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We analyze the Illustris-1 hydrodynamical cosmological simulation to explorethe stellar velocity dispersion of quiescent galaxies as an observational probeof dark matter halo velocity dispersion and mass. Stellar velocity dispersionis proportional to dark matter halo velocity dispersion for both central andsatellite galaxies. The dark matter halos of central galaxies are in virialequilibrium and thus the stellar velocity dispersion is also proportional todark matter halo mass. This proportionality holds even when a line-of-sightaperture dispersion is calculated in analogy to observations. In contrast, at agiven stellar velocity dispersion, the dark matter halo mass of satellitegalaxies is smaller than virial equilibrium expectations. This deviation fromvirial equilibrium probably results from tidal stripping of the outer darkmatter halo. Stellar velocity dispersion appears insensitive to tidal effectsand thus reflects the correlation between stellar velocity dispersion and darkmatter halo mass prior to infall. There is a tight relation (<0.2 dex scatter)between line-of-sight aperture stellar velocity dispersion and dark matter halomass suggesting that the dark matter halo mass may be estimated from themeasured stellar velocity dispersion for both central and satellite galaxies.We evaluate the impact of treating all objects as central galaxies if therelation we derive is applied to a statistical ensemble. A large fraction(>2/3) of massive quiescent galaxies are central galaxies and systematicuncertainty in the inferred dark matter halo mass is <0.1 dex thus simplifyingapplication of the simulation results to currently available observations.

Stellar Velocity Dispersion: Linking Quiescent Galaxies to their Dark Matter Halos