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Building Late-Type Spiral Galaxies by In-Situ and Ex-Situ Star Formation

We analyze the formation and evolution of the stellar components in "Eris", a 120 pc-resolution cosmological hydrodynamic simulation of a late-type spiral galaxy. The simulation includes the effects of a uniform UV background, a delayed-radiative-cooling scheme for supernova feedback, and a star formation recipe based on a high gas density threshold. It allows a detailed study of the relative contributions of "in-situ" (within the main host) and "ex-situ" (within satellite galaxies) star formation to each major Galactic component in a close Milky Way analog. We investigate these two star-formation channels as a function of galactocentric distance, along different lines of sight above and along the disk plane, and as a function of cosmic time. We find that: 1) approximately 70 percent of today's stars formed in-situ; 2) more than two thirds of the ex-situ stars formed within satellites after infall; 3) the majority of ex-situ stars are found today in the disk and in the bulge; 4) the stellar halo is dominated by ex-situ stars, whereas in-situ stars dominate the mass profile at distances < 5 kpc from the center at high latitudes; and 5) approximately 25% of the inner, r < 20 kpc, halo is composed of in-situ stars that have been displaced from their original birth sites during Eris' early assembly history.