The ACS Nearby Galaxy Survey Treasury. XI. The Remarkably Undisturbed NGC 2403 Disk

We present detailed analysis of color-magnitude diagrams of NGC 2403, obtained from a deep ( m 28) Hubble Space Telescope ( HST ) Wide Field Planetary Camera 2 observation of the outer disk of NGC 2403, supplemented by several shallow ( m 26) HST Advanced Camera for Surveys fields. We derive the spatially resolved star formation history of NGC 2403 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 24μ fluxes, finding excellent agreement between the methods. Our measurements also show that the radial gradient in recent SFR mirrors 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 (~2 × 10 –6 M ☉  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 NGC 2403 to those of its "morphological twins," NGC 300 and M 33, showing that the structure and age distributions of the NGC 2403 disk are more similar to those of the relatively isolated system NGC 300 than to those of the Local Group analog M 33. We also discuss the environments and H I 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 NGC 2403 suggest that the galaxy has had no close encounters with other M 81 group members and may be falling into the group for the first time.