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 ≈ 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 Lz, 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, fN/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 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 ($\rm [Fe/H]\lesssim -1.5$) when up to $\approx 50~{{\ \rm per\ cent}}-70~{{\ \rm per\ cent}}$ of the in situ stars formed in massive bound clusters. The dramatic drop of fN/O at $\rm [Fe/H]\gtrsim -0.9$ indicates that after the disc emerges the fraction of stars forming in massive bound clusters decreases by two orders of magnitude.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- November 2023
- DOI:
- 10.1093/mnras/stad2241
- arXiv:
- arXiv:2306.00060
- Bibcode:
- 2023MNRAS.525.4456B
- Keywords:
-
- Galaxy: abundances;
- Galaxy: disc;
- Galaxy: evolution;
- globular clusters: general;
- galaxies: high-redshift;
- galaxies: star clusters: general;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 18 pages, 13 figures, submitted to MNRAS