CLASSY XII: Nitrogen Enrichment Shaped by Gas Density and Feedback

TL;DR

The CLASSY study uses direct-$T_e$ abundances for 45 local star-forming galaxies to map the $N/O$–$O/H$ chemical evolution and identify drivers of scatter, including electron density, star-formation activity, and feedback. It finds a plateau in $N/O$ at low metallicity, with substantial dispersion at higher $O/H$, and a robust link between $n_e$ and $N/O$, indicating density structure as a key scatter contributor. Comparison with high-$z$ galaxies shows no significant redshift evolution in $N/O$ at fixed metallicity up to $z\sim6$ (though $z>6$ UV-based nitrogen abundances can be offset), suggesting similar enrichment pathways across cosmic time. The work also demonstrates that feedback, compactness, and density together regulate nitrogen enrichment, and it presents chemical-evolution models in which wind retention and massive-star winds can explain the observed $N/O$ scatter, with implications for interpreting JWST-era observations of the early universe.

Abstract

We investigate the chemical evolution of N/O using a sample of 45 local star-forming galaxies (SFGs) from the CLASSY survey. This sample spans a wide range of galaxy properties, with robust determinations of nitrogen and oxygen abundances via the direct-$T_{\rm e}$ method. We explore how N/O relates to density structure, stellar mass, star formation rate (SFR), stellar age, compactness, and gas kinematics. In addition, we compare our results with those of galaxies at $z =2-10$ where N/O ratios were derived from optical or UV nitrogen lines, aiming to identify chemical enrichment pathways across cosmic time. Our analysis shows that the N/O-O/H relation in CLASSY galaxies aligns with the trends seen in local galaxies and extragalactic HII regions, and that galaxies at $z = 2-6$ exhibit similar N/O values, indicating no significant redshift evolution in N/O for a fixed metallicity. We identify a significant correlation between electron density $n_{\rm e}$([S II]) and N/O, suggesting that density structure contributes to the scatter in the N/O-O/H relation. The CLASSY galaxies with high SFRs or compact star formation show elevated N/O, though no strong correlation with stellar mass is found. We also find that high-velocity outflows (v$_{out}$ > 350 km/s) and low mass-loading factors are linked to elevated N/O, indicating that feedback plays a significant role. These results highlight the importance of density, star formation, and feedback from young stellar populations in shaping N/O enrichment and provide key insights for interpreting high-$z$ galaxies observed with JWST.

Figures (12)

• Figure 1: Comparison between log(N/O) derived using $T_{\rm e}$[Oii] and log(N/O) derived using $T_{\rm e}$[Nii], labeled as TO2 and TN2, respectively. The solid line represents the 1:1 relation, and the results are color-coded with log($n_{\rm e}$[Sii]). Note that the uncertainties in the measurements of log(N/O) are larger when using $T_{\rm e}$[Nii] compared to $T_{\rm e}$[Oii]. It shows that the two values are consistent within the uncertainties, with a slight offset toward lower values of log(N/O) at low $n_{\rm e}$ when using $T_{\rm e}$[Nii].
• Figure 2: The N/O-O/H relation for the CLASSY sample. CLASSY galaxies hosting WR stars are also identified (star symbols), and they show similar N/O ratios to the bulk of the sample, with the exception of the WR galaxy Mrk 996 james09, which exhibits a N/O enhancement. The comparison sample of local SFGs and Hii regions (grey symbols) was compiled and re-analyzed in arellanocordova24. A sample of high-redshift galaxies is included for comparison. SFGs at $z = 2-6$ with N/O ratios derived from optical [Nii] lines are shown as pentagons sanders23welch25rogers24arellanocordova25scholte25Zhang25, while squares represent $z > 6$ galaxies with N/O derived from UV nitrogen lines sanders23bmarqueschaves24jones23arellanocordova24schaerer24isobe23anakajima23topping24b. The different curves show predictions from chemical evolution models with an infall mass of log($M_{\rm inf}) = 10$ M$_\odot$, star formation efficiencies (SFE) of 0.5, 1, and 5 Gyr$^{-1}$, and a time infall of $\tau_{\rm inf} = 0.1$ Gyr represented by solid, dashed, and dotted lines, respectively vincenzo16. The CLASSY galaxies (orange circles) follow the expected N/O-O/H relation and occupy a similar parameter space as SFGs at $z=2-6$. In contrast, they differ from the $z>6$ population, showing no apparent redshift evolution.
• Figure 3: The N/H-O/H relation for CLASSY and the sample of high redshift galaxies. SFGs at $z=2-6$ from sanders23bwelch25rogers24arellanocordova25scholte25 and Zhang25 in cyan pentagons, while the green squares represent $z>6$ galaxies from marqueschaves24topping24topping24bcurti24bschaerer24. For comparison, we include the local sample in gray circles. High redshift galaxies are shifted to higher values of N/H, mainly those galaxies using UV nitrogen lines (squares), while the optical N sample at $z =2-6$ follows the trend of CLASSY at $z\sim0$. The purple triangles show those CLASSY galaxies with N/O derived using UV [Niv] or [Niii] lines (see Martinez et al. 2025). The different curves show the predictions of chemical evolution models for the N/H–O/H relation, as in Fig. \ref{['fig:NO_highz']}, while the dash-dotted line represents an empirical calibration to SDSS galaxies from flury20.
• Figure 4: Left N/O abundances as a function of low-ionization density, $n_{\rm e}$[Sii]. SFGs at $z>2$ are also included for comparison from sanders23welch25arellanocordova25stanton2025scholte25. We find that high density derived from $n_{\rm e}$[Sii] correlates with the high values of N/O for local galaxies (with a Kendall's coefficient of $\tau=0.372$ and p-value = 0.001), while $z=2-6$ galaxies show a similar trend. The yellow stars are galaxies with WR stars features. The best fit to the data for the N/O vs. log($n_{\rm e}$[Sii]) is shown in an orange solid line for CLASSY. Right: The N/O ratios as a function of high-ionization density, $n_{\rm e}$[Ariv]. N$^{+}$ and O$^{+}$ were derived using $n_{\rm e}$[Ariv] for 13 CLASSY galaxies. Only three CLASSY galaxies have measurements of both $n_{\rm e}$[Sii] and $n_{\rm e}$[Ariv] within a similar range of $n_{\rm e}$$=$ 200–600 cm$^{-3}$. J0808+3948 shows the highest values, with $n_{\rm e}$$=$ 1200–6310 cm$^{-3}$ and log(N/O) = –0.47 to –0.65 when derived using both density diagnostics.
• Figure 5: Comparison of the N/O and N/H ratios as a function of the stellar mass (left), SFR (middle) and sSFR (right) for CLASSY. The circles indicate the sample of SFGs from berg12berg16berg19izotov17. The dashed line shows the N/O vs M$\star$ for SDSS galaxies derived by hayden-pawson22, while the dashed-dotted line represents the N/O vs. M$\star$ and N/H vs. M$\star$ relations from strom22 for galaxies at $z \sim 2-3$. Such relations were derived using strong-line methods. The dotted lines represents the solar value of N/O and N/H from asplund21. The pentagons and squares represent galaxies at $z =2-6$ and their N/O abundances on optical lines by sanders23welch25arellanocordova25scholte25Zhang25, while that the squares represent galaxies at $z >6$ based their N/O abundances on UV lines by isobe23amarqueschaves24topping24topping24bcurti24bschaerer24. Our best fit to the CLASSY data is shown in a solid blue line.