A Bimodal Metallicity Distribution Function in the Ultra-Faint Dwarf Galaxy Reticulum II

TL;DR

This study reveals a bimodal metallicity distribution in the ultra-faint dwarf galaxy Reticulum II, indicating complex star formation history and enrichment processes in such low-mass galaxies.

Contribution

First spectroscopic metallicities of main sequence turn-off stars in an ultra-faint dwarf galaxy, revealing a bimodal MDF and supporting a two-burst star formation model.

Findings

MDF of Reticulum II is bimodal with peaks at [Fe/H]=-3.0 and -2.1.

80% of stars are in the metal-poor peak, 20% in the metal-rich peak.

Evidence of Type Ia supernova enrichment during a quiescent period.

Abstract

Star formation in ultra-faint dwarf galaxies (UFDs, $M_* <10^5M_\odot$) is suppressed by reionization, but may not be completely quenched. The metallicity distribution function (MDF) of stars in ultra-faint dwarf galaxies could show these signatures of reionization. However, past studies of UFD MDFs have been limited, because there are only a few dozen red giant branch (RGB) stars in such low-mass galaxies. We present low-resolution Magellan/IMACS spectroscopy of 167 stars in the UFD Reticulum II ($M_* \approx 3000 M_\odot$), increasing the number of stellar metallicities by 6.5 times and resulting in the most populated spectroscopic metallicity distribution function of any UFD. This is possible because we determined the first spectroscopic metallicities of main sequence turn-off stars in any UFD. The MDF of Reticulum II is clearly a bimodal distribution, displaying two peaks with about $80\%$ of the stars in the metal-poor peak at $\rm[Fe/H]=-3.0$ and $20\%$ of the stars in the more metal-rich peak at $\rm[Fe/H]=-2.1$. Such a large metallicity gap can be explained by Type Ia supernova enrichment during a long quiescent period. This supports the currently-favored two-burst star formation history for Reticulum II and shows that such low-mass galaxies clearly can form stars after reionization.