  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. By contrast, the velocity dispersion measured
today, sigma_now, increases monotonically with age because of stronger
post-formation heating for Pre-Disk stars. Importantly, most of sigma_now was
in place at formation, not added post-formation, for stars younger than ~ 10
Gyr. We compare the evolution of the three velocity components: at all times,
sigma_R,form > sigma_phi,form > sigma_Z,form. Post-formation heating primarily
increased sigma_R at ages < 4 Gyr but acted nearly isotropically for older
stars. The kinematics of young stars in FIRE-2 broadly agree with the range
observed across the MW, M31, M33, and PHANGS-MUSE galaxies. The lookback time
that the disk began to settle correlates with its dynamical state today:
earlier-settling galaxies currently form colder disks. Including stellar
cosmic-ray feedback does not significantly change disk rotational support at
fixed stellar mass.
