Stellar Populations in the Outer Disk of M101

The outskirts of disk galaxies hold a remarkable range of information on processes driving galaxy evolution. Correlations between outer disk structure and galaxy Hubble type argue for the interaction-driven growth of galaxy disks, but the specific mechanism by which this occurs remains unclear. Characterizing the age and metallicity of stars in the outskirts of disk galaxies can differentiate between models of disk building via induced star formation versus those involving stripping or migration of older stars outwards from the inner disk.The nearby giant spiral M101 (NGC 5457) provides an ideal opportunity to study the outer disk populations of a giant Sc galaxy in great detail. Our deep ground-based imaging has mapped M101's stellar disk out to nearly 50 kpc (10 scale lengths), where we see signatures of diverse stellar populations tracing recent disk building. However, the constraints provided by integrated colors are limited; direct imaging of the stellar populations with HST will provide much stronger constraints on the evolutionary history of the outer disk.We propose deep ACS imaging of the outskirts of M101's disk, to construct resolved stellar CMDs for the outer disk and use the relative distribution of stars in different evolutionary phases to constrain the ages and metallicities of the stellar populations. Our deep (I=28.7, V=29.9) imaging will extend far enough down the stellar luminosity function to probe a variety of stellar evolutionary tracers for both young and old stars. A parallel WFC3 pointing will image a nearby blank field, to allow for detection and separation of any M101 halo populations from those in the outer disk field.