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

Abstract The detections of bright UV nitrogen emission lines in some high-redshift galaxies suggest unexpectedly high nitrogen-to-oxygen ratios ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">N</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≳</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1.0</mml:mn> </mml:math> ) compared to local values ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">N</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≳</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1.5</mml:mn> </mml:math> ) at similar metallicities ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>12</mml:mn> <mml:mo>+</mml:mo> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≲</mml:mo> <mml:mn>8.0</mml:mn> </mml:math> ). 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 &gt; 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">N</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≳</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.4</mml:mn> </mml:math> ), while JADES can only detect galaxies with moderately elevated N/O ratios compared to local values ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">N</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≳</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1.0</mml:mn> </mml:math> ). Even for the deepest exposure in JADES, UV nitrogen lines produced by “N-normal” galaxies at z &gt; 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>t</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>exp</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>40</mml:mn> <mml:mrow> <mml:mo>–</mml:mo> </mml:mrow> <mml:mn>500</mml:mn> <mml:mspace width="0.25em"/> </mml:math> hr) spectroscopic surveys will be crucial for building a complete sample to study nitrogen enrichment mechanisms in the early Universe.