  Galaxy mergers are instrumental in dictating the final mass, structure,
stellar populations, and kinematics of galaxies. Cosmological galaxy
simulations indicate that the most massive galaxies at z=0 are dominated by
high fractions of `ex-situ' stars, which formed first in distinct independent
galaxies, and then subsequently merged into the host galaxy. Using spatially
resolved MUSE spectroscopy we quantify and map the ex-situ stars in thirteen
massive Early Type galaxies. We use full spectral fitting together with
semi-analytic galaxy evolution models to isolate the signatures in the
galaxies' light which are indicative of ex-situ populations. Using the large
MUSE field of view we find that all galaxies display an increase in ex-situ
fraction with radius, with massive and more extended galaxies showing a more
rapid increase in radial ex-situ fraction, (reaching values between 30% to 100%
at 2 effective radii) compared to less massive and more compact sources
(reaching between 5% to 40% ex-situ fraction within the same radius). These
results are in line with predictions from theory and simulations which suggest
ex-situ fractions should increase significantly with radius at fixed mass for
the most massive galaxies.
