Simulations of multifield ultralight axionlike dark matter

As constraints on ultralight axionlike particles (ALPs) tighten, models with multiple species of ultralight ALP are of increasing interest. We perform simulations of two-ALP models with particles in the currently supported range of plausible masses. The code we modified, UltraDark.jl, not only allows for multiple species of ultralight ALP with different masses, but also different self-interactions and interfield interactions. This allows us to perform the first three-dimensional simulations of two-field ALPs with self-interactions and interfield interactions. Our simulations show that having multiple species and interactions introduces different phenomenological effects as compared to a single field, noninteracting scenarios. In particular, we explore the dynamics of solitons. Interacting multispecies ultralight dark matter has different equilibrium density profiles as compared to single-species and/or noninteracting ultralight ALPs. As seen in earlier work, attractive interactions tend to contract the density profile while repulsive interactions spread out the density profile. We also explore collisions between solitons comprised of distinct axion species. We observe a lack of interference patterns in such collisions, and that resulting densities depend on the relative masses of the ALPs and their interactions.