The ACS Nearby Galaxy Survey Treasury IV. The Star Formation History of
  NGC 2976
  We present resolved stellar photometry of NGC 2976 obtained with the Advanced
Camera for Surveys (ACS) as part of the ACS Nearby Galaxy Survey Treasury
(ANGST) program. The data cover the radial extent of the major axis of the disk
out to 6 kpc, or ~6 scale lengths. The outer disk was imaged to a depth of
M_F606W ~ 1, and an inner field was imaged to the crowding limit at a depth of
M_F606W ~ -1. Through detailed analysis and modeling of these CMDs we have
reconstructed the star formation history of the stellar populations currently
residing in these portions of the galaxy, finding similar ancient populations
at all radii but significantly different young populations at increasing radii.
In particular, outside of the well-measured break in the disk surface
brightness profile, the age of the youngest population increases with distance
from the galaxy center, suggesting that star formation is shutting down from
the outside-in. We use our measured star formation history, along with H I
surface density measurements, to reconstruct the surface density profile of the
disk during previous epochs. Comparisons between the recovered star formation
rates and reconstructed gas densities at previous epochs are consistent with
star formation following the Schmidt law during the past 0.5 Gyrs, but with a
drop in star formation efficiency at low gas densities, as seen in local
galaxies at the present day. The current rate and gas density suggest that
rapid star formation in NGC 2976 is currently in the process of ceasing from
the outside-in due to gas depletion. This process of outer disk gas depletion
and inner disk star formation was likely triggered by an interaction with the
core of the M81 group >~1 Gyr ago that stripped the gas from the galaxy halo
and/or triggered gas inflow from the outer disk toward the galaxy center.
