Self-Consistent Direct Method for Chemical Abundances in High-z Galaxies with JWST

Abstract

The unprecedented rest-frame UV and optical coverage provided by JWST enables simultaneous constraints on the electron density (n${}_{\mathrm{e}}$) and temperature (T${}_{\mathrm{e}}$) of ionized gas in galaxies at z>5. We present a self-consistent direct method based on multiple OIII]1661,66) and [OIII] ($\hat{I}»$4363, and $\hat{I}»$5007) transitions to characterize the physical conditions of the high-ionization zone. This new approach is insensitive to a wide range of n${}_{\mathrm{e}}$ due to the high critical densities of the OIII] and [OIII] transitions. Applying this technique to six galaxies at z=5-9, we find electron densities up to n${}_{\mathrm{e}}$$\sim 3\times 10^5$ cm${}^{-3}$ and temperatures of T${}_{\mathrm{e}}$ $\sim 20,000$ K in systems at $z>6$. Accounting for these self-consistent densities changes the derived T${}_{\mathrm{e}}$ and modifies the inferred metallicities by up to 0.29 dex relative to previous estimates. We discuss the reported N/O overabundances in the high-$z$ galaxies from our sample, which arise entirely from the high N${}^{3+}$/H${}^{+}$ values inferred from NIV] lines. We point out that a T${}_{\mathrm{e}}$-stratification, in which the N${}^{3+}$ zone has a slightly higher T${}_{\mathrm{e}}$ than T${}_{\mathrm{e}}$([OIII]), could substantially reduce the inferred N/O. Quantitatively, if T${}_{\mathrm{e}}$(N${}^{3+}$) were 10\% higher than T${}_{\mathrm{e}}$([OIII]), this could induce a systematic overestimation of N${}^{3+}$/O${}^{2+}$ of nearly 50\%. Classical N/O diagnostics such as N${}^{+}$/O${}^{+}$, due to their critical densities, can significantly impact the inferred N/O abundance in the presence of high-density gas, whereas N${}^{2+}$/O${}^{2+}$ place these galaxies closer to $z\sim 0$ systems in the N/O-O/H plane. Future JWST programs with larger and more diverse samples will be essential to test the universality and robustness of these results.

13 pages, 6 figures. Accepted for publication in MNRAS