Disc settling and dynamical heating: histories of Milky Way-mass stellar discs across cosmic time in the FIRE simulations
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY(2024)
摘要
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.
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关键词
methods: numerical,Galaxy: evolution,Galaxy: formation,galaxies: disc,galaxies: evolution,galaxies: kinematics and dynamics
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