Stellar haloes at the EDGE of galaxy formation

Ultra-faint dwarf galaxies (UFDs) are commonly found in close proximity to the Milky Way and other massive spiral galaxies. Recently, many of these nearby dwarf galaxies have garnered attention for hosting faint, metal-poor stars at significant distances from the centres of their galaxies, suggesting the existence of stellar haloes—a longstanding uncertainty for galaxies of these masses. In this talk, we share insights from our investigation into the stellar outskirts and accretion histories of 13 isolated dwarf galaxies drawn from the 'Engineering Dwarfs at Galaxy Formation's Edge' (EDGE) cosmological simulation suite. Our EDGE dwarfs span halo mass sizes of M_200 ~ 1e9 − 1e10 MSun and stellar mass sizes M_* ~ 1e4 − 1e7 Sun, and are simulated within a cosmological Lambda Cold Dark Matter (ΛCDM) framework, enabling direct comparison with observational data. Using stellar particle IDs in the EDGE simulations, we trace back the formation histories of these dwarf galaxies to unveil the mechanism responsible for stellar material located far from their galactocentres. Our analysis reveals that all 13 isolated dwarf galaxies harbour stellar haloes resulting from the late-time dry accretion of lower-mass companions. Furthermore, we observe a prevalence of metal-poor (-2.5 < [Fe/H] < -7) stellar content at large galactocentric radii, a characteristic signature of stellar haloes, consistent with observations such as the recent discovery of distant, metal-poor stars lying > 2 half-light radii from the centre of the Tucana II UFD (M_* ~ 3000 MSun), along with many other UFDs. Our findings underscore the ubiquity of stellar haloes in dwarf galaxies, even at the smallest scales. We establish that the assembly process to form UFDs involves multiple small dark matter haloes merging in a hierarchical fashion, concluding that stellar haloes in tidally undisturbed dwarf galaxies are a function of the merger history and can be expected across all mass ranges.