A relationship between stellar metallicity gradients and galaxy age in dwarf galaxies

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 105.5 and 108.6 M⊙. 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.

Chemical and stellar properties of star-forming dwarf galaxies

“…Most of them show, within observational uncertainties of ∼0.1-0.2 dex, a rather homogeneous spatial distribution of metals within ∼1 kpc scale [126][127][128], suggesting a rapid dispersion and mixing of the ejecta from stellar winds and supernovae across the ISM. This behavior is in agreement with the predictions of cosmological, three-dimensional hydrodynamical simulations of dwarf galaxies with a realistic feedback treatment and the inclusion of metal diffusion [19,129,130], which produce typically flat gas-phase metallicity gradients, with the rare occurrence of steep gradients limited to the inner galaxy regions [131].…”

Predictions for Complex Distributions of Stellar Elemental Abundances in Low-Mass Galaxies

“…The highestmetallicity stars are at smaller radii (< 4 kpc), while the most metalpoor stars are found in larger radial bins (4-8 kpc). This generally agrees with the radial dependencies of abundances and ages of FIRE-2 dwarf galaxies found in Graus et al (2019) and Mercado et al (2020). The bottom panel shows the [Mg/Fe] for stars in the same radial bins.…”

“…Additionally, the radial dependence of the overall distributions in [Mg/Fe] and [Fe/H] vary little (< 0.1 dex) and are generally selfsimilar across all radii. Graus et al (2019) and Mercado et al (2020) investigated the radial dependencies of FIRE-2 dwarf galaxies and characterized their radial age variations and mean metallicity gradients. Stars in central regions of these galaxies tend to be younger and metal-rich compared to stars at larger radii, consistent with several observations.…”

Elemental Abundances in M31: Gradients in the Giant Stellar Stream*

“…In comparison, smaller Local Group dwarf galaxies (M å  10 8.5 M e ) have diverse gradients, ranging from flat to as steep as −0.4 dex per half-light radius (e.g., Kirby et al 2011Kirby et al , 2017Leaman et al 2013;Vargas et al 2014b;Ho et al 2015;Kacharov et al 2017), with no clear relationship between the magnitude of a gradient and luminosity, host distance, or morphology (Ho et al 2015, cf. Leaman et al 2013, although there may be a trend with median stellar age (Mercado et al 2021).…”