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We present resolved stellar photometry of NGC 2976 obtained with the AdvancedCamera 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 diskout to 6 kpc, or ~6 scale lengths. The outer disk was imaged to a depth ofM_F606W ~ 1, and an inner field was imaged to the crowding limit at a depth ofM_F606W ~ -1. Through detailed analysis and modeling of these CMDs we havereconstructed the star formation history of the stellar populations currentlyresiding in these portions of the galaxy, finding similar ancient populationsat all radii but significantly different young populations at increasing radii.In particular, outside of the well-measured break in the disk surfacebrightness profile, the age of the youngest population increases with distancefrom the galaxy center, suggesting that star formation is shutting down fromthe outside-in. We use our measured star formation history, along with H Isurface density measurements, to reconstruct the surface density profile of thedisk during previous epochs. Comparisons between the recovered star formationrates and reconstructed gas densities at previous epochs are consistent withstar formation following the Schmidt law during the past 0.5 Gyrs, but with adrop in star formation efficiency at low gas densities, as seen in localgalaxies at the present day. The current rate and gas density suggest thatrapid star formation in NGC 2976 is currently in the process of ceasing fromthe outside-in due to gas depletion. This process of outer disk gas depletionand inner disk star formation was likely triggered by an interaction with thecore of the M81 group >~1 Gyr ago that stripped the gas from the galaxy haloand/or triggered gas inflow from the outer disk toward the galaxy center.

The ACS Nearby Galaxy Survey Treasury IV. The Star Formation History of NGC 2976