From dawn till disc: Milky Way's turbulent youth revealed by the APOGEE plus Gaia data

We use accurate estimates of aluminium abundance from the APOGEE Data Release 17 and Gaia Early Data Release 3 astrometry to select a highly pure sample of stars with metallicity -1.5 less than or similar to [Fe/H] less than or similar to 0.5 born in-situ in the Milky Way proper. The low-metallicity ([Fe/H] less than or similar to -1.3) in-situ component we dub Aurora is kinematically hot with an approximately isotropic velocity ellipsoid and a modest net rotation. Aurora stars exhibit large scatter in metallicity and in many element abundance ratios. The median tangential velocity of the in-situ stars increases sharply with metallicity between [Fe/H] = -1.3 and -0.9, the transition that we call the spin-up. The observed and theoretically expected age-metallicity correlations imply that this increase reflects a rapid formation of the MW disc over approximate to 1-2 Gyr. The transformation of the stellar kinematics as a function of [Fe/H] is accompanied by a qualitative change in chemical abundances: the scatter drops sharply once the Galaxy builds up a disc during later epochs corresponding to [Fe/H] > -0.9. Results of galaxy formation models presented in this and other recent studies strongly indicate that the trends observed in the MW reflect generic processes during the early evolution of progenitors of MW-sized galaxies: a period of chaotic pre-disc evolution, when gas is accreted along cold narrow filaments and when stars are born in irregular configurations, and subsequent rapid disc formation. The latter signals formation of a stable hot gaseous halo around the MW progenitor, which changes the mode of gas accretion and allows development of coherently rotating disc.

Automated summary

This study uses accurate measurements of aluminium abundance to select a pure sample of stars born in the Milky Way with specific metallicity range. The low-metallicity in-situ component, named Aurora, exhibits distinct kinematic and chemical properties. The observed kinematic transformation and change in chemical abundances suggest a rapid formation of the Milky Way disc. The results also imply that these trends are common in the early evolution of Milky Way-sized galaxies.