New Ionization Models and the Shocking Nitrogen Excess at z > 5

TL;DR

The paper addresses the puzzling nitrogen excess in $z>5$ galaxies by building a uniform, empirically anchored library of ionization models (SFGs, AGN, and radiative shocks) using MAPPINGS V with Nicholls2017 abundance patterns and dust depletion. It introduces UV-line diagnostics showing that slow to intermediate shocks ($v_s \lesssim 200$ km s$^{-1}$) can reproduce observed line ratios, and demonstrates that accounting for shocks reduces the inferred $\log(\mathrm{N/O})$ by about $0.3$–$0.5$ dex, bringing results into line with local abundance patterns and suggesting Wolf–Rayet enrichment as a contributor. The study also proposes JWST-friendly rest-UV diagnostic diagrams to distinguish shocks from AGN and star formation at $z>5$, emphasizing the need for a self-consistent treatment of shocks and dust. Overall, the work reframes the nitrogen excess as a consequence of excitation physics (shocks and density effects) rather than requiring extreme nucleosynthetic yields, and it provides practical tools for interpreting high-redshift ISM in forthcoming surveys. The findings have significant implications for ISM conditions and chemical evolution in the early universe and highlight the role of winds and WR-driven enrichment in shaping observed UV emission.

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 log(N/O)~1 dex or more than expected from log(N/O) vs 12+log(O/H) trends in the local Universe. A variety of novel enrichment pathways have been presented to explain the apparent nitrogen excess, invoking a wide range of processes from very massive stars to stripped binaries to fine-tuned star-formation histories. However, understanding the excitation mechanism responsible for the observed nebular emission is necessary to accurately infer chemical abundances. 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] 1660,66 and He ii 1640, N iii] 1747-54 / C iii] 1907,09 best selects shock-dominated galaxies while N iv] 1483,86 / C iii] 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 km/s) 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.

Figures (5)

• Figure 1: Ionizing photon spectral energy distribution models for oxaf disk-powerlaw AGN (optxagnf, red), stellar populations (blue and purple), and a shock precursor (green). AGN models here indicate combinations of MBH mass and Eddington accretion rate. Stellar populations include instantaneous (solid blue and purple lines) and continuous (dotted purple line) for Starburst99 (starburst99v1starburst99v2starburst99v2a, blue) and BPASS (bpass2.2, purple). MAPPINGS V upstream ionizing spectrum consisting of bremsstrahlung continuum plus X-ray cooling lines shown in green for a 500 $\rm km~s^{-1}$ shock, scaled for visibility. Ionization potential energies of various ions are labeled and indicated by grey lines (see Table \ref{['tab:ionPot']}). The difference in ionizing photon SED becomes most pronounced blueward of 228 Å, corresponding to the ionization potentials for He$^{+}$ and O$^{+2}$. The nearby C$^{+2}$ and N$^{+2}$ ionization potentials at 263 Å make these ions sensitive to the differences as well due to their photoionization cross-sections.
• Figure 2: BPT-style diagnostics for Ciii]/Heii (top) and Civ/Ciii] (bottom) vs Civ/Heii. Our model predictions for $\zeta_{\rm O}\leq0.5$ ($\la$50% $Z_\odot$) are shown in purple for SFGs (stars and pentagons for instantaneous and continuous star formation, respectively), red for AGN (up and down triangles for the Jin2012 and optxagnf SED models, respectively), and green for shocks (crosses and exes for $v_s\leq200$$\rm km~s^{-1}$ and $v_s>200$$\rm km~s^{-1}$, the "low" and "high" shock velocity regimes, respectively; triradii for dusty shocks with $v_{s}=50$-$200$$\rm km~s^{-1}$). Yellow squares correspond to nitrogen-excess galaxies at $z>5$ observed with JWST. Pink diamond indicates the giant extragalactic Hii region Mrk 71-A, a Green Pea analog exhibiting density enhancement and signatures of massive stars. White hexagons indicate the $z\sim0$ star-forming galaxies from CLASSY Berg2022Mingozzi2022 and from Jung2024. Grey line is the 10% shock mixture boundary given by Jaskot2016. Black lines indicate our limits for SFGs (solid), AGN (dashed), and shocks (dotted) as given by Equations \ref{['eqn:c3he2c4he2']} (top) and \ref{['eqn:c4c3c4he2']} (bottom).
• Figure 3: BPT-style diagnostic for Civ/Heii (top), Ciii]/Heii (center), and Civ/Ciii] (bottom) vs Oiii]/Heii. Symbols as in Figure \ref{['fig:c3he2c4he2']}. Blue diamond indicates the low-redshift galaxy Mrk 996 exhibiting nitrogen excess and signatures of massive stars. Cyan circle indicates the nitrogen-excess knot of the Sunburst arc ($z=2.39$) exhibiting winds and Lyman continuum escape. Solid and dotted black lines in the top and bottom panels indicate demarcations given by Equations \ref{['eqn:o3he2c4he2']} and \ref{['eqn:o3hec4c3']}, respectively. Solid and dotten grey lines in the center panel indicate demarcations from Mingozzi2024.
• Figure 4: BPT-style diagnostics for Civ/Ciii] vs Civ/(Heii+Oiii]). Yellow squares correspond to nitrogen-excess galaxies at $z>5$ observed with JWST. Symbols as in Figure \ref{['fig:c3he2c4he2']}. Solid, and dotted lines correspond to Equation \ref{['eqn:c4c3c4o3he2']} to distinguish excitation regimes, which we have labeled.
• Figure 5: BPT-style diagnostics for Niii]/Ciii] vs Oiii]/Heii (top) and Niv]/Ciii] vs Oiii]/Heii (bottom). Symbols as in Figures \ref{['fig:c3he2c4he2']}-\ref{['fig:c4c3he2o3']}. Solid, dotted, and long-dashed lines correspond to Equations \ref{['eqn:n3c3o3he2']} and \ref{['eqn:n4c3o3he2']} in the top and bottom panels, respectively, to distinguish excitation regimes. The CLASSY galaxy appearing as a filled white hexagon in both panels is J1253-3012 (SHOC391). Radiative shocks occupy a unique space where extreme densities and sufficiently extreme radiation can populate the diagram above the solid, dashed, and dotted lines. The location of the high redshift galaxies in these diagnostic diagrams suggests that radiative shocks are the dominant excitation mechanism for these objects and may account for some, if not all, of the apparent nitrogen excess.