The Structural Evolution of Substructure

We investigate the evolution of substructure in cold dark matter (CDM) halos using N-body simulations of tidal stripping of substructure halos within a static host potential. We find that halos modeled following the Navarro, Frenk, & White (NFW) mass profile lose mass continuously because of tides from the massive host, leading to the total disruption of satellite halos with small tidal radii. Although mass is predominantly stripped from the outer regions, tidal heating also causes the halo to expand and the central density to decrease after each pericentric passage, when mass loss preferentially occurs. As a result, simple models based on the tidal-limit approximation underestimate significantly the tidal mass loss over several orbits. The equilibrium structure of stripped NFW halos depends mainly on the fraction of mass lost and can be expressed in terms of a simple correction to the original NFW profile. We apply these results to substructure in the Milky Way and conclude that the dark matter halos surrounding its dwarf spheroidal (dSph) satellites have circular velocity curves that peak well beyond the luminous radius at velocities significantly higher than expected from the stellar line-of-sight velocity dispersion. Our modeling suggests that the true tidal radii of dSphs lie well beyond the putative tidal cutoff observed in the surface brightness profile, suggesting that the latter are not really tidal in origin but rather features in the light profile of limited dynamical relevance. For Draco in particular, our modeling implies that its tidal radius is much larger than derived by Irwin & Hatzidimitriou, lending support to the interpretation of recent Sloan Digital Sky Survey data by Odenkirchen et al. Similarly, our model suggests that Carina's halo has a peak circular velocity of order ~55 km s^-1, which may help explain how this small galaxy has managed to retain enough gas to undergo several bursts of star formation. Our results imply a close correspondence between the most massive substructure halos expected in a CDM universe and the known satellites of the Milky Way and suggest that only substructure halos with peak circular velocities below 35 km s^-1 lack readily detectable luminous counterparts.