Nitrogen enrichment and clustered star formation at the dawn of the Galaxy

Anomalously high nitrogen-to-oxygen abundance ratios [N/O] are observed in globular clusters (GCs), among the field stars of the Milky Way (MW), and even in the gas in a z approximate to 11 galaxy. Using data from the APOGEE Data Release 17 and the Gaia Data Release 3, we present several independent lines of evidence that most of the MW's high-[N/O] stars were born in situ in massive bound clusters during the early, pre-disc evolution of the Galaxy. Specifically, we show that distributions of metallicity [Fe/H], energy, the angular momentum L-z, and distance of the low-metallicity high-[N/O] stars match the corresponding distributions of stars of the Aurora population and of the in situ GCs. We also show that the fraction of in situ field high-[N/O] stars, f(N/O), increases rapidly with decreasing metallicity. During epochs when metallicity evolves from [Fe /H] = -1.5 to [Fe /H] = -0.9, the Galaxy spins up and transitions from a turbulent Aurora state to a coherently rotating disc. This transformation is accompanied by many qualitative changes. In particular, we show that high N/O abundances similar to those observed in GN-z11 were common before the spin-up ([Fe/H] less than or similar to -1.5) when up to approximate to 50 per cent - 70 per cent of the in situ stars formed in massive bound clusters. The dramatic drop of f N/O at [Fe/H] greater than or similar to -0.9 indicates that after the disc emerges the fraction of stars forming in massive bound clusters decreases by two orders of magnitude.