Flybys, orbits, splashback: subhalos and the importance of the halo boundary

Understand

The classification of dark matter halos as isolated hosts or subhalos is critical for our understanding of structure formation and the galaxy-halo connection.

• Most commonly, subhalos are defined to reside inside a spherical overdensity boundary such as the virial radius.
• The resulting host-subhalo relations depend sensitively on the somewhat arbitrary overdensity threshold, but the impact of this dependence is rarely quantified.
• The recently proposed splashback radius tends to be larger and to include more subhalos than even the largest spherical overdensity boundaries.