Accreted stars and stellar haloes of simulated galaxies in TNG50

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, Reff, depend strongly on the total accreted mass. At given galaxy mass, the more massive stellar haloes are systematically more concentrated (smaller Reff) and have steeper density profiles (larger n). The accreted component generally has a larger Reff 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.