Constraining dark halos with tidal tails

Massive and extended dark halos can inhibit the formation of long tidal tails in galaxy collisions. We examine this effect using an extensive survey of simulations with different dark halo potentials to constrain halo properties of interacting galaxies. These constraints are compared to other observational limits and theoretical predictions of halo structure. The dark halos predicted by $\Omega=1$ cosmological models like CDM are too massive and extended to produce the long tidal tails seen in nearby galaxy collisions. There is also a conflict with the halo potentials inferred from satellite kinematics; such halos would likewise inhibit tail formation in galaxy collisions.