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 \sim 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 \lesssim 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 \sim 2 \times 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 ] \sim - 1.1 \pm 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 ) \propto r ^ { - \gamma } , with \gamma \sim 2 .
If this component were separate from the bulge and from the bright optical disk , then it would contain \sim 10 - 15 \% of M81 ’ s total V-band luminosity .
We discuss the possibility that this is M81 ’ s halo or thick disk , and in particular highlight its similarities and differences with these components in the Milky Way .
Other possibilities for its nature , such as a perturbed disk or the faint extension of the bulge , can not be completely ruled out , though our data disfavor the latter .
These observations add to the growing body of evidence for faint , complex extended structures beyond the bright disks of spiral galaxies .