The Dynamics of Old Inner Galaxy Stars in Milky Way-mass Galaxies Using FIRE-2 Simulations

Abstract Understanding how galaxies like the Milky Way assembled over cosmic time remains a central question in astrophysics. Understanding the processes that shaped their formation and evolution is greatly enhanced by the joint use of observational data and high-resolution cosmological simulations. Old stars in the inner regions of galaxies serve as powerful tracers of early dynamical events, having formed during the initial stages of galaxy assembly and retaining the kinematic imprints of those formative periods. We investigate the kinematic properties of old (age >10 Gyr) inner galaxy (rGC<5 kpc) stars in thirteen Milky Way-mass galaxies from the FIRE-2 cosmological zoom-in simulations, focusing on their origin, orbital structure, and kinematic alignment with the disk. Our analysis reveals that old stars in the inner galaxy are more likely to have been formed in their host galaxy, although accretion is seen most prominently during the earliest stages of galaxy formation. Many of these accreted stars tend to occupy kinematically hot orbits compared to their counterparts formed in the host galaxy, although some stars formed in-galaxy also retain kinematically hot orbits. Disk-like dynamics are present throughout all age bins, and are most prominent as age decreases. Although some old stellar populations retain disk-like structure, the prominence of this rotational component varies significantly across galaxies and between star populations. These results emphasize the diversity of early galaxy assembly histories and suggest that coherent angular momentum in accreted material can leave detectable kinematic signatures in present-day stellar halos.