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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 ≳ 8 Gyr ago), when stars formed on dispersion-dominated orbits; an Early-Disc Era (≈8–4 Gyr ago), when stars started to form on rotation-dominated orbits but with high velocity dispersion, σform; and a Late-Disc Era (≲ 4 Gyr ago), when stars formed with low σform. σform increased with time during the Pre-Disc Era, peaking ≈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, σnow, increases monotonically with age because of stronger post-formation heating for Pre-Disc stars. Importantly, most of σnow 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, σR, form > σϕ, form > σZ, form. Post-formation heating primarily increased σR 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 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.

Observational studies are finding stars believed to be relics of the earliest stages of hierarchical mass assembly of the Milky Way (i.e. proto-galaxy). In this work, we contextualize these findings by studying the masses, ages, spatial distributions, morphology, kinematics, and chemical compositions of proto-galaxy populations from the 13 Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations. Our findings indicate that proto-Milky Way populations: (i) can have a stellar mass range between 1 × 108 < M⋆ < 2 × 1010 [M⊙], a virial mass range between 3 × 1010 < M⋆ < 6 × 1011 [M⊙], and be as young as 8 ≲ Age ≲ 12.8 [Gyr] (1 ≲ z ≲ 6); (ii) are pre-dominantly centrally concentrated, with $\sim 50~{{\ \rm per\ cent}}$ of the stars contained within 5–10 kpc; (iii) on average show weak but systematic net rotation in the plane of the host’s disc at z = 0 (i.e. 0.25 ≲ 〈κ/κdisc〉 ≲ 0.8); (iv) present [α/Fe]-[Fe/H] compositions that overlap with the metal-poor tail of the host’s old disc; and (v) tend to assemble slightly earlier in Local Group-like environments than in systems in isolation. Interestingly, we find that $\sim 60~{{\ \rm per\ cent}}$ of the proto-Milky Way galaxies are comprised by 1 dominant system (1/5 ≲M⋆/M⋆, proto-MilkyWay≲ 4/5) and 4–5 lower mass systems (M⋆/M⋆, proto-MilkyWay≲ 1/10); the other $\sim 40~{{\ \rm per\ cent}}$ are comprised by 2 dominant systems and 3–4 lower mass systems. These massive/dominant proto-Milky Way fragments can be distinguished from the lower mass ones in chemical-kinematic samples, but appear (qualitatively) indistinguishable from one another. Our results could help observational studies disentangle if the Milky Way formed from one or two dominant systems.