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Modern observational and analytic techniques now enable the directmeasurement of star formation histories and the inference of galaxy assemblyhistories. However, current theoretical predictions of assembly are not ideallysuited for direct comparison with such observational data. We therefore extendthe work of prior examinations of the contribution of ex-situ stars to thestellar mass budget of simulated galaxies. Our predictions are specificallytailored for direct testing with a new generation of observational techniquesby calculating ex-situ fractions as functions of galaxy mass and morphologicaltype, for a range of surface brightnesses. These enable comparison with resultsfrom large FoV IFU spectrographs, and increasingly accurate spectral fitting,providing a look-up method for the estimated accreted fraction. We furthermoreprovide predictions of ex-situ mass fractions as functions of galaxy mass,galactocentric radius and environment. Using $z=0$ snapshots from the100cMpc$^3$ and 25cMpc$^3$ EAGLE simulations we corroborate the findings ofprior studies, finding that ex-situ fraction increases with stellar mass forcentral and satellite galaxies in a stellar mass range of 2$\times$10$^{7}$ -1.9$\times$10$^{12}$ M$_{\odot}$. For those galaxies of massM$_*$>5$\times$10$^{8}$M$_{\odot}$, we find that the total ex-situ massfraction is greater for more extended galaxies at fixed mass. When categorisingsatellite galaxies by their parent group/cluster halo mass we find that theex-situ fraction decreases with increasing parent halo mass at fixed galaxymass. This apparently counter-intuitive result may be the result of highpassing velocities within large cluster halos inhibiting efficient accretiononto individual galaxies.

An EAGLE's View of Ex-situ Galaxy Growth