Stellar Populations in the Outer Disk and Halo of the Spiral Galaxy M101
  We use deep Hubble Space Telescope imaging in the outskirts of the nearby
spiral M101 to study stellar populations in the galaxy's outer disk and halo.
Our ACS field lies 17.6 arcmin (36 kpc) from the center of M101 and targets the
blue "NE Plume" of M101's outer disk, while the parallel WFC3 field lies at a
distance of 23.3 arcmin (47 kpc) to sample the galaxy's stellar halo. The WFC3
halo field shows a well-defined red giant branch characterized by low
metallicity ([M/H]=-1.7 $\pm$ 0.2), with no evidence of young stellar
populations. In contrast, the ACS disk field shows multiple stellar
populations, including a young main sequence, blue and red helium burning
stars, and old RGB and AGB populations. The mean metallicity of these disk
stars is quite low: [M/H]=-1.3 $\pm$ 0.2 for the RGB population, and -1.15
$\pm$ 0.2 for the younger helium burning sequences. Of particular interest is a
bunching of stars along the BHeB sequence, indicative of an evolving cohort of
massive young stars. We show that the young stellar populations in this field
are well-described by a decaying burst of star formation that peaked ~ 300-400
Myr ago, along with a more extended star formation history to produce the older
RGB and AGB populations. These results confirm and extend the results from our
previous deep surface photometry of M101's outer disk, providing an important
cross-check on stellar population studies using resolved stellar populations
versus integrated light photometry. We discuss our results in the context of
halo formation models and the interaction history of M101 and its companions.
