The ACS Nearby Galaxy Survey Treasury XI. The Remarkably Undisturbed NGC
  2403 Disk
  We present detailed analysis of color-magnitude diagrams of NGC2403, obtained
from a deep (m<28) Hubble Space Telescope (HST) Wide Field Planetary Camera 2
observation of the outer disk of NGC2403, supplemented by several shallow
(m<26) HST Advanced Camera for Surveys fields. We derive the spatially resolved
star formation history of NGC2403 out to 11 disk scale lengths. In the inner
portions of the galaxy, we compare the recent star formation rates (SFRs) we
derive from the resolved stars with those measured using GALEX FUV + Spitzer
24-micron fluxes, finding excellent agreement between the methods. Our
measurements also show that the radial gradient in recent SFR mirror s the disk
exponential profile to 11 scale lengths with no break, extending to SFR
densities a factor of 100 lower than those that can be measured with GALEX and
Spitzer (2x10^{-6} M_{\sun} yr^{-1} kpc^{-2}). Furthermore, we find that the
cumulative stellar mass of the disk was formed at similar times at all radii.
We compare these characteristics of NGC2403 to those of its "morphological
twins," NGC300 and M33, showing that the structure and age distributions of the
NGC2403 disk are more similar to those of the relatively isolated system NGC300
than to those of the Local Group analog M33. We also discuss the environments
and HI morphologies of these three nearby galaxies, comparing them to
integrated light studies of larger samples of more distant galaxy disks. Taken
together, the physical properties and evolutionary history of NGC2403 suggest
that the galaxy has had no close encounters with other M81 group members and
may be falling into the group for the first time.
