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