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

Abstract: 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 $z\approx 11$ 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-disk 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, $f_{\rm N/O}$, increases rapidly with decreasing metallicity. During epochs when metallicity evolves from $\rm [Fe/H]=-1.5$ to $\rm [Fe/H]=-0.9$, the Galaxy spins up and transitions from a turbulent Aurora state to a coherently rotating disk. 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 ($\rm [Fe/H]\lesssim -1.5$) when up to $\approx 50\%-70\%$ of the in-situ stars formed in massive bound clusters. The dramatic drop of $f_{\rm N/O}$ at $\rm [Fe/H]\gtrsim -0.9$ indicates that after the disk emerges the fraction of stars forming in massive bound clusters decreases by two orders of magnitude.