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Predictions for Complex Distributions of Stellar Elemental Abundances in Low-Mass Galaxies

We investigate stellar elemental abundance patterns at z = 0 in 8 low-mass (M_* = 10^6 - 10^9 M_sun) galaxies in the Feedback in Realistic Environments (FIRE-2) cosmological simulations. Using magnesium (Mg) as a representative alpha-element, we explore stellar abundance patterns in [Mg/Fe] versus [Fe/H], which follow an overall monotonic trend that evolved slowly over time. Beyond this, we explore 3 notable secondary features in enrichment (found in three different case-study galaxies) that arise from a galaxy merger or bursty star formation. First, we observe a secondary track with a lower [Mg/Fe] than the main trend. At z = 0, stars from this track are predominantly found within 2-6 kpc of the center; they were accreted in a 1:3 total-mass-ratio merger ~ 0.4 Gyr ago. Second, we find a distinct elemental bi-modality that forms following a strong burst in star formation in a galaxy at t_lookback ~ 10 Gyr. This burst quenched star formation for ~ 0.66 Gyr, allowing Ia supernovae to enrich the system with iron before star formation resumed. Third, we examine stripes in enrichment that run roughly orthogonal to the dominant [Mg/Fe] versus [Fe/H] trend; these stripes correspond to short bursts of star formation during which core-collapse supernovae enrich the surrounding medium with Mg (and Fe) on short timescales. If observed, these features would substantiate the utility of elemental abundances in revealing the assembly and star formation histories of dwarf galaxies. We explore the observability of these features for upcoming spectroscopic studies. Our results show that precise measurements of elemental abundance patterns can reveal critical events in the formation histories of low-mass galaxies.