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

Abstract

Self-interacting dark matter (SIDM) cosmologies admit an enormous diversity of dark matter (DM) halo density profiles, from low-density cores to high-density core-collapsed cusps. The possibility of the growth of high central density in low-mass haloes, accelerated if haloes are subhaloes of larger systems, has intriguing consequences for small-halo searches with substructure lensing. Ho we ver, follo wing the e volution of ≲10 8 M ⊙subhaloes in lens-mass systems ( ∼10 13 M ⊙) is computationally e xpensiv e with traditional N -body simulations. In this work, we develop a new hybrid semi-analytical + N -body method to study the evolution of SIDM subhaloes with high fidelity, from core formation to core-collapse, in staged simulations. Our method works best for small subhaloes ( ≲ 1/1000 host mass), for which the error caused by dynamical friction is minimal. We are able to capture the e v aporation of subhalo particles by interactions with host halo particles, an effect that has not yet been fully explored in the context of subhalo core-collapse. We find three main processes drive subhalo evolution: subhalo internal heat outflo w, host-subhalo e v aporation, and tidal ef fects. The subhalo central density gro ws only when the heat outflow outweighs the energy gain from e v aporation and tidal heating. Thus, e v aporation delays or even disrupts subhalo core-collapse. We map out the parameter space for subhaloes to core-collapse, finding that it is nearly impossible to drive core-collapse in subhaloes in SIDM models with constant cross-sections. An y disco v ery of ultracompact dark substructures with future substructure lensing observations fa v ours additional de grees of freedom, such as v elocity-dependence, in the cross-section.