Cosmological simulations of two-component wave dark matter

ABSTRACT Wave (fuzzy) dark matter (DM) consists of ultralight bosons, featuring a solitonic core within a granular halo. Here we extend 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 DM contains 75 per cent major component and 25 per cent minor component, fix the major-component particle mass to m ₌₀₉₎ₑ=1 10^-22\, 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 DM model can resolve observational tensions over the single-component model while retaining its key features.