Inside Out and Upside Down: Tracing the Assembly of a Simulated Disk
  Galaxy Using Mono-Age Stellar Populations
  We analyze the present-day structure and assembly history of a high
resolution hydrodynamic simulation of the formation of a Milky Way (MW)-like
disk galaxy, from the "Eris" simulation suite, dissecting it into cohorts of
stars formed at different epochs of cosmic history. At z=0, stars with t_form <
2 Gyr mainly occupy the stellar spheroid, with the oldest (earliest forming)
stars having more centrally concentrated profiles. The younger age cohorts
populate disks of progressively longer radial scale length and shorter vertical
scale height. At a given radius, the vertical density profiles and velocity
dispersions of stars vary smoothly as a function of age, and the superposition
of old, vertically-extended and young, vertically-compact cohorts gives rise to
a double-exponential profile like that observed in the MW. Turning to formation
history, we find that the trends of spatial structure and kinematics with
stellar age are largely imprinted at birth, or immediately thereafter. Stars
that form during the active merger phase at z>3 are quickly scattered into
rounded, kinematically hot configurations. The oldest disk cohorts form in
structures that are radially compact and relatively thick, while subsequent
cohorts form in progressively larger, thinner, colder configurations from gas
with increasing levels of rotational support. The disk thus forms "inside-out"
in a radial sense and "upside-down" in a vertical sense. Secular heating and
radial migration influence the final state of each age cohort, but the changes
they produce are small compared to the trends established at formation. The
predicted correlations of stellar age with spatial and kinematic structure are
in good qualitative agreement with the correlations observed for mono-abundance
stellar populations in the MW.
