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

Abstract

The James Webb Space Telescope (JWST) has revealed a growing number of $z>6$ galaxies with enhanced N/O ratios that challenge standard chemical enrichment frameworks. We test whether this is exclusive to extreme systems or a generic feature of $6 \leq z \leq 10$ populations through a stacking analysis of 135 galaxies with JWST/NIRSpec $R\sim1000$ spectroscopy, probing the origin of N, C and O enrichment and its dependence on recent star formation. A full-sample composite reveals prominent C III], C IV, N IV] and He II emission, demonstrating a ubiquity of high-ionisation UV lines. Auroral [O III]$λ$4363 electron temperatures and multi-ionisation zone modelling yield direct-$T_e$ abundances with supersolar $\log(\mathrm{N/O}) = -0.57^{+0.09}_{-0.10}$ and subsolar $\log(\mathrm{C/O}) = -0.80^{+0.03}_{-0.03}$ at $12 + \log(\mathrm{O/H}) = 7.79^{+0.03}_{-0.03}$, in contrast with local samples and suggesting significant nitrogen enhancement within the first billion years. Stacking further by recent star formation activity over burst timescales of $Δt = 3$--$20$~Myr, we find that sources 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. We interpret these observations as the result of delayed AGB enrichment across multiple burst episodes, together with dilution by pristine gas inflows. Cosmological simulations reproduce this trend with CCSNe and AGB yields alone, without the need for 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 requiring new physical frameworks.