Abstract The new era of galaxy evolution studies hearkened in by JWST has led to the discovery of z 5 galaxies exhibiting excess nitrogen with (N/O) ∼1 dex or more than expected from (N/O) - 12+ (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 5, finding that, when combined with O iii] λλ1660, 66 and He ii λ1640, N iii] λλ1747-54 / C iii] λλ1907, 09 best selects shock-dominated galaxies while N iv] λλ1483, 86 / C iii] λλ1907, 09 best distinguishes between active black holes and star forming galaxies. From our diagnostics, we find that slow/intermediate radiative shocks (v = 75-150 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.
Flury et al. (2025) studied this question.