Low-mass galaxy evolution is driven by both internal (e.g., star formation and feedback) and external (e.g., tidal interaction) processes; however, few nearby systems show evidence of both mechanisms operating in tandem. The Local Group dwarf galaxy NGC 6822 presents a unique opportunity to study important evolutionary processes because of proximity and timing: it harbors one of the largest known holes in its neutral ISM (a signpost of violent stellar feedback), and it appears to be undergoing a tidal interaction with a very low mass companion dwarf galaxy. To capitalize on this opportunity, we propose to undertake an ACS and WFC3 imaging study of the stellar populations throughout NGC 6822. We will study the role of both internal and external processes by sampling the stellar populations associated with the main body (using archival WFPC2 data), the giant HI hole, the companion dwarf galaxy, and the tidal material at the ends of the disk. From these data we will extract precise color magnitude diagrams; we will measure the full 13 Gyr star formation history of each field, with high time resolution (20-250 Myr) over the past 1 Gyr, and quantify the relative contributions of internal and external drivers of dwarf galaxy evolution. Specifically, we will study: 1) the nature of the companion object (differentiating between infalling gas and a genuine low-mass galaxy); 2) the energetics of giant HI hole creation; 3) the role of interactions in the system's evolution; 4) the nature of feedback in governing the star formation process; and 5) the patterns of star formation over time. We have optimized our field placements to extract the most information possible about the competing forces that drive galaxy evolution; in only six orbits, we will gain fundamental insights into the contributions of both internal and external processes that bear on the evolution of low-mass galaxies.

To obtain detailed SFHs back to 1 Gyr, we need photometry deep enough to reach M_V = +2.5. The distance of NGC 6822 allows us to reach our requisite photometric depth quickly and eciently. A single 52-minute orbit per pointing will be used to obtain imaging in the F475W and F814W filters (ACS - all fields; WFC3 - non-overlapping fields) and in the F336W filter (WFC3 - overlapping fields). This provides exceptional quality photometry to the depth we require (e.g., S/N > 8, 13 for a star of M_V = +2.5 in the G and I bands, respectively, using either ACS or WFC3; S/N ~ 7 for a star of M_V +1 in the WFC3 U band). We request 6 pointings (3 each at two dierent roll angles), for a total of 6 orbits. NGC 6822 is close enough to allow efficient photometry, but yet distant enough that the ACS and WFC3 detectors can image a significant fraction of the galaxy in a modest number of orbits. We are interested in the nature of multiple stellar populations in various regions of the galaxy, and in how these populations are related. We will combine suciently deep archival WFPC2 pointings with these proposed ACS and WFC3 observations in order to study all regions of interest. RESOLVED STELLAR POPULATIONS 12 false 0.0 0.0 false false false false false false SUPPORTED GALAXY MAGELLANIC IRREGULAR J2000 19 43 33.5007 -14 33 41.00 J2000 GALAXY MAGELLANIC IRREGULAR J2000 19 43 54.0219 -14 37 9.62 J2000 GALAXY MAGELLANIC IRREGULAR J2000 19 44 14.8717 -14 40 39.74 J2000 GALAXY MAGELLANIC IRREGULAR J2000 19 45 10.2455 -14 49 46.42 J2000 GALAXY MAGELLANIC IRREGULAR J2000 19 45 30.2646 -14 53 14.32 J2000 GALAXY MAGELLANIC IRREGULAR J2000 19 45 51.1279 -14 56 36.66 J2000 ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F814W The scheduling data was generated by CASM 18.3.1. 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