Strong Outflows and Inefficient Star Formation in the Reionization-Era Ultrafaint Dwarf Galaxy Eridanus II

We present novel constraints on the underlying galaxy formation physics (e.g. mass-loading factor, star formation history, and metal retention) at z greater than or similar to 7 for the low-mass (M-* similar to 10(5) M-circle dot) Local Group ultrafaint dwarf galaxy (UFD) Eridanus II (Eri II). Using a hierarchical Bayesian framework, we apply a one-zone chemical evolution model to Eri II's CaHK-based photometric metallicity distribution function (MDF; [Fe/H]) and find that the evolution of Eri II is well characterized by a short, exponentially declining star formation history (tau(SFH) = 0.39 +/-(0.18)(0.13) Gyr), a low star formation efficiency (tau(SFE) = 27.56 +/-(25.14)(12.92) Gyr), and a large mass-loading factor (eta = 194.53 +/-(33.37)(42.67)). Our results are consistent with Eri II forming the majority of its stars before the end of reionization. The large mass-loading factor implies strong outflows in the early history of Eri II and is in good agreement with theoretical predictions for the mass scaling of galactic winds. It also results in the ejection of >90 per cent of the metals produced in Eri II. We make predictions for the distribution of [Mg/Fe]-[Fe/H] in Eri II as well as the prevalence of ultra metal-poor stars, both of which can be tested by future chemical abundance measurements. Spectroscopic follow-up of the highest metallicity stars in Eri II ([Fe/H] > -2) will greatly improve model constraints. Our new framework can readily be applied to all UFDs throughout the Local Group, providing new insights into the underlying physics governing the evolution of the faintest galaxies in the reionization era.