The stellar mass assembly of galaxies in the Illustris simulation: growth by mergers and the spatial distribution of accreted stars

We use the Illustris simulation to study the relative contributions of <it>in situ</it> star formation and stellar accretion to the build-up of galaxies over an unprecedentedly wide range of masses (<it>M</it><inf>*</inf> = 109-1012 M<inf>⊙</inf>), galaxy types, environments, and assembly histories. We find that the ‘two-phase’ picture of galaxy formation predicted by some models is a good approximation only for the most massive galaxies in our simulation – namely, the stellar mass growth of galaxies below a few times 1011 M<inf>⊙</inf> is dominated by <it>in situ</it> star formation at all redshifts. The fraction of the total stellar mass of galaxies at <it>z</it> = 0 contributed by accreted stars shows a strong dependence on galaxy stellar mass, ranging from about 10 per cent for Milky Way-sized galaxies to over 80 per cent for <it>M</it><inf>*</inf> ≈ 1012 M<inf>⊙</inf> objects, yet with a large galaxy-to-galaxy variation. At a fixed stellar mass, elliptical galaxies and those formed at the centres of younger haloes exhibit larger fractions of <it>ex situ</it> stars than disc-like galaxies and those formed in older haloes. On average, ∼50 per cent of the <it>ex situ</it> stellar mass comes from major mergers (stellar mass ratio μ > 1/4), ∼20 per cent from minor mergers (1/10 < μ < 1/4), ∼20 per cent from very minor mergers (μ < 1/10), and ∼10 per cent from stars that were stripped from surviving galaxies (e.g. flybys or ongoing mergers). These components are spatially segregated, with <it>in situ</it> stars dominating the innermost regions of galaxies, and <it>ex situ</it> stars being deposited at larger galactocentric distances in order of decreasing merger mass ratio.