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We study the kinematics of stars both at their formation and today within 14Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations.We quantify the relative importance of cosmological disk settling andpost-formation dynamical heating. We identify three eras: a Pre-Disk Era(typically >8 Gyr ago), when stars formed on dispersion-dominated orbits; anEarly-Disk Era (~ 8 - 4 Gyr ago), when stars started to form onrotation-dominated orbits but with high velocity dispersion, sigma_form; and aLate-Disk Era (< 4 Gyr ago), when stars formed with low sigma_form. sigma_formincreased with time during the Pre-Disk Era, peaking ~ 8 Gyr ago, thendecreased throughout the Early-Disk Era as the disk settled and remained lowthroughout the Late-Disk Era. By contrast, the velocity dispersion measuredtoday, sigma_now, increases monotonically with age because of strongerpost-formation heating for Pre-Disk stars. Importantly, most of sigma_now wasin place at formation, not added post-formation, for stars younger than ~ 10Gyr. We compare the evolution of the three velocity components: at all times,sigma_R,form > sigma_phi,form > sigma_Z,form. Post-formation heating primarilyincreased sigma_R at ages < 4 Gyr but acted nearly isotropically for olderstars. The kinematics of young stars in FIRE-2 broadly agree with the rangeobserved across the MW, M31, M33, and PHANGS-MUSE galaxies. The lookback timethat the disk began to settle correlates with its dynamical state today:earlier-settling galaxies currently form colder disks. Including stellarcosmic-ray feedback does not significantly change disk rotational support atfixed stellar mass.

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