Core-collapse, evaporation and tidal effects: the life story of a self-interacting dark matter subhalo

Abstract: Self-interacting dark matter (SIDM) cosmologies could admit an enormous diversity of dark matter halo density profiles, from low-density cores to high-density core-collapsed cusps. This could provide plausible solutions to small-scale problems of cold dark matter (CDM), such as the diversity of dwarf galaxy rotation curves. The possibility of the growth of high central density in low-mass halos also has intriguing consequences for small-halo searches with upcoming substructure lensing observations. However, following the evolution of $\lesssim 10^8 M_\odot$ subhalos in lens-mass systems ($\sim 10^{13}M_\odot$) is computationally expensive with traditional N-body simulations. We develop a new hybrid semi-analytical + N-body method to study the evolution of SIDM subhalos with high fidelity, from core formation to core-collapse, in staged simulations.

We find three main processes that drive subhalo evolution: subhalo internal heat outflow, host-subhalo evaporation, and tidal effects. The subhalo central density grows only when the heat outflow (cooling) outweighs the energy gain (heating) from evaporation and tidal heating. In other words, interactions with the host halo could lead to delay or even disruption of subhalo core-collapse. We map out the parameter space for subhalos to core-collapse, finding that it is nearly impossible to drive core-collapse in subhalos in SIDM models with constant cross sections. Any discovery of ultra-compact dark substructures would favor additional degrees of freedom, such as velocity-dependence, in the cross section.