Using the Millennium II simulation to test CDM predictions for the structure of massive galaxies

We have combined the semi-analytic galaxy formation model of Guo et al. (2011) with a novel particle-tagging technique to predict galaxy surface brightness profiles in a representative sample of ~1900 massive dark matter haloes (10¹²-10¹⁴ M_⊙) from the Millennium II ΛCDM N body simulation. We focus on the outer regions of galaxies and stars accreted in mergers. Our simulations cover scales from the stellar haloes of Milky Way-like galaxies to the `cD envelopes' of groups and clusters, and resolve low surface brightness substructure such as the tidal streams of dwarf galaxies. We find that the spatial distribution of stars in low surface brightness regions is tightly correlated with DM halo mass and that collisionless merging during the hierarchical assembly of galaxies largely determines the structure of spheroidal stellar components. Our ΛCDM model agrees well with the available data.