The Extended Optical Disk of M101
  We have used deep, wide-field optical imaging to study the faint outskirts of
the luminous spiral galaxy M101 (NGC 5457), as well as its surrounding
environment. Over six square degrees, our imaging has a limiting surface
brightness of mu_B ~ 29.5 mag/arcsec^2, and has revealed the stellar structure
of M101's disk out to nearly 25 arcminutes (50 kpc), three times our measured
R25 isophotal size of the optical disk. At these radii, the well-known
asymmetry of the inner disk slews 180 degrees, resulting in an asymmetric plume
of light at large radius which follows the very extended HI disk to the
northeast of M101. This plume has very blue colors (B-V ~ 0.2), suggesting it
is the somewhat more evolved (few hundred Myr to ~ 1 Gyr) counterpart of the
young far ultraviolet emitting population traced by GALEX imaging. We also
detect another, redder spur of extended light to the east of the disk, and both
structures are reminiscent of features produced during fly-by galaxy
interactions. However, we see no evidence of very extended tidal tails around
M101 or any of its companions which might be expected from a recent encounter
with a massive companion. We consider the properties of M101's outer disk in
light of possible past interactions with the nearby companion galaxies NGC 5477
and NGC 5474. The detection of optical starlight at such large radii gives us
the ability to study star formation histories and stellar populations in outer
disks over a longer timescales than those traced by the UV or Halpha emitting
populations. Our data suggest ongoing buildup of the M101's outer disk due to
encounters in the group environment triggering extended star formation and
tidal heating of existing disk populations.
