Formation of In Situ Stellar Haloes in Milky Way-Mass Galaxies
  We study the formation of stellar haloes in three Milky Way-mass galaxies
using cosmological smoothed particle hydrodynamics simulations, focusing on the
subset of halo stars that form in situ, as opposed to those accreted from
satellites. In situ stars in our simulations dominate the stellar halo out to
20 kpc and account for 30-40 per cent of its total mass. We separate in situ
halo stars into three straightforward, physically distinct categories according
to their origin: stars scattered from the disc of the main galaxy ('heated
disc'), stars formed from gas smoothly accreted on to the halo ('smooth' gas)
and stars formed in streams of gas stripped from infalling satellites
('stripped' gas). We find that most belong to the stripped gas category. Those
originating in smooth gas outside the disc tend to form at the same time and
place as the stripped-gas population, suggesting that their formation is
associated with the same gas-rich accretion events. The scattered disc star
contribution is negligible overall but significant in the Solar neighbourhood,
where ~90 per cent of stars on eccentric orbits once belonged to the disc.
However, the distinction between halo and thick disc in this region is highly
ambiguous. The chemical and kinematic properties of the different components
are very similar at the present day, but the global properties of the in situ
halo differ substantially between the three galaxies in our study. In our
simulations, the hierarchical buildup of structure is the driving force behind
not only the accreted stellar halo, but also those halo stars formed in situ.
