The Nature of Nitrogen Enhanced High Redshift Galaxies

TL;DR

The paper tackles the puzzle of nitrogen-enhanced, low-metallicity galaxies at $z\gtrsim5$ by constructing nitrogen-enhanced photoionization models and systematically comparing AGN- and H II-region excitation to eight galaxies observed by JWST. It finds seven galaxies are best described by nitrogen-enhanced AGN models, while RXCJ2248-ID is consistent with a nitrogen-enhanced H II model; crucially, allowing for enhanced N/O yields does not bias the derived $\log(\mathrm{N/O})$ by more than $\sim0.1$ dex. EW-based UV diagrams emerge as particularly robust for separating AGN from star-forming sources when $\mathrm{N/O}$ varies. All nitrogen-enhanced systems show high gas pressure ($7.0 \leq \log(P/k) \leq 9.8$) and high ionization parameter ($\log(U) \gtrsim -2.0$), implying dense, compact environments, with Wolf-Rayet-containing super star clusters identified as the most plausible nitrogen sources.

Abstract

Recent JWST observations have revealed a population of high-redshift galaxies ($z\gtrsim5$) exhibiting unexpectedly bright ultraviolet (UV) nitrogen emission lines. The strong N III] and N IV] features imply nitrogen-to-oxygen abundance ratios (N/O) as high as $-0.8 \lesssim \log(\mathrm{N/O}) \lesssim 0.4$ in these low-metallicity galaxies ($12+\log(\mathrm{O/H}) \lesssim 8.2$), compared to the local value of $\log(\mathrm{N/O})\approx-1.5$. If confirmed, this level of nitrogen enrichment challenges existing models of nucleosynthesis and galaxy evolution. However, the presence of active galactic nuclei (AGNs) can affect spectral diagnostics, and previous studies often excluded AGN contamination using photoionization models based on local N/O ratios. In this work, we compare nitrogen-enhanced AGN and H II region models to observed spectra of eight high-redshift galaxies to constrain their nitrogen abundance, excitation source, gas-phase metallicity, ionization parameter, and gas pressure, simultaneously. We find seven galaxies (GHZ9, GS 3073, GN-z9p4, CEERS-1019, GHZ2, GN-z11, and GS-z9-0) are best described by nitrogen-enhanced AGN models, while RXCJ2248-ID is best reproduced by the nitrogen-enhanced H II model. The presence of AGN does not significantly impact ($\lesssim0.1\,$dex) the derived N/O ratio. We also find that equivalent width (EW)-based diagrams are the most robust UV diagnostic diagrams to distinguish AGNs and star-forming galaxies for situations where the nitrogen abundance is varying. All nitrogen-enhanced galaxies have moderate to high gas pressure ($7.0\leq\log (P/k)\leq9.8$) and high ionization parameter ($\log(U)\gtrsim-2.0$), indicating a dense and compact environment. We suggest that super star clusters containing Wolf-Rayet stars and massive stars are the most likely contributors to the elevated nitrogen abundance in these galaxies.