Radial Star Formation Histories in 32 Nearby Galaxies
  The spatially resolved star formation histories are studied for 32 normal
star-forming galaxies drawn from the the Spitzer Extended Disk Galaxy
Exploration Science survey. At surface brightness sensitivities fainter than 28
mag arcsec$^{-2}$, the new optical photometry is deep enough to complement
archival ultraviolet and infrared imaging and to explore the properties of the
emission well beyond the traditional optical extents of these nearby galaxies.
Fits to the spectral energy distributions using a delayed star formation
history model indicate a subtle but interesting average radial trend for the
spiral galaxies: the inner stellar systems decrease in age with increasing
radius, consistent with inside-out disk formation, but the trend reverses in
the outermost regions with the stellar age nearly as old as the innermost
stars. These results suggest an old stellar outer disk population formed
through radial migration and/or the cumulative history of minor mergers and
accretions of satellite dwarf galaxies. The subset of S0 galaxies studied here
show the opposite trend compared to what is inferred for spirals:
characteristic stellar ages that are increasingly older with radius for the
inner portions of the galaxies, and increasingly younger stellar ages for the
outer portions. This result suggests that either S0 galaxies are not well
modeled by a delayed-$\tau$ model, and/or that S0 galaxies have a more
complicated formation history than spiral galaxies.
