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We have obtained resolved stellar photometry from Hubble Space Telescope(HST) Advanced Camera for Surveys (ACS) observations of a field in the outerdisk of M81 as part of the ACS Nearby Galaxy Survey Treasury (ANGST). Motivatedby the recent discovery of extended UV (XUV) disks around many nearby spiralgalaxies, we use the observed stellar population to derive the recent starformation histories of five ~0.5 kpc-sized regions within this field. Theseregions were selected on the basis of their UV luminosity from GALEX andinclude two HII regions, two regions which are UV-bright but Halpha-faint, andone "control" region faint in both UV and Halpha. We estimate our effective SFRdetection limit at ~2 x 10^-4 Msun/yr, which is lower than that of GALEX forregions of this size. As expected, the HII regions contain massive mainsequence stars (in the mass range 18-27 Msun, based on our best extinctionestimates), while similar massive main sequence stars are lacking in theUV-bright/Halpha-faint regions. The observations are consistent with stellarages <10 Myr in the HII regions, and >16 Myr in the UV-bright/Halpha-faintregions. All regions but the control have formed ~10^4 Msun of stars over thepast ~65 Myr. Thus, our results, for at least one small area in the outer diskof M81, are consistent with an age difference being sufficient to explain theobserved discrepancy between star-forming regions detected in Halpha and thosedetected exclusively in UV. However, our data cannot conclusively rule outother explanations, such as a strongly truncated initial mass function (IMF).

The ACS Nearby Galaxy Survey Treasury II. Young Stars and their Relation to Halpha and UV Emission Timescales in the M81 Outer Disk