Disk settling and dynamical heating: histories of Milky Way-mass stellar disks across cosmic time in the FIRE simulations

Understand

We study the kinematics of stars both at their formation and today within 14 Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations.

• We quantify the relative importance of cosmological disk settling and post-formation dynamical heating.
• We identify three eras: a Pre-Disk Era (typically >8 Gyr ago), when stars formed on dispersion-dominated orbits; an Early-Disk Era (~ 8 - 4 Gyr ago), when stars started to form on rotation-dominated orbits but with high velocity dispersion, sigma_form; and a Late-Disk Era (< 4 Gyr ago), when stars formed with low sigma_form.
• sigma_form increased with time during the Pre-Disk Era, peaking ~ 8 Gyr ago, then decreased throughout the Early-Disk Era as the disk settled and remained low throughout the Late-Disk Era.