Formation of Milky Way-type stellar haloes in a Λ-CDM cosmology

Abstract

Recent observations suggest that the Milky Way stellar halo has a ‘dual nature’, meaning that both dissipational and dissipationless processes play a role in its build-up. The GIMIC suite of cosmological hydro-dynamical simulations show that, for Milky Way-mass haloes, in situ star formation is the dominant factor in the inner < 20 − 30 kpc, while tidal disruption of satellite galaxies contributes primarily to the outer regions. The in situ stars are found to originate in the earlier disc, at redshifts ~ 1– 1.5, and subsequently diffusing out of the disc by dynamical heating associated with mergers. The in situ component has a more flattened shape, a net prograde rotation and more metal-rich populations, in quantitative agreement with the observations. We conclude that the dual nature of the stellar halo is entirely compatible with the currently favoured Λ-CDM model.