2024MNRAS.534L..35M

Under the assumption of hierarchical galaxy formation, dwarf galaxies are the closest existing analogues to the high-redshift protogalaxies that merged to form the Milky Way. These low-mass systems serve as unique laboratories for studying nucleosynthetic channels given that the chemical compositions of their stars play a pivotal role in constraining their chemical enrichment history. To date, stellar abundances in dwarf galaxies have focused almost exclusively on elemental abundance ratios. While important, elemental abundances omit critical information about the isotopic composition. Here, we compute the Mg isotopic ratios of six accreted dwarf galaxy stars (low $\alpha$) and seven Milky Way stars (high $\alpha$) using a set of high-resolution (65 000 < R < 160 000) and high signal-to-noise ratio ($\rm {S/N} \gt 250$) optical spectra. We show, for the first time, that at a given [Fe/H] stars born in a dwarf galaxy differ in their Mg isotopic ratios from stars born in the Milky Way. However, when comparing isotopic ratios at a given [Mg/H] rather than [Fe/H], a powerful diagnostic emerges that suggests nucleosynthesis processes are consistent across different stellar environments. This universality of Mg isotopic abundances provides additional dimensionality for chemical evolution models and helps to constrain massive star nucleosynthesis across cosmic time.