SHH method for SIDM: An SIDM-hydro hybrid method for simulating self-interacting dark matter

We present a new scheme to couple existing numerical methods for elastic self-interacting dark matter (SIDM) to the hydrodynamic equations via a continuous function of the local Knudsen number. The method, an SIDM-hydro hybrid (SHH), allows more efficient simulation of the evolution of inhomogeneous halos deep into the regime of gravothermal collapse. With the improved efficiency gained by moving to a hydrodynamical description in high-density regions, the SHH method allows central densities of two orders of magnitude higher to be reached in considerably less simulation time than traditional methods. Our implementation should be considered as the first step toward a robust SHH method, as we interpolate the first and second moments of the Boltzmann equation in the ideal-fluid limit only. The simulation results are qualitatively similar to those found with other methods, although there are differences in the implementation of the primary physics driving the dynamics, and in the details of the resulting halo profiles. However, our results indicate that the SHH technique shows promise to investigate gravothermal collapse in diverse, dynamical environments. The method can be extended to incorporate non-ideal fluid terms and dissipation, as needed for dark-matter scenarios where interactions beyond the elastic regime may be important in the dense interiors of some halos.