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

TL;DR

This paper introduces a hybrid semi-analytical and N-body method to study the complex evolution of self-interacting dark matter subhalos, revealing how evaporation and tidal effects influence core-collapse and challenging the likelihood of such collapse in constant cross-section SIDM models.

Contribution

The authors develop a new high-fidelity hybrid simulation approach to track SIDM subhalo evolution, including evaporation effects, from core formation to collapse, which was not previously feasible.

Findings

Evaporation delays or prevents subhalo core-collapse.

Core-collapse is nearly impossible in SIDM models with constant cross sections.

Future observations of ultra-compact substructures could indicate velocity-dependent interactions.

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 halos, accelerated if halos are subhalos of larger systems, has intriguing consequences for small-halo searches with substructure lensing. 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. In this work, 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. Our method works best for small subhalos ($\lesssim 1/1000$ host mass), for which the error caused by dynamical friction is minimal. We are able to capture the evaporation 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 outflow, host-subhalo evaporation, and tidal effects. The subhalo central density grows only when the heat outflow outweighs the energy gain from evaporation and tidal heating. Thus, evaporation delays or even disrupts 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 with future substructure lensing observations favors additional degrees of freedom, such as velocity-dependence, in the cross section.