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

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 disc settling and post-formation dynamical heating. We identify three eras: a Pre-Disc Era (typically greater than or similar to 8 Gyr ago), when stars formed on dispersion-dominated orbits; an Early-Disc Era (approximate to 8-4 Gyr ago), when stars started to form on rotation-dominated orbits but with high velocity dispersion, sigma(form); and a Late-Disc Era (less than or similar to 4 Gyr ago), when stars formed with low sigma(form). sigma(form) increased with time during the Pre-Disc Era, peaking approximate to 8 Gyr ago, then decreased throughout the Early-Disc Era as the disc settled and remained low throughout the Late-Disc Era. By contrast, the dispersion measured today, sigma(now), increases monotonically with age because of stronger post-formation heating for Pre-Disc stars. Importantly, most of sigma(now) was in place at formation, not added post-formation, for stars younger than approximate to 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 less than or similar to 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 disc began to settle correlates with its dynamical state today: earlier-settling galaxies currently form colder discs. Including stellar cosmic-ray feedback does not significantly change disc rotational support at fixed stellar mass.