2025ApJ...994L..29Z

The detections of bright UV nitrogen emission lines in some high-redshift galaxies suggest unexpectedly high nitrogen-to-oxygen ratios ($\mathrm{log}({\rm{N}}/{\rm{O}})\gtrsim -1.0$) compared to local values ($\mathrm{log}({\rm{N}}/{\rm{O}})\gtrsim -1.5$) at similar metallicities ($12+\mathrm{log}({\rm{O}}/{\rm{H}})\lesssim 8.0$). Although the presence of these “N-enhanced” galaxies indicates signatures of atypical chemical enrichment processes in the early Universe, the prevalence of nitrogen enhancement in high-z galaxies is unclear. So far, only ∼10 z > 5 galaxies have nitrogen abundance measurements, and they all suggest elevated N/O ratios. Do all high-redshift galaxies exhibit elevated N/O ratios, or are we simply missing “N-normal” galaxies whose nitrogen abundances follow the local N/O scaling relation? To tackle these questions, we calculate the detection limits of UV N III] or N IV] lines in current JWST surveys CEERS and JADES, and compare them to predictions from both “N-enhanced” and “N-normal” active galactic nuclei (AGN) narrow-line region and H II region photoionization models. We find that CEERS can only detect galaxies with significant nitrogen enhancement ($\mathrm{log}({\rm{N}}/{\rm{O}})\gtrsim -0.4$), while JADES can only detect galaxies with moderately elevated N/O ratios compared to local values ($\mathrm{log}({\rm{N}}/{\rm{O}})\gtrsim -1.0$). Even for the deepest exposure in JADES, UV nitrogen lines produced by “N-normal” galaxies at z > 5 are too faint and thus not detectable, making their nitrogen abundance unmeasurable. Our results suggest that the existing sample of galaxies with measurable nitrogen abundances at z ≳ 5 is incomplete and biased toward galaxies with significantly elevated N/O ratios. Deep (${t}_{\exp }\sim 40\mbox{--}500\,$hr) spectroscopic surveys will be crucial for building a complete sample to study nitrogen enrichment mechanisms in the early Universe.