2025ApJ...993..204H

We present a JWST and Atacama Large Millimeter/submillimeter Array (ALMA) detailed study of the interstellar medium properties of high-redshift galaxies. Our JWST/NIRSpec integral field unit spectroscopy targeting three galaxies at z = 6-7 detects key rest-frame optical emission lines, allowing us to derive [O II]λλ3726, 3729-based electron densities of n_e,optical ∼ 1000 cm^–3 on average and [O III]λ4363-based metallicities of $12+\mathrm{log}({\rm{O}}/{\rm{H}})=8.0-8.2$ in two galaxies. New ALMA Band 9 and 10 observations detect the [O III] 52 μm line in one galaxy but do not in the others, resulting in far-infrared (FIR)-based densities of n_e,FIR ≲ 500 cm^–3 from the [O III] 52 μm/[O III] 88 μm ratio, systematically lower than the optical [O II]-based measurements. These low FIR-based densities are comparable to those at both z ∼ 0 and z > 6 in the literature, including JADES-GS-z14-0 at z = 14.18, suggesting little evolution up to z ∼ 14, 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 [O III] 52 and 88 μm luminosities are too high to be explained by the optical-based densities at which they would be significantly collisionally de-excited. Instead, a two-zone model with distinct high- and low-density regions is required to reproduce all observed lines, indicating that FIR [O III] emission arises predominantly from low-density gas, while the optical [O III] and [O II] lines trace both regions. We further demonstrate that the direct-T_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.