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Motivated by the observed bottom-light initial mass function (IMF) in faintdwarfs, we study how a metallicity-dependent IMF affects the feedback budgetand observables of an ultra-faint dwarf galaxy. We model the evolution of alow-mass ($\approx 8 \, \times \, 10^{8} \, \rm M_{\odot}$) dark matter halowith cosmological, zoomed hydrodynamical simulations capable of resolvingindividual supernovae explosions, which we complement with an empiricallymotivated subgrid prescription for systematic IMF variations. In thisframework, at the low gas metallicities typical of faint dwarfs, the IMF ofnewborn stellar populations becomes top-heavy, increasing the efficiency ofsupernova and photoionization feedback in regulating star formation. Thisresults in a 100-fold reduction of the final stellar mass of the dwarf comparedto a canonical IMF, at fixed dynamical mass. The increase in the feedbackbudget is nonetheless met by increased metal production from more numerousmassive stars, leading to nearly constant iron content at $z=0$. Ametallicity-dependent IMF therefore provides a mechanism to produce low-mass($\rm M_{\star}\sim 10^3 \rm M_{\odot}$), yet enriched ($\rm [Fe/H]\approx -2$)field dwarf galaxies, thus opening a self-consistent avenue to populate theplateau in $\rm [Fe/H]$ at the faintest end of the mass--metallicity relation.

EDGE: The sensitivity of ultra-faint dwarfs to a metallicity-dependent initial mass function