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

TL;DR

This study uses simulations to analyze the age and metallicity gradients of low-mass galaxy stellar halos, revealing complex formation histories and the influence of satellite accretion events.

Contribution

It provides new insights into the radial metallicity and age profiles of low-mass galaxies and their halos, highlighting the role of satellite infall and merger history.

Findings

All galaxies exhibit negative metallicity gradients.

More massive halos tend to be older than less massive ones.

The age profiles often show a U-shape driven by in situ star formation and mergers.

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 in dex/R$_h$ 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.