Bar‐Halo Friction in Galaxies. I. Scaling Laws

It has been known for some time that rotating bars in galaxies slow due todynamical friction against the halo. However, recent attempts to use thisprocess to place constraints on the dark matter density in galaxies andpossibly also to drive dark matter out of the center have been challenged. Thispaper uses simplified numerical experiments to clarify several aspects of thefriction mechanism. I explicitly demonstrate the Chandrasekhar scaling of thefriction force with bar mass, halo density, and halo velocity dispersion. Ipresent direct evidence that exchanges between the bar and halo orbits at majorresonances are responsible for friction and study both individual orbits andthe net changes at these resonances. I also show that friction alters the phasespace density of particles in the vicinity of a major resonance, which is thereason the magnitude of the friction force depends on the prior evolution. Idemonstrate that bar slow down can be captured correctly in simulations havingmodest spatial resolution and practicable numbers of particles. Subsequentpapers in this series delineate the dark matter density that can be toleratedin halos of different density profiles.Comment: 15 pages, 13 figures, to appear in ApJ - major revisions from version