Secular Evolution in Disk Galaxies
  Disk galaxies evolve over time through processes that may rearrange both the
radial mass profile and the metallicity distribution within the disk. This
review of such slow changes is largely, though not entirely, restricted to
internally-driven processes that can be distinguished from evolution driven by
galaxy interactions. It both describes our current understanding of disk
evolution, and identifies areas where more work is needed. Stellar disks are
heated through spiral scattering, which increases random motion components in
the plane, while molecular clouds redirect some fraction of the random energy
into vertical motion. The recently discovered process of radial migration at
the corotation resonance of a transient spiral mode does not alter the
underlying structure of the disk, since it neither heats the disk nor causes it
to spread, but it does have a profound effect on the expected distribution of
metallicities among the disk stars. Bars in disks are believed to be major
drivers of secular evolution through interactions with the outer disk and with
the halo. Once the material that makes up galaxy disks is converted into stars,
their overall angular momentum distribution cannot change by much, but that of
the gas is generally far more liable to rearrangement, allowing rings and
pseudo-bulges to form. While simulations are powerful tools from which we have
learned a great deal, those of disks may suffer from collisional relaxation
that requires some results to be interpreted with caution.
