Accreted stars and stellar haloes of simulated galaxies in TNG50

¹ Facultad de Matemática, Astronomía, Física y Computación, UNC, Medina Allende s/n, X5000HUA Córdoba, Argentina
² Instituto de Astronomía Teórica y Experimental, CONICET–UNC, Laprida 854, X5000BGR Córdoba, Argentina
³ Observatorio Astronómico de Córdoba, UNC, Laprida 854, X5000BGR Córdoba, Argentina
⁴ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 5C2, Canada

^⋆ Corresponding author: bruno.celiz@mi.unc.edu.ar

Received: 28 July 2025
Accepted: 20 October 2025

Abstract

We used the TNG50 cosmological hydrodynamic simulation to study the accreted stellar component and stellar haloes of isolated galaxies spanning a wide range of masses (10⁸ < M_*/M_⊙ < 10¹¹). We find that stars formed in the main progenitor (i.e. in situ stars) typically dominate the inner regions as far as ∼10 half-light radii from the centre, implying that detecting uncontrovertible evidence for the presence of an accreted stellar halo requires the probing of the far outskirts of a galaxy. Stars from accreted, disrupted satellites (i.e. ex situ stars) dominate beyond that radius (roughly 25% of the virial radius, r₂₀₀), which we identify as the inner boundary of the outer stellar halo. The fraction of accreted stars decreases monotonically with decreasing galaxy mass, M_*, from ∼20% on average in ∼2 × 10¹² M_⊙ haloes (M_* ∼ 10¹¹ M_⊙) to 2–3% in ∼2 × 10¹⁰ M_⊙ haloes (M_* ∼ 10⁸ M_⊙). The outer halo has a mass comparable to roughly 10% of all accreted stars. Fewer than ∼30% of stars in the outer halo are in situ stars, many of which originate from star-forming satellites during the late stages of disruption, especially in low-mass systems. Accreted stars are systematically metal poorer in less massive systems, which makes the outer haloes of dwarf galaxies a fertile hunting ground for extremely metal-poor stars. The density profile of accreted stars can be well approximated by a Sérsic law, whose index, n, and effective radius, R_eff, depend strongly on the total accreted mass. At given galaxy mass, the more massive stellar haloes are systematically more concentrated (smaller R_eff) and have steeper density profiles (larger n). The accreted component generally has a larger R_eff than the main galaxy, although the two can have similar characteristic radii in the most massive systems with the largest accreted fractions. Our results provide a blueprint for interpreting observations of the outskirts of isolated galaxies in terms of their assembly histories.