Accreted stars and stellar haloes of simulated galaxies in TNG50

Abstract

We use the TNG50 cosmological hydrodynamic simulation to study the accreted stellar component and stellar haloes of isolated galaxies spanning a wide range of masses ($10^8<M_*/M_\odot<10^{11}$). We find that stars formed in the main progenitor (i.e., in-situ stars) typically dominate the inner regions as far as $\sim$10 half-light radii from the centre, implying that detecting uncontrovertible evidence for the presence of an accreted stellar halo requires probing 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, $r_{200}$), 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 $\sim$$20\%$ on average in $\sim$$2\times 10^{12}\, M_\odot$ haloes ($M_*\sim$$10^{11}\, M_\odot$) to $2$-$3\%$ in $\sim$$2\times 10^{10}\, M_\odot$ haloes ($M_*\sim$$10^{8}\, M_\odot$). The outer halo has a mass comparable to roughly $10\%$ of all accreted stars. Fewer than $\sim$$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 more metal poor in less massive systems, which makes the outer haloes of dwarf galaxies a fertile hunting ground for extremely metal-poor stars. At given galaxy mass, the more massive stellar haloes are systematically more concentrated (smaller $R_{\rm eff}$) and have steeper density profiles (larger $n$). Our results provide a blueprint for interpreting observations of the outskirts of isolated galaxies in terms of their assembly histories.