📝 SummarXiv

Abstract

Context. The mass-metallicity relation is a fundamental galaxy scaling law that has been extended to the faintest systems in the Local Group. The small scatter in this relation has been used to argue against severe tidal mass loss in Local Group satellites. Aims. We aim to show that the observed scatter is consistent with tidal mass loss seen in cosmological hydrodynamical zoom-in simulations, using simulated Milky Way-mass haloes from the Auriga project as an example. Methods. We analyze the stellar masses and mean metallicites for all accreted systems in Auriga. We compute these quantities two ways: considering the total system (bound + lost material) and only considering the bound progenitor. For both cases we fit the mass-metallicity relation and consider properties that drive scatter about these relations. Results. Accreted systems in Auriga have a tight relation between total stellar mass and metallicity, with scatter at a fixed stellar mass driven by age. When only considering the bound progenitor, the tidally evolved mass-metallicity relation has similar scatter ($\sim$0.27 dex) as observed for the Milky Way and M31 satellites ($\sim$0.23 dex). Satellites that lie above the relation have experienced substantial mass loss and typically have low metallicity for their total stellar mass. Even satellites that fall exactly on the evolved relation can lose over half of their stellar mass. Only satellites that are substantially below the evolved relation are reliably intact. Conclusions. The small scatter in the observed mass-metallicity relation is compatible with tidal mass loss. Based on their offset from the observed relation, we predict which Milky Way and M31 satellites have experienced substantial mass loss. These predictions will be tested with deep resolved-star observations from upcoming facilities including Rubin LSST, Euclid, and Roman.