New ionization models and the shocking nitrogen excess at <i>z</i> &amp;gt; 5

ABSTRACT The new era of galaxy evolution studies hearkened in by James Webb Space Telescope has led to the discovery of $z&amp;gt;5$ galaxies exhibiting excess nitrogen with $\rm \log (N/O)$$\sim$1 dex or more than expected from $\rm \log (N/O)$–$\rm 12+\log (O/H)$ trends in the local Universe. As of yet, the ionization sources of these galaxies have not been thoroughly explored, with radiative shocks left out of the picture. We present a suite of homogeneous excitation models for star-forming galaxies, active galactic nuclei, and radiative shocks, with which we explore possible explanations for the apparent nitrogen excess. We propose new BPT-style diagnostics to classify galaxies at $z&amp;gt;5$, finding that, when combined with O iii] $\lambda \lambda$1660,66 and He ii $\lambda$1640, N iii] $\lambda \lambda$1747–54 / C iii] $\lambda \lambda$1907,09 best selects shock-dominated galaxies while N iv] $\lambda \lambda$1483,86/C iii] $\lambda \lambda$1907,09 best distinguishes between active black holes and star-forming galaxies. From our diagnostics, we find that slow/intermediate radiative shocks ($v=75$–150 $\rm km~s^{-1}$) are most consistent with observed UV emission line flux ratios in nitrogen-bright galaxies. Accounting for the effects of shocks can bring nitrogen estimates into better agreement with abundance patterns observed in the local Universe and may be attributable to Wolf-Rayet populations actively enriching these galaxies with nitrogen and possibly driving winds responsible for these shocks.