Redshift evolution of the ex situ stellar mass fraction. Left: the fraction of ex situ stars across the whole Illustris volume as a function of redshift, obtained by summing over all galaxy stellar masses (black lines). The green and magenta curves denote the contributions from stars that were accreted via completed mergers (of any mass ratio) and from those that were stripped from surviving galaxies, respectively. The solid, dashed, and dotted lines correspond to the Illustris-1, 2, and 3 simulations, of which Illustris-1 has the highest resolution. Right: the ex situ stellar mass fraction as a function of galaxy stellar mass, shown at different redshifts. The black line corresponds to z = 0 and is shown for reference (it is identical to the solid black line in Figs 3 and 4), while the coloured lines show the ex situ stellar mass fraction at z = 1, 2, 3, and 4.