JWST & ALMA Joint Analysis with [OII]$λλ$3726,3729, [OIII]$λ$4363, [OIII]88$μ$m, and [OIII]52$μ$m: Multi-Zone Evolution of Electron Densities at $\mathbf{z\sim0-14}$ and Its Impact on Metallicity Measurements

TL;DR

Using JWST/NIRSpec IFU, the study targets galaxies at $z=6-7$ to obtain [OII]$\lambda$3726,3729 and [OIII]$\lambda$4363, enabling direct-$T_e$ metallicities $12+\log(O/H)\approx 8.0-8.2$; ALMA FIR [OIII] 52 μm detections in one source indicate FIR electron densities lower than optical values. A joint optical+FIR analysis reveals that observed FIR [OIII] 52,88 μm luminosities cannot be explained by the optical-density regime, requiring a two-zone ISM model with distinct high- and low-density regions, where FIR arises from low-density gas while optical lines trace both. The direct-$T_e$ metallicities can be biased low by up to ~0.8 dex if low-density gas is present but not fully traced optically, underscoring the need to combine optical and FIR diagnostics to obtain accurate gas-phase metallicities in the early universe and to better constrain its metal enrichment history. These findings have significant implications for the inferred metal enrichment history and the evolution of the ISM structure up to $z\sim14$ and demonstrate a practical, joint JWST-ALMA approach for robust metallicity measurements in the epoch of early galaxy formation.

Abstract

We present a JWST and ALMA detailed study of the ISM properties of high-redshift galaxies. Our JWST/NIRSpec IFU spectroscopy targeting three galaxies at $z=6-7$ detects key rest-frame optical emission lines, allowing us to derive [OII]$λλ$3726,3729-based electron densities of $n_\mathrm{e,optical}\sim1000$ cm$^{-3}$ on average and [OIII]$λ$4363-based metallicities of $\mathrm{12+log(O/H)}=8.0-8.2$ in two galaxies. New ALMA Band 9/10 observations detect the [OIII]52$μ$m line in one galaxy but do not in the others, resulting in FIR-based densities of $n_\mathrm{e,FIR}\lesssim500$ cm$^{-3}$ from the [OIII]52$μ$m/[OIII]88$μ$m ratios, systematically lower than the optical [OII]-based measurements. These low FIR-based densities are comparable to those at both $z\sim0$ and $z>6$ in the literature, including JADES-GS-z14-0 at $z=14.18$, suggesting little evolution up to $z\sim14$, in contrast to the increasing trend of optical-based densities with redshift. By conducting a JWST and ALMA joint analysis using emission lines detected with both telescopes, we find that the observed FIR [OIII]52,88$μ$m luminosities are too high to be explained by the optical-based densities at which they would be significantly collisionally de-excited. Instead, a 2-zone model with distinct high- and low-density regions is required to reproduce all observed lines, indicating that FIR [OIII] emission arises predominantly from low-density gas, while optical [OIII] and [OII] lines trace both regions. We further demonstrate that the direct-$T_\mathrm{e}$ method can sometimes significantly underestimate metallicities up to 0.8 dex due to the presence of the low-density gas not fully traced by optical lines alone, highlighting the importance of combining optical and FIR lines to accurately determine gas-phase metallicities in the early universe.