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 _ { \mathrm { 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 .