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

We have obtained resolved stellar photometry from Hubble Space Telescope Advanced Camera for Surveys observations of a field in the outer disk of M81 as part of ANGST. Motivated by the recent discovery of extended UV disks around many nearby spiral galaxies, we use the observed stellar population to derive the recent star formation histories of five ~ 0.5 kpc-sized regions within this field. These regions were selected on the basis of their UV luminosity from GALEX and include two H II regions, two regions that are UV-bright but Hα-faint, and one "control" region faint in both UV and Hα. We estimate our effective star formation rate detection limit at ~2 × 10^-4 M _sun yr^-1, which is lower than that of GALEX for regions of this size. As expected, the H II regions contain massive main-sequence stars (in the mass range 18-27 M _sun, based on our best extinction estimates), while similar massive main-sequence stars are lacking in the UV-bright/Hα-faint regions. The observations are consistent with stellar ages lsim 10 Myr in the H II regions, and gsim 16 Myr in the UV-bright/Hα-faint regions. All regions but the control have formed ~ 10⁴ M _sun of stars over the past ~ 65 Myr. Thus, our results, for at least one small area in the outer disk of M81, are consistent with an age difference being sufficient to explain the observed discrepancy between star forming regions detected in Hα and those detected exclusively in UV. However, our data cannot conclusively rule out other explanations, such as a strongly truncated initial mass function.