Unveiling and Characterising Ubiquitous Nitrogen Enhancement in $6 \leq z \leq 10$ Galaxies with JWST Spectroscopy

Recent JWST observations have revealed a growing number of z>6 galaxies with elevated N/O ratios that challenge standard chemical enrichment frameworks. This study tests whether such enhancement is exclusive to extreme systems or instead a generic feature of galaxies at 6 ≤ z ≤ 10, using a stacking analysis of 135 galaxies observed with JWST/NIRSpec R~1000 spectroscopy.

The analysis probes the origin of nitrogen, carbon, and oxygen enrichment and its dependence on recent star formation. The full-sample composite spectrum reveals prominent C III], C IV, N IV], and He II emission, demonstrating that high-ionisation UV lines are ubiquitous in these galaxies. Using auroral [O III] λ4363 electron temperatures and multi-ionisation zone modelling, the authors derive direct-Te abundances: supersolar log(N/O) = −0.57 (+0.09/−0.10) and subsolar log(C/O) = −0.80 (+0.03/−0.03) at 12 + log(O/H) = 7.79 (+0.03/−0.03). These values contrast with local samples and imply significant nitrogen enhancement within the first billion years.

When stacking by recent star formation activity over burst timescales of Δt = 3–20 Myr, the authors find that galaxies caught in a recent burst are less enriched in both N/O and metallicity than those in a relative lull, with the largest contrasts confined to the shortest timescales. C/O, however, remains largely invariant across all metrics. The observations are interpreted as the result of delayed AGB enrichment across multiple burst episodes combined with dilution by pristine gas inflows.

Cosmological simulations reproduce the observed trends using CCSNe and AGB yields alone, without requiring shorter-lived (super-)massive stars. Enhanced N/O therefore appears to be a generic feature of high-redshift systems, produced by standard enrichment processes rather than by exotic stellar populations.