ANATOMY OF THE BAR INSTABILITY IN CUSPY DARK MATTER HALOS

Abstract

We examine the bar instability in galactic models with an exponential disk and a cuspy dark matter (DM) halo with a Navarro-Frenk-White (NFW) cosmological density profile. We construct equilibrium models from a 3-integral composite distribution function that are subject to the bar instability. We generate a sequence of models with a range of mass resolution from 1.8K to 18M particles in the disk and 10K to 100M particles in the halo along with a multi-mass model with an effective resolution of ~10^10 particles. We describe how mass resolution affects the bar instability, including its linear growth phase, the buckling instability, pattern speed decay through the resonant transfer of angular momentum to the DM halo, and the possible destruction of the halo cusp. Our higher resolution simulations show a converging spectrum of discrete resonance interactions between the bar and DM halo orbits. As the pattern speed decays, orbital resonances sweep through most of the DM halo phase space and widely distribute angular momentum among the halo particles. The halo does not develop a flat density core and preserves the cusp, except in the region dominated by gravitational softening. The formation of the bar increases the central stellar density and the DM is compressed adiabatically increasing the halo central density by 1.7X. Overall, the evolution of the bar displays a convergent behavior for halo particle numbers between 1M and 10M particles, when comparing bar growth, pattern speed evolution, the DM halo density profile and a nonlinear analysis of the orbital resonances. Higher resolution simulations clearly illustrate the importance of discrete resonances in transporting the angular momentum from the bar to the halo.