Description
The stellar mass-metallicity relation (MZR) is a strong correlation between $M_*$ and [Fe/H], but below $M_* \sim 10^5$\,M$_\odot$, it flattens with increased scatter. This has been linked to a top-heavy initial mass function, tidal stripping, or Population III stars, but no definitive cause is known. We explore whether environmental effects explain this plateau.
We show that tidal stripping moves galaxies leftward in the MZR, inducing scatter, but most Milky Way satellites show little stellar mass loss, making tides an unlikely primary cause. Quenching during early evolution can freeze in metallicity scatter in isolated EDGE dwarfs at $M_* < 10^4$\,M$_\odot$. This owes to scatter in their central densities, which affects star formation and metal enrichment rates, and in the accretion of low-metallicity gas (`dilution’). However, it fails to explain the highest metallicities observed ([Fe/H] $\sim -2$).
Instead, "dwarfs-of-dwarfs" in EDGE simulations naturally populate the plateau at $M_* < 10^4$, with scatter comparable to real data, sometimes reaching [Fe/H] $\sim -1$. This results from two effects: pollution of metal-rich gas from a nearby host dwarf and confinement of outflows by the circumgalactic medium. Since these effects occur in satellite dwarfs long before they encounter a larger galaxy like the Milky Way, we predict that the plateau and its scatter should be present in most environments, except in extreme isolation.
