On the Tidal Evolution of Dark Matter Substructure

The statistics of dark matter (DM) substructure are sensitive to the underlying DM model. In particular, the DM thermal velocity sets the cut-off scale for low-mass halos, and the (potentially non-zero) cross-section for DM self-interaction can core out the otherwise cuspy slopes of subhalo inner density profiles, making them less resilient to the strong tidal forces of the host halo. A variety of techniques are being used to determine the DM substructure abundance, and therewith the nature of DM, including gravitational lensing, gaps in stellar streams, and dark matter annihilation. In order for these methods to be successful, it is prudent that we can make reliable theoretical predictions. However, N-body simulations, which is the main tool used to predict the abundance of DM substructure, are hampered by artificial disruption due to inadequate force softening and discreteness noise. I discuss analytical treatments of tidal stripping and heating that can be used to overcome these shortcomings. In particular, I demonstrate that isotropic subhalos in CDM are resilient to complete disruption, even if the central halo contains a central disk. I explicitly show that, contrary to claims in the literature, tidal shocking due to impulsive encounters with a central disk does not lead to sub-halo disruption, and that simulations drastically under-predict the abundance of subhalos close to the central galaxy. I present new N-body simulations, at unprecedented resolution, that follow the tidal evolution of dark matter subhalos without being affected by numerical artefacts. These are used to study, for the first time, the dependence on the orbital anisotropy of the subhalo. It is shown that, contrary to isotropic subhalos, subhalos with a radially anisotropic center can undergo complete disruption. We also show that tidal stripping leads to a rapid isotropization of the bound remnant, and that the orbital anisotropy of the subhalo has a strong impact on the clumpiness of the resulting tidal stream. We discuss implications for the ability to use the demographics of dark matter substructure to inform the nature of dark matter.