As constraints on ultralight axion-like 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
[arXiv:1307.1705] 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 inter-field interactions. This allows us
to perform the first three-dimensional simulations of two-field ALPs with
self-interactions and inter-field interactions. Our simulations show that
having multiple species and interactions introduces different phenomenological
effects as compared to a single field, non-interacting scenarios. In
particular, we explore the dynamics of solitons. Interacting multi-species
ultralight dark matter has different equilibrium density profiles as compared
to single-species and/or non-interacting ultralight ALPs. As seen in earlier
work [arXiv:2011.09510], 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.
