Metallicity Distribution Function of the Eridanus II Ultra-faint Dwarf Galaxy from Hubble Space Telescope Narrowband Imaging

We use deep narrowband Ca H&K ($F395N$) imaging taken with the Hubble Space Telescope (HST) to construct metallicity distribution function (MDF) of Local Group (LG) ultra-faint dwarf (UFD) galaxy Eridanus II (Eri II). When combined archival $F475W$ and $F814W$ data, we measure metallicities for 60 resolved red giant branch stars as faint $m_{F475W}\sim24$ mag, a factor $\sim4$x more than current spectroscopic MDF determinations. find that Eri has mean [Fe/H]$=$-2.50$^{+0.07}_{-0.07}$ dispersion $\sigma_{\mbox{[Fe/H]}}=0.42^{+0.06}_{-0.06}$, which are consistent MDFs, though precisely constrained owing larger sample. identify handful extremely metal-poor star candidates (EMP; [Fe/H] $< -3$) marginally bright enough follow up. II's appears well-described by leaky box chemical evolution model. also compute an updated orbital history using Gaia eDR3 proper motions, it is likely on first infall into Milky Way. Our findings suggest underwent evolutionary similar isolated galaxy. Compared MDFs select cosmological simulations mass galaxies, lower fraction -3$, such comparisons should currently be treated caution due paucity simulations, selection effects, known limitations EMPs. This study demonstrates power HST CaHK measuring UFDs.