Metallicity Distribution Function of the Eridanus~II Ultra-Faint Dwarf Galaxy from Hubble Space Telescope Narrow-band Imaging

TL;DR

This study uses deep Hubble Space Telescope narrow-band imaging to accurately determine the metallicity distribution of stars in the Eridanus II ultra-faint dwarf galaxy, revealing insights into its chemical evolution and orbital history.

Contribution

It presents the first detailed MDF of Eri II using a larger sample of stars from HST imaging, and compares its chemical evolution and orbital history with models and simulations.

Findings

Eri II has a mean metallicity of [Fe/H] = -2.50 and a dispersion of 0.42.

Identifies candidate extremely metal-poor stars for follow-up.

Eri II likely on first infall into the Milky Way, with an evolution similar to isolated galaxies.

Abstract

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