A New Mechanism for Radial Migration in Galactic Disks: Spiral-Bar
  Resonance Overlap
  While it has long been known that a large number of short-lived transient
spirals can cause stellar migration, here we report that another mechanism is
also effective at mixing disks of barred galaxies. The resonance overlap of the
bar and spiral structure induces a nonlinear response leading to a strong
redistribution of angular momentum in the disk. We find that, depending on the
amplitudes of the perturbers, the changes in angular momentum, dL, could occur
up to an order of magnitude faster than in the case of recurrent spirals. The
signature of this mechanism is a bimodality in dL with maxima near the bar's
corotation and its outer Lindblad resonance; this is independent of the
properties of the spiral structure. For parameters consistent with the Milky
Way the disk mixes in about 3 Gyr and the stellar velocity dispersion increases
with time in a manner roughly consistent with observations. This new mechanism
could account for both the observed age-velocity relation and the absence of
age-metallicity relation in the solar neighborhood. Spiral-bar interaction
could also explain observations showing that strongly barred galaxies have
weaker metallicity gradients than weakly barred or non-barred galaxies.
