[Paper Review] Totally metal: A relationship between stellar metallicity gradients and galaxy age in dwarf galaxies

Francisco J. Mercado, James S. Bullock+8 more|arXiv (Cornell University)|Sep 2, 2020

Galaxies: Formation, Evolution, Phenomena4 references4 citations

TL;DR

This study links stellar metallicity gradients in dwarf galaxies to their median stellar age via feedback-driven processes: old stars are puffed outward by feedback, creating metallicity gradients, while late-time star formation—especially of metal-rich stars—flattens these gradients. Observations of Local Group dwarfs confirm the predicted inverse correlation between age and gradient strength, implicating internal baryonic feedback as the dominant driver over environmental effects.

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 $10^{8.6} \msun$. We find that stellar metallicity gradients are common in our simulations, 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. The strengths of these gradients are set by two competing processes: (1) the steady puffing of old, metal-poor stars by feedback-driven potential fluctuations, and (2) extended, late-time star formation. Late-time star formation tends to be metal rich, so galaxies with significant late-time star formation will mitigate the gradients formed by the puffing process. We use published results from ten Local Group dwarf galaxies to show that our predicted relationship between age and stellar metallicity-gradient strength is consistent with existing observations. This suggests that observed stellar metallicity gradients may be driven largely by the baryon/feedback cycle rather than by external environmental effects.

Motivation & Objective

• To investigate the origin of stellar metallicity gradients in dwarf galaxies, focusing on internal baryonic processes rather than external environmental effects.
• To determine how the age of stellar populations influences the strength of metallicity gradients in both simulated and observed dwarf galaxies.
• To test whether feedback-driven potential fluctuations and late-time star formation are the dominant mechanisms shaping metallicity gradients.
• To validate simulation predictions against observational data from 10 Local Group dwarf galaxies.
• To establish a quantitative relationship between galaxy-wide median stellar age and the steepness of metallicity gradients.

Proposed method

• Utilized FIRE-2 cosmological baryonic zoom-in simulations of 26 isolated dwarf galaxies with stellar masses between $10^{5.5}$ and $10^{8.6} m{M}_igodot$.
• Tracked the evolution of stellar metallicity gradients by analyzing spatial distributions of stars and their metallicities across radial bins.
• Quantified the competing effects of feedback-driven puffing of old, metal-poor stars and late-time star formation of metal-rich stars.
• Applied a median stellar age metric across each galaxy to correlate with gradient strength and assess temporal trends.
• Compared simulated gradients with observed metallicity gradients from 10 Local Group dwarf galaxies using published data.
• Used statistical analysis to test the significance of the correlation between median stellar age and gradient steepness.

Experimental results

Research questions

• RQ1 What drives the formation of stellar metallicity gradients in dwarf galaxies—internal feedback or external environmental effects?
• RQ2 How does the median age of a galaxy's stellar population correlate with the strength of its metallicity gradient?
• RQ3 To what extent does late-time star formation mitigate metallicity gradients formed by feedback-driven radial mixing?
• RQ4 Is the predicted relationship between stellar age and metallicity gradient strength consistent with observations of Local Group dwarf galaxies?
• RQ5 Can feedback-driven potential fluctuations alone explain the observed gradient patterns, or is additional star formation required?

Key findings

• Stellar metallicity gradients are common in simulated dwarf galaxies, with central regions being more metal-rich than outer regions.
• Galaxies with younger stellar populations exhibit flatter metallicity gradients, indicating a strong inverse correlation between median stellar age and gradient strength.
• The strength of metallicity gradients is governed by a competition between feedback-driven puffing of old, metal-poor stars and late-time star formation of metal-rich stars.
• Late-time star formation acts to flatten metallicity gradients by introducing metal-rich stars into outer regions.
• The predicted correlation between younger stellar populations and flatter gradients is consistent with observational data from 10 Local Group dwarf galaxies.
• The results suggest that internal baryon/feedback cycles are the primary drivers of observed metallicity gradients, not external environmental effects.