Heated disc stars in the stellar halo

ABSTRACT

Minor accretion events with mass ratio M_sat : M_host≃ 1 : 10 are common in the context of Λ cold dark matter cosmology. We use high-resolution simulations of Galaxy-analogue systems to show that these mergers can dynamically eject disc stars into a diffuse light component that resembles a stellar halo both spatially and kinematically. For a variety of initial orbital configurations, we find that ∼3–5 × 10⁸ M_⊙ of primary stellar disc material is ejected to a distance larger than 5 kpc above the Galactic plane. This ejected contribution is similar to the mass contributed by the tidal disruption of the satellite galaxy itself, though it is less extended. If we restrict our analysis to the approximate solar neighbourhood in the disc plane, we find that ∼1 per cent of the initial disc stars in that region would be classified kinematically as halo stars. Our results suggest that the inner parts of galactic stellar haloes contain ancient disc stars and that these stars may have been liberated in the very same events that delivered material to the outer stellar halo.