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

Abstract In the two-phase scenario of galaxy formation, a galaxy’s stellar mass growth is first dominated by in-situ star and subsequently accretion. We analyze radial distribution accreted in ∼500 galaxies from (48 Mpc/ h ) 3 box volume hydrodynamical cosmological simulation Magneticum, stellar-mass range 10 to 12 M ⊙ . find that higher-mass have larger fractions, as found previous works, but predict generally higher accretion fractions for low-mass galaxies. Based on 3D components, we define six classes, completely dominated, measure transition radii between accretion-dominated regions reveal transition. About 70% our one radius. However, about 10% are everywhere, 13% two radii, with center outskirts both being dominated. show these classes strongly correlated merger histories, especially cold gas fraction at time merging. high total (low accretion) be associated smaller, lower-mass galaxies, lower central dark-matter radii. Finally, dips observed surface brightness profiles seen many early-type do not correspond regions, any inferred indicative true contain information galaxies’ dry-merger history.