An EAGLE's View of Ex-situ Galaxy Growth
  Modern observational and analytic techniques now enable the direct
measurement of star formation histories and the inference of galaxy assembly
histories. However, current theoretical predictions of assembly are not ideally
suited for direct comparison with such observational data. We therefore extend
the work of prior examinations of the contribution of ex-situ stars to the
stellar mass budget of simulated galaxies. Our predictions are specifically
tailored for direct testing with a new generation of observational techniques
by calculating ex-situ fractions as functions of galaxy mass and morphological
type, for a range of surface brightnesses. These enable comparison with results
from large FoV IFU spectrographs, and increasingly accurate spectral fitting,
providing a look-up method for the estimated accreted fraction. We furthermore
provide predictions of ex-situ mass fractions as functions of galaxy mass,
galactocentric radius and environment. Using $z=0$ snapshots from the
100cMpc$^3$ and 25cMpc$^3$ EAGLE simulations we corroborate the findings of
prior studies, finding that ex-situ fraction increases with stellar mass for
central and satellite galaxies in a stellar mass range of 2$\times$10$^{7}$ -
1.9$\times$10$^{12}$ M$_{\odot}$. For those galaxies of mass
M$_*$>5$\times$10$^{8}$M$_{\odot}$, we find that the total ex-situ mass
fraction is greater for more extended galaxies at fixed mass. When categorising
satellite galaxies by their parent group/cluster halo mass we find that the
ex-situ fraction decreases with increasing parent halo mass at fixed galaxy
mass. This apparently counter-intuitive result may be the result of high
passing velocities within large cluster halos inhibiting efficient accretion
onto individual galaxies.
