Speaker
Description
The well-known stellar mass-metallicity relation observed in galaxies shows significant increase in scatter and a metallicity plateau in the regime of ultra-faint dwarfs (UFDs). Two mechanisms may lead to this: pre-enrichment of accreted gas, or internal enrichment and modulation by metal outflow. We consider the role both processes play in driving the observed relation. We study pre-enrichment by applying the semi-analytical model GRUMPY to mass assembly histories of UFD halos in MW environments and draw metallicity of accreted intergalactic gas $(Z_{IGM})$ from the distribution of gas likely to be accreted by UFDs when they are expected to be star forming. We quantify $Z_{IGM}$ using several cosmological simulations of galaxy formation with different star formation, feedback, and pop. III prescriptions (e.g. FIRE-2 and MEGATRON). We find that the overwhelming majority $(\gt 90-95 \%)$ of gas that would be accreted by UFDs is unenriched, well below a typical metallicity floor of $\mathrm{[Fe/H]}=-4$. Using these $Z_{IGM}$ distributions, the GRUMPY model predicts lower mean UFD metallicities, and insufficient scatter to reproduce the observed mass-metallicity relation. Assuming this largely unenriched $Z_{IGM}$, we further explore outflow properties and their stochastic variation which may reproduce the observed metallicity properties of UFDs. We find that variation in outflow parameters (e.g. mass loading factor), strongly shape the mass-metallicity relation, and effectively produce scatter in mass-metallicity. Therefore, metallicity regulation by outflow properties is more likely to shape the metallicity properties of UFDs than pre-enrichment of accreted gas, and we place constraints on the outflow parameters of these low mass relic systems.
