Accretion history of an ‘average’ MW, in the space of lookback time and peak halo mass. We have marked some important events in the accretion history of the MW known to date, with scatter points and uncertainties based on values from the literature (see Sect. 3.1 for details). These represent regions of interest in this space, chosen to capture the characteristics of some of the key accretion events, with four selections in Table 1. The red line is the median M₂₀₀ mass of the main halo across the level-4 AURIGA simulations against time, with green shaded regions indicating the 1σ and 2σ spreads (defined as the 18–84% and 5–95% ranges).