Dynamical Friction and the Distribution of Dark Matter in Barred Galaxies

We use fully self-consistent N-body simulations of barred galaxies to show that dynamical friction from a dense dark matter halo dramatically slows the rotation rate of bars. Our result supports previous theoretical predictions for a bar rotating within a massive halo. On the other hand, low-density halos, such as those required for maximum disks, allow the bar to continue to rotate at a high rate. There is somewhat meager observational evidence indicating that bars in real galaxies do rotate rapidly, and we use our result to argue that dark matter halos must have a low central density in all high surface brightness disk galaxies, including the Milky Way. Bars in galaxies that have larger fractions of dark matter should rotate slowly, and we suggest that a promising place to look for such candidate objects is among galaxies of intermediate surface brightness. Subject headings: galaxies: evolution — galaxies: halos — galaxies: kinematics and dynamics — Galaxy: halo — Galaxy: structure