The ACS Nearby Galaxy Survey Treasury V. Radial Star Formation History
  of NGC 300
  We present new Hubble Space Telescope (HST) observations of NGC 300 taken as
part of the ACS Nearby Galaxy Survey Treasury (ANGST). Individual stars are
resolved in these images down to an absolute magnitude of M_F814W = 1.0 (below
the red clump). We determine the star formation history of the galaxy in 6
radial bins by comparing our observed color-magnitude diagrams (CMDs) with
synthetic CMDs based on theoretical isochrones. We find that the stellar disk
out to 5.4 kpc is primarily old, in contrast with the outwardly similar galaxy
M33. We determine the scale length as a function of age and find evidence for
inside-out growth of the stellar disk: the scale length has increased from 1.1
+/- 0.1 kpc 10 Gyr ago to 1.3 +/- 0.1 kpc at present, indicating a buildup in
the fraction of young stars at larger radii. As the scale length of M33 has
recently been shown to have increased much more dramatically with time, our
results demonstrate that two galaxies with similar sizes and morphologies can
have very different histories. With an N-body simulation of a galaxy designed
to be similar to NGC 300, we determine that the effects of radial migration
should be minimal. We trace the metallicity gradient as a function of time and
find a present day metallicity gradient consistent with that seen in previous
studies. Consistent results are obtained from archival images covering the same
radial extent but differing in placement and filter combination.
