Abstract

We further develop recent efforts to characterize the axion as an ultralight dark matter candidate by simulating the effects of a range of attractive particle self-interactions in an idealized halo merging event. This type of bosonic scalar field dark matter is a viable alternative to the standard cold dark matter (CDM) paradigm, as it approximates the predictions of CDM on large scales while potentially resolving some of its small-scale problems via kiloparsec-scale quantum interference. In our self-interacting "fuzzy'' dark matter (SIFDM) simulations, halos merge together and condense into a soliton, characteristic of collisionless fuzzy dark matter. An attractive particle self-interaction introduces a critical halo mass threshold for the soliton, above which it is predicted to become unstable and undergo a phase transition. We evolve a particular initial configuration of halos in simulations with various self-interaction strengths to study the cases of weak and strong self-interactions with respect to a collisionless (zero SI) case. We find that weakly self-interacting halos have smaller soliton cores and higher central densities than collisionless halos, while the energy decomposition and density profile power-law tail remain invariant. Strongly self-interacting halos exhibit a soliton for a period of time (dependent on the interaction strength), then rapidly collapse into a dense, compact object.