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.
