Core-collapse, evaporation and tidal effects: the life story of a
  self-interacting dark matter subhalo
  Recently, work on self-interacting dark matter (SIDM) cosmologies has shown
that an enormous diversity of dark matter (DM) halo density profiles is
possible for a fixed SIDM model, ranging from the development of 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. With this method, 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 driving subhalo evolution: subhalo internal heat outflow,
host-subhalo evaporation, and tidal effects. We conclude that 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, and find 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
disfavors SIDM models with constant cross sections, indicating instead the
presence of additional degrees of freedom, such as velocity-dependence or
dissipation of energy.
