A Relationship Between Stellar Metallicity Gradients and Galaxy Age in Dwarf Galaxies

TL;DR

This study investigates how stellar metallicity gradients in dwarf galaxies are influenced by galaxy age and feedback processes, combining simulations and observations to reveal that recent star formation and feedback shape these gradients.

Contribution

It demonstrates that stellar metallicity gradients are primarily driven by internal feedback and star formation history, rather than external environmental factors, in dwarf galaxies.

Findings

Younger galaxies tend to have flatter metallicity gradients.

Feedback-driven puffing and late-time metal-rich gas accretion influence gradients.

Observed gradients correlate with galaxy age, supporting simulation results.

Abstract

We explore the origin of stellar metallicity gradients in simulated and observed dwarf galaxies. We use FIRE-2 cosmological baryonic zoom-in simulations of 26 isolated galaxies as well as existing observational data for 10 Local Group dwarf galaxies. Our simulated galaxies have stellar masses between $10^{5.5}$ and $10^{8.6} \msun$. Whilst gas-phase metallicty gradients are generally weak in our simulated galaxies, we find that stellar metallicity gradients are common, with central regions tending to be more metal-rich than the outer parts. The strength of the gradient is correlated with galaxy-wide median stellar age, such that galaxies with younger stellar populations have flatter gradients. Stellar metallicty gradients are set by two competing processes: (1) the steady "puffing" of old, metal-poor stars by feedback-driven potential fluctuations, and (2) the accretion of extended, metal-rich gas at late times, which fuels late-time metal-rich star formation. If recent star formation dominates, then extended, metal-rich star formation washes out pre-existing gradients from the "puffing" process. We use published results from ten Local Group dwarf galaxies to show that a similar relationship between age and stellar metallicity-gradient strength exists among real dwarfs. This suggests that observed stellar metallicity gradients may be driven largely by the baryon/feedback cycle rather than by external environmental effects.