Resolving the Stellar Outskirts of M81: Evidence for a Faint, Extended
  Structural Component
  We present a wide field census of resolved stellar populations in the
northern half of M81, conducted with Suprime-Cam on the 8-m Subaru telescope
and covering an area ~ 0.3 square degrees. The resulting color-magnitude
diagram reaches over one magnitude below the red giant branch (RGB) tip,
allowing a detailed comparison between the young and old stellar spatial
distributions. The surface density of stars with ages <~ 100 Myr is correlated
with that of neutral hydrogen in a manner similar to the disk-averaged
Kennicutt-Schmidt relation. We trace this correlation down to gas densities of
~ 2 x 10^20 cm^{-2}, lower than typically probed with H-alpha flux. Both
diffuse light and resolved RGB star counts show compelling evidence for a
faint, extended structural component beyond the bright optical disk, with a
much flatter surface brightness profile. The star counts allow us to probe this
component to significantly fainter levels than is possible with the diffuse
light alone. From the colors of its RGB stars, we estimate this component has a
peak global metallicity [M/H] ~ -1.1 +/- 0.3 at deprojected radii 32 - 44 kpc
assuming an age of 10 Gyr and distance of 3.6 Mpc. The spatial distribution of
its RGB stars follows a power-law surface density profile, I(r) ~ r^{-gamma},
with gamma ~ 2. [Abridged]
