I review the arguments for the importance of halo structure in driving galaxy
evolution and coupling a galaxy to its environment. We begin with a general
discussion of the key dynamics and examples of structure dominated by modes. We
find that simulations with large numbers of particles (N > 1e6) are required to
resolve the dynamics. Finally, I will describe some new results which
demonstrates that a disk bar can produce cores in a cuspy CDM dark-matter
profile within a gigayear. An inner Lindblad-like resonance couples the
rotating bar to halo orbits at all radii through the cusp, rapidly flattening
it. This resonance disappears for profiles with cores and is responsible for a
qualitative difference in bar-driven halo evolution with and without a cusp.
Although the bar gives up the angular momentum in its pattern to make the core,
the formation epoch is rich in accretion events to recreate or trigger a
classic stellar bar. The evolution of the cuspy inner halo by the
first-generation bar paves the way for a long-lived subsequent bar with low
torque and a stable pattern speed.Comment: 12 pages, 5 figures, to appear in "Astrophysical Supercomputing Using
Particles", eds J. Makino and P. Hut, Proc. IAU Symposium 208, Tokyo, July
10-13, 200
