Metallicity Distributions of the Faintest Dwarf Galaxies

Ultra-faint dwarf galaxies (UFDs) appear to be the oldest, the most dark matter dominated, and the most metal-poor galaxies known. They appear to have formed all of their stars during the universe’s first 1-2 billion years of existence. As such, their stellar populations retain important information concerning processes at work during this early epoch, including the formation of the first generation of stars (Population III) and reionization. While high redshift UFDs are too faint to be studied by even the JWST, nearby examples should in principle be capable of yielding enough information to reconstruct their formation history and constrain galaxy formation. The principal investigator (PI) and his team will use deep spectroscopy and photometry obtained with the Magellan telescope to derive metallicities of 50-200 Main-Sequence turn-off (MSTO) stars for each of four nearby UFDs, more than doubling the number with measured chemical abundances. Analysis of the resulting metallicity distribution functions (MDFs) promises to yield precise star formation histories for these objects and thus provide important constraints on the formation of the first galaxies. An exciting secondary goal of this work is the possibility of detecting individual Population III stars among the observed MSTO stars and constraining their mass function. This award will support the training of a graduate student as well as research experiences for underrepresented undergraduate students. The PI will also implement a series of talks during the summer for undergraduate researchers. The researchers will obtain deep spectroscopy capable of enabling metallicity measurements for 50- 200 MSTO stars in each of the targeted UFD galaxies. This will result in the largest metallicity sample to date for chemical evolution modeling of these systems. The team will break age-metallicity degeneracies in isochrone-based star formation histories, which will enable sub-gigayear age precision for individual MSTOs. Ages and metallicities will be determined by simultaneously fitting stellar photometry and line equivalent widths. These will lead to the best-measured formation histories of UFDs, which will make the data invaluable as tests of chemical evolution or hydrodynamic galaxy formation simulations. These formation histories will advance understanding of the edge of galaxy formation by showing how quickly star formation is halted in the faintest dwarf galaxies. Given the very low UFD stellar masses and large number of metallicity measurements, there is also a substantial chance to discover a surviving Population III star, which would be the first of its kind. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.