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

Abstract

Self-interacting dark matter (SIDM) cosmologies admit an enormous diversity\nof dark matter (DM) halo density profiles, from low-density cores to\nhigh-density core-collapsed cusps. The possibility of the growth of high\ncentral density in low-mass halos, accelerated if halos are subhalos of larger\nsystems, has intriguing consequences for small-halo searches with substructure\nlensing. However, following the evolution of $\\lesssim 10^8 M_\\odot$ subhalos\nin lens-mass systems ($\\sim 10^{13}M_\\odot$) is computationally expensive with\ntraditional N-body simulations. In this work, we develop a new hybrid\nsemi-analytical + N-body method to study the evolution of SIDM subhalos with\nhigh fidelity, from core formation to core-collapse, in staged simulations. Our\nmethod works best for small subhalos ($\\lesssim 1/1000$ host mass), for which\nthe error caused by dynamical friction is minimal. We are able to capture the\nevaporation of subhalo particles by interactions with host halo particles, an\neffect that has not yet been fully explored in the context of subhalo\ncore-collapse. We find three main processes drive subhalo evolution: subhalo\ninternal heat outflow, host-subhalo evaporation, and tidal effects. The subhalo\ncentral density grows only when the heat outflow outweighs the energy gain from\nevaporation and tidal heating. Thus, evaporation delays or even disrupts\nsubhalo core-collapse. We map out the parameter space for subhalos to\ncore-collapse, finding that it is nearly impossible to drive core-collapse in\nsubhalos in SIDM models with constant cross sections. Any discovery of\nultra-compact dark substructures with future substructure lensing observations\nfavors additional degrees of freedom, such as velocity-dependence, in the cross\nsection.\n