The global and radial stellar mass assembly of Milky Way-sized galaxies

Abstract: We study the global and radial stellar mass assembly of eight zoomed-in MW-sized galaxies produced in Hydrodynamics cosmological simulations. The disk-dominated galaxies (4) show a fast initial stellar mass growth in the innermost parts, driven mostly by in-situ SF, but since $z\sim2-1$ the SF enters in a long-term quenching phase. The outer regions follow this trend but more gentle as more external they are. As the result, the radial stellar mass growth is highly inside-out due to both the inside-out structural growth and inside-out SF quenching. The half-mass radius evolves fast; for instance, $R_{0.5}$($z=1$)$<0.5$$R_{0.5}$($z=0$). Two other runs resemble lenticular galaxies. One shows also a pronounced inside-out growth and the other one presents a nearly uniform radial mass assembly. The other two galaxies suffered late major mergers. Their normalized radial mass growth histories (MGHs) are nearly close among them but with periods of outside-in assembly during or after the mergers. For all the simulations, the archaeological radial MGHs calculated from the $z = 0$ stellar-particles age distribution are similar to the current MGHs, which evidences that the mass assembly by ex-situ stars and the radial mass transport do not change significantly their radial mass distributions. Our results agree qualitatively with observational inferences from the fossil record method applied to a survey of local galaxies and from look-back observations of progenitors of MW-sized galaxies. However, the inside-out growth mode is more pronounced and the $R_{0.5}$ growth is faster in simulations than in observational inferences.