Unveiling the formation channels of stellar halos through their chemical fingerprints

Abstract

Context. Stellar halos around galaxies contain key information about their formation and assembly history. Using simulations, we can trace the origins of different stellar populations in these halos, contributing to our understanding of galaxy evolution. Aims. We aim to investigate the assembly of stellar halos and their chemical abundances in 28 galaxies from CIELO cosmological hydrodynamical zoom-in simulations, spanning a broad range of stellar masses, M⋆,gal ∈ 109 ‑ 1011 M⊙. Methods. Stellar halos were identified using the AM-E method, focusing on the outer regions between the 1.5 optical radius and the virial radius. We divided the stellar populations based on their formation channel: ex-situ, endo-debris, and in-situ, and analyzed their chemical abundances, ages, and spatial distributions. Additionally, we explored correlations between halo mass, metallicity, and alpha-element enrichment. Results. CIELO simulations reveal that stellar halos are predominantly composed of accreted material, including both ex-situ and endo-debris stars, in agreement with previous works. The mass fraction of these populations is independent of stellar halo mass, though their metallicities scale linearly with it. Ex-situ stars tend to dominate the outskirts and be more α-rich and older, while endo-debris stars are more prevalent at lower radii and tend to be less α-rich and slightly younger. Massive stellar halos (M⋆,halo > 109.5 M⊙ ) require a median of five additional satellites to build 90% of their mass, compared to lower-mass halos, which typically need fewer (median of 2.5) and lower-mass satellites and are assembled earlier. The diversity of accreted satellite histories results in well-defined stellar halo mass-metallicity and [α/Fe]-[Fe/H] relations, offering a detailed view of the chemical evolution and assembly history of stellar halos. We find that the [α/Fe]-[Fe/H] is more sensitive to the characteristics and star formation history of the contributing satellites than the stellar halo mass-metallicity relationship.