THE ACS NEARBY GALAXY SURVEY TREASURY. I. THE STAR FORMATION HISTORY OF THE M81 OUTER DISK

The Advanced Camera for Surveys (ACS) Nearby Galaxy Survey Treasury is a large Hubble Space Telescope (HST)/ACS treasury program to obtain resolved stellar photometry for a volume-limited sample of galaxies out to 4 Mpc. As part of this program, we have obtained deep ACS imaging of a field in the outer disk of the large spiral galaxy M81. The field contains the outskirts of a spiral arm as well as an area containing no current star formation. Our imaging results in a color-magnitude diagram (CMD) reaching to m(F814W) = 28.8 and m(F606W) = 29.5, one magnitude fainter than the red clump. Through detailed modeling of the full CMD, we quantify the age and metallicity distribution of the stellar populations contained in the field. The mean metallicity in the field is -1 < [M/H] < 0 and only a small fraction of stars have ages less than or similar to 1 Gyr. The results show that most of the stars in this outer disk field were formed by z similar to 1 and that the arm structure at this radius has a lifetime of greater than or similar to 100 Myr. We discuss the measured evolution of the M81 disk in the context of surveys of high redshift disk galaxies and deep stellar photometry of other nearby galaxies. All of these indicate that massive spiral disks are mostly formed by z similar to 1 and that they have experienced rapid metal enrichment.