Presentation #108.01 in the session Dwarf Galaxies in the Local Group.
The metallicities and abundance patterns of stars in ultra-faint dwarf galaxies (UFDs) are rich in information about the baryonic processes that shaped the formation of the lowest-mass galaxies in the Universe. However, the paucity of bright stars in UFDs means that remarkably little is known about even their basic chemical properties, such as their metallicity distribution functions (MDFs). Here, I present results from a 43-orbit imaging survey of 16 UFDs using narrowband Ca H & K filter on the Hubble Space Telescope. I highlight the power of Ca H&K by comparing our inferred stellar metallicities in Eri II with those inferred from spectroscopy. Then, I will present preliminary results for the other ultra-faints, most of which have no previously published MDFs, and discuss the insights that MDFs provide into processes that shaped their formation.