  The ACS Nearby Galaxy Survey Treasury (ANGST) is a large Hubble Space
Telescope (HST) Advanced Camera for Surveys (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 F814W = 28.8 and
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 <~1 Gyr. The
results show that most of the stars in this outer disk field were formed by z~1
and that the arm structure at this radius has a lifetime of >~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~1 and that they have experienced rapid metal enrichment.
