The Star Formation History and Evolution of the Ultra-Diffuse M81 Satellite F8D1

We present deep Hubble Space Telescope (HST) imaging of one of the nearest ultra-diffuse galaxies (UDGs) outside of the Local Group: F8D1, a satellite of M81 known to be tidally disrupting. UDGs are an enigmatic and diverse population, with evolutionary pathways ranging from tidal processing to bursty feedback and high initial angular momentum. To determine F8D1's evolutionary drivers, we resolve stars in F8D1's central similar to 1 kpc and in a parallel field similar to 6 kpc along its major axis to deep photometric limits, reaching below the red clump. We also image eight shallower fields along F8D1's major and minor axes. We calculate the star formation history (SFH) in the two deep fields, finding that while currently quiescent, both regions experienced a substantial burst similar to 2 Gyr ago and a smaller burst similar to 500 Myr ago, which likely formed F8D1's nuclear star cluster. In the shallow fields, using the ratio of evolved asymptotic giant branch and red giant branch stars out to similar to 13 kpc along F8D1's known stellar stream, we confirm that F8D1 was globally star-forming until at least similar to 2 Gyr ago. We estimate a total progenitor stellar mass, including the stream, of similar to 1.3 & times; 108 M circle dot, with an average [M/H] similar to -0.8. We compare F8D1's properties to those of Local Group galaxies with similar initial stellar mass. We find that F8D1 is consistent with a progenitor star-forming galaxy similar to NGC 6822 that is in the midst of a transition to a Sagittarius-like system. Notably, this evolutionary sequence can be accomplished through tidal processing alone in galaxies that have experienced sufficiently bursty feedback to have created cored profiles.