The Red and Featureless Outer Disks of Nearby Spiral Galaxies
  We present results from deep, wide-field surface photometry of three nearby
(D=4--7 Mpc) spiral galaxies: M94 (NGC 4736), M64 (NGC 4826), and M106 (NGC
4258). Our imaging reaches limiting surface brightnesses of $\mu_{B} \sim$ 28
-- 30 mag arcsec$^{-2}$ and probes colors down to $\mu_{B} \sim$ 27.5 mag
arcsec$^{-2}$. We compare our broadband optical data to available ultraviolet
and high column-density HI data to better constrain the star forming history
and stellar populations of the outermost parts of each galaxy's disk. Each
galaxy has a well-defined radius beyond which little star formation occurs and
the disk light appears both azimuthally smooth and red in color, suggestive of
old, well-mixed stellar populations. Given the lack of ongoing star formation
or blue stellar populations in these galaxies' outer disks, the most likely
mechanisms for their formation are dynamical processes such as disk heating or
radial migration, rather than inside-out growth of the disks. This is also
implied by the similarity in outer disk properties despite each galaxy showing
distinct levels of environmental influence, from a purely isolated galaxy (M94)
to one experiencing weak tidal perturbations from its satellite galaxies (M106)
to a galaxy recovering from a recent merger (M64), suggesting that a variety of
evolutionary histories can yield similar outer disk structure. While this
suggests a common secular mechanism for outer disk formation, the large extent
of these smooth, red stellar populations---which reach several disk
scalelengths beyond the galaxies' spiral structure---may challenge models of
radial migration given the lack of any non-axisymmetric forcing at such large
radii.
