Only Nitrogen-enhanced Galaxies Have Detectable Ultraviolet Nitrogen Emission Lines at High Redshift

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Abstract
The detections of bright UV nitrogen emission lines in some high-redshift galaxies suggest unexpectedly high nitrogen-to-oxygen ratios ( log ( N / O ) ≳ − 1.0 ) compared to local values ( log ( N / O ) ≳ − 1.5 ) at similar metallicities ( 12 + log ( O / H ) ≲ 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 ( log ( N / O ) ≳ − 0.4 ), while JADES can only detect galaxies with moderately elevated N/O ratios compared to local values ( log ( N / O ) ≳ − 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 ∼ 40 – 500 hr) spectroscopic surveys will be crucial for building a complete sample to study nitrogen enrichment mechanisms in the early Universe.

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