Age and metallicity of low-mass galaxies: from their centres to their stellar halos

TL;DR

We study metallicity and age distributions from galaxy centers to stellar halos in 17 low-mass Auriga dwarfs to understand formation histories. Using high-resolution cosmological zoom-in simulations in a $\Lambda$CDM framework, we find negative $[Fe/H]$ gradients, no simple mass-based gradient trend, and a timing-driven metallicity variation in accreted halos; U-shaped radial age profiles are common and largely in situ–driven, while halo ages scale with halo mass and accreted halos show younger ages at higher metallicity. A strong age–metallicity relation is evident for accreted halos, reflecting late-time chemical enrichment before disruption, while in situ material can mask this trend in total halos. Overall, the results reveal a wide diversity of evolutionary pathways for dwarf galaxies and their halos and highlight the outskirts as a key venue for constraining their assembly histories, with future surveys and facilities poised to test these predictions.

Abstract

We aim to analyse the metallicity and the ages of the stellar halos of low-mass galaxies to better understand their formation history. We use 17 simulated low-mass galaxies from the Auriga Project ($\sim 3 \times 10^8 \, M_\odot \leq M_* \lesssim 2 \times 10^{10} \, M_\odot$). We study the metallicity and the ages of these galaxies and their stellar halos, as well as the relation between these two properties. We find that all galaxies have negative radial [Fe/H] gradients, and that the centres of less massive dwarfs are generally more metal poor than those of more massive dwarfs. We find no correlation between the metallicity gradients and intrinsic galaxy properties, such as stellar mass or accreted stellar mass, suggesting that these gradients are not a simple byproduct of galaxy evolution in the low-mass regime. We also find that the dispersion in the mass-metallicity relation found in the stellar halos of low-mass galaxies can be explained with the infall time of their most dominant satellite: at a fixed accreted stellar halo mass, dwarf galaxies that accreted this satellite at later times have more metal-rich accreted stellar halos. Regarding the ages of the analysed galaxies, we find a prominent U-shape in the radial mean age profiles of $\sim 65\%$ of them, which is mainly driven by the in situ stellar material. This presence of a U-shape in the age profiles is due to the combination of the cessation of recent star formation at large radial distances and the merger events these galaxies undergo, which redistribute the stellar material to their outer regions. When focusing on the ages of the stellar halos, we find that more massive ones are older than less massive ones. Our results show a wide variety in ages and metallicities of low-mass galaxies and their stellar halos, reflecting the complex and non-uniform evolutionary pathways these systems can follow.

Figures (12)

• Figure 1: Total median [Fe/H] (black dotted line) and accreted median [Fe/H] (pink dash-dotted line) profiles of the galaxies in our sample. Individual [Fe/H] profiles for $6$ galaxies of our sample are also shown, normalised by their respective $R_h$ and colour-coded by their stellar masses.
• Figure 2: Metallicity gradients of the galaxies of our sample computed when considering the metallicity profile of the galaxy within $4 \, R_h$ (blue), and the metallicity profile of the total stellar halo (pink) and the accreted stellar halo (green).
• Figure 3: Left panel: Metallicity gradient of the galaxies of our sample computed within $4 \, R_h$ (empty circles) and $10 \, R_h$ (filled circles), as a function of their stellar mass. Right panel: Metallicity gradients of the total stellar halo and the accreted stellar halo as a function of the galaxies' stellar mass, represented with filled circles and empty triangles respectively. The arrow represents the value corresponding to the accreted stellar halo of Auriga 17, which is $\sim -0.3$. Both panels are colour-coded by the galaxies accreted mass fraction.
• Figure 4: Median [Fe/H] of the accreted stellar halo of the galaxies as a function of their most dominant satellite's infall time, colour-coded by these satellites' stellar mass.
• Figure 5: Left panel: median [Fe/H] of the stellar halos of the galaxies as a function of their masses. Right panel: median [Fe/H] of the accreted stellar halos of the galaxies as a function of their masses. Both panels are colour-coded by the mean infall look-back time of the significant progenitors of the stellar halos. We note a correlation between the metallicity of the accreted stellar halo and the infall time of its significant progenitors: more metal rich accreted halos have accreted their significant progenitors at later times.