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 formation, and subsequently by accretion. We analyze the radial distribution of the accreted stellar mass in ∼500 galaxies from the (48 Mpc/h)3 box volume of the hydrodynamical cosmological simulation Magneticum, in a stellar-mass range of 1010 to 1012 M ⊙. We find that higher-mass galaxies have larger accreted fractions, as found in previous works, but predict generally higher accretion fractions for low-mass galaxies. Based on the 3D radial distribution of the accreted and in-situ components, we define six galaxy classes, from completely accretion to completely in-situ dominated, and measure the transition radii between in-situ and accretion-dominated regions for galaxies that reveal a transition. About 70% of our galaxies have one transition radius. However, about 10% of the galaxies are accretion dominated everywhere, and about 13% have two transition radii, with the center and the outskirts both being accretion dominated. We show that these classes are strongly correlated with the galaxy merger histories, especially with the cold gas fraction at the time of merging. We find high total in-situ (low accretion) fractions to be associated with smaller, lower-mass galaxies, lower central dark-matter fractions, and larger transition radii. Finally, we show that the dips in observed surface brightness profiles seen in many early-type galaxies do not correspond to the transition from in-situ to accretion-dominated regions, and that any inferred mass fractions are not indicative of the true accreted mass but contain information about the galaxies’ dry-merger history.