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In the two-phase scenario of galaxy formation, a galaxy's stellar mass growthis first dominated by in-situ star formation, and subsequently by accretion. Weanalyse the radial distribution of the accreted stellar mass in ~500 galaxiesfrom the hydrodynamical cosmological simulation Magneticum. Generally, we findgood agreement with other simulations in that higher mass galaxies have largeraccreted fractions, but we predict higher accretion fractions for low-massgalaxies. Based on the radial distribution of the accreted and in-situcomponents, we define 6 galaxy classes, from completely accretion dominated tocompletely in-situ dominated, and measure the transition radii between in-situand accretion-dominated regions for galaxies that have such a transition. About70% of our galaxies have one transition radius. However, we also find about 10%of the galaxies to be accretion dominated everywhere, and about 13% to have twotransition radii, with the centre and the outskirts both being accretiondominated. We show that these classes are strongly correlated with the galaxymerger histories, especially with the mergers' cold gas fractions. We find hightotal in-situ (low accretion) fractions to be associated with smaller, lowermass galaxies, lower central dark matter fractions, and larger transitionradii. Finally, we show that the dips in observed surface brightness profilesseen in many early-type galaxies do not correspond to the transition fromin-situ to accretion-dominated regions, and any inferred mass fractions are notindicative of the true accreted mass. Instead, these dips contain informationabout the galaxies' dry minor merger assembly history.

Accreted or Not Accreted? The Fraction of Accreted Mass in Galaxies from Simulations and Observations