Cosmological Simulations of Two-Component Wave Dark Matter

Wave (fuzzy) dark matter ($\psi$DM) consists of ultralight bosons, featuring a solitonic core within a granular halo. Here we extend $\psi$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 $\psi$DM contains $75$ per cent major component and $25$ per cent minor component, fix the major-component particle mass to $m_{\rm major}=1\times10^{-22}\,{\rm eV}$, and explore two different minor-component particle masses with $m_{\rm major}:m_{\rm minor}=3:1$ and $1:3$, respectively. For $m_{\rm major}:m_{\rm minor}=3:1$, 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 $m_{\rm major}:m_{\rm minor}=1:3$, 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 the extra flexibility added to the multi-component $\psi$DM model can resolve observational tensions over the single-component model while retaining its key features.