The role of accreted and in situ populations in shaping the stellar halos of low-mass galaxies

¹ Departamento de Astronomía, Universidad de La Serena, Av. Raúl Bitrán 1305, La Serena, Chile
² Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany
⁴ Cardiff Hub for Astrophysics Research and Technology, School of Physics and Astronomy, Cardiff University, Queen’s Buildings, Cardiff CF24 3AA, UK
⁵ Instituto de Astrofísica, Pontificia Univerisidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
⁶ Centro de AstroIngeniería, Pontificia Univerisidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
⁷ Núcleo Milenio ERIS, ANID, Chile
⁸ Department of Physics and Astronomy “Augusto Righi”, University of Bologna, Via P. Gobetti 93/2, I-40129 Bologna, Italy
⁹ INAF, Astrophysics and Space Science Observatory Bologna, Via P. Gobetti 93/3, I-40129 Bologna, Italy
¹⁰ Institut für Astrophysik, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland

^⋆ Corresponding author: elisa.tau@userena.cl

Received: 17 December 2024
Accepted: 20 May 2025

Abstract

Context. The stellar halos of low-mass galaxies (M_* ≤ 10¹⁰ M_⊙) are becoming objects of interest among the extragalactic community due to a recent set of observations with the capacity to detect such structures. Additionally, new and very-high-resolution cosmological simulations have been performed, enabling the study of this faint component in low-mass galaxies. The presence of stellar halos in low-mass systems could help shed light on our understanding of the assembly of low-mass observed galaxies and their evolution. It could also allow us to test whether the hierarchical model for the formation of structures is applicable at small scales.

Aims. In this work, we aim to characterise the stellar halos of simulated low-mass galaxies and analyse their evolution and accretion history.

Methods. We used a sample of 17 simulated low-mass galaxies from the Auriga Project with a stellar mass range from 3.28 × 10⁸ M_⊙ to 2.08 × 10¹⁰ M_⊙. These are cosmological magneto-hydrodynamical zoom-in simulations that have a very high resolution 5 × 10⁴ M_⊙ in dark matter (DM) mass and ∼6 × 10³ M_⊙ in baryonic mass. We defined the stellar halo as the stellar material located outside of an ellipsoid with semi-major axes equal to four times the half-light radius of each galaxy. We analysed the stellar halos of these galaxies and studied their formation channels.

Results. We find that the inner regions of the stellar halo (between four and six times the half-light radius) are dominated by in situ material. For the less massive simulated dwarfs (M_* ≤ 4.54 × 10⁸ M_⊙), this dominance extends to all radii. We find that this in situ stellar halo is mostly formed in the inner regions of the galaxies and was subsequently ejected into the outskirts during interactions and merger events with satellite galaxies. In ∼50% of the galaxies, the stripped gas from satellite galaxies (likely mixed with the gas from the host dwarf) contributed to the formation of this in situ halo. The stellar halos of the galaxies more massive than M_* ≥ 1 × 10⁹ M_⊙ are dominated by the accreted component beyond six half-light radii. We find that the more massive dwarf galaxies (M_* ≥ 6.30 × 10⁹ M_⊙) accrete stellar material until later times (τ₉₀ ≈ 4.44 Gyr ago, with τ₉₀ as the formation time) than the less massive ones (τ₉₀ ≈ 8.17 Gyr ago). This has an impact on the formation time of the accreted stellar halos. These galaxies have between one and seven significant progenitors that contribute to the accreted component of these galaxies; however, there is no clear correlation between the amount of accreted mass of the galaxies and their number of significant progenitors.