UC Davis

How disk galaxies, including our own Milky Way, formed and evolved over cosmic time remains an outstanding question in astrophysics. A powerful approach is to treat a galaxy's stellar populations as an archaeological record, one in which the present-day kinematics of stars retains a memory of their galaxy’s past dynamical state. In particular, the age–velocity dispersion relation (AVR), σ(τ) has long served as a probe of stellar dynamical evolution, reflecting both the kinematics with which stars formed – as set by the star-forming interstellar medium – and the dynamical processes that have subsequently altered their orbits. However, interpreting AVRs is inherently ambiguous: similar present-day trends can arise from stars forming with different initial kinematics or from cumulative dynamical heating over time.I use cosmological zoom-in simulations from the FIRE-2 project to disentangle these effects. I find that disk assembly proceeds through three distinct eras, during which stars form with systematically evolving kinematics as galaxies transition from turbulent, dispersion-dominated systems to dynamically cold, rotationally supported disks. Although the strength and character of dynamical heating vary across these eras, I show that, for all but the oldest stellar populations, present-day velocity dispersions are largely set at formation rather than acquired through subsequent evolution. I further find that galaxies that form disks earlier host systematically colder star-forming gas at late times, linking present-day conditions to the timing of disk assembly.Recent observations have extended AVR measurements of σ(τ) well beyond the solar neighborhood, enabling comparisons across a growing sample of nearby disk galaxies. I compile these measurements and use simulations to quantify how observational systematics -- such as aperture size, galactocentric radius, and age uncertainties -- affect the inferred AVR, placing these data on a common footing. In this framework, the simulations broadly reproduce the kinematics of external systems, while the Milky Way exhibits systematically colder stellar populations than our simulations and all but one observed galaxy. This suggests that the Milky Way, long used as the benchmark for disk evolution and for testing simulations, instead appears to be a kinematic outlier with an atypical dynamical history.