  We examine the central stellar velocity dispersion of subhalos based on
IllustrisTNG cosmological hydrodynamic simulations. The central velocity
dispersion is a fundamental observable that links galaxies with their dark
matter subhalos. We carefully explore simulated stellar velocity dispersions
derived with different definitions to assess possible systematics. We explore
the impact of variation in the identification of member stellar particles, the
viewing axes, the velocity dispersion computation technique, and simulation
resolution. None of these issues impact the velocity dispersion significantly;
any systematic uncertainties are smaller than the random error. We examine the
stellar mass-velocity dispersion relation as an observational test of the
simulations. At fixed stellar mass, the observed velocity dispersions
significantly exceed the simulation results. This discrepancy is an interesting
benchmark for the IllustrisTNG simulations because the simulations are not
explicitly tuned to match this relation. We demonstrate that the stellar
velocity dispersion provides measures of the dark matter velocity dispersion
and the dark matter subhalo mass.
