soliton; the total density profile, for both halo and soliton, is thus dominated by the major component and closely follows the single-component case. To support this finding, we propose a toy model illustrating that it is difficult to form a soliton in a hot environment associated with a deep gravitational potential. The work demonstrates that the extra flexibility added to the multi-component $\psi \rm {DM}$ model can resolve observational tensions over the single-component model while retaining its key features.