2025ApJ...991L..38U

We report the discovery of a complex density-stratified interstellar medium (ISM) in the star-forming galaxy COS-2987 at z = 6.81, revealed by the unprecedented synergy between James Webb Space Telescope (JWST)/NIRSpec integral field spectroscopy and Atacama Large Millimeter/submillimeter Array (ALMA) observations. These observations detect key emission lines, including [O II] λλ 3727, 3730, [O III] 4364, [O III] λλ 4960, 5008, and [O III] 88 μm, as well as Hα and Hβ. JWST spectroscopy alone indicates ISM properties that are typical for galaxies at z ∼ 7. These include low dust extinction (A_V ≈ 0.14 mag), moderate electron density (n_e ≈ 500 cm^–3), and low gas-phase metallicity (∼10%). However, the strong far-infrared [O III] 88 μm emission detected by ALMA cannot be explained by a single-component ionized medium with uniform electron density and temperature. Instead, a two-component ISM model-comprising compact, high-temperature, and high-density gas components (T_e ≈ 26,000 K; n_e ≈ 600 cm^–3) and an extended, cooler, and lower-density component (T_e ≈ 8​​​​​​000 K; n_e ≈ 50 cm^–3)-successfully reproduces the observed line ratios of [O III] 88 μm/[O III] 5008 Å and [O III] 4364/[O III] 5008 Å, with a volume ratio of 1 : 300 between the two components. Our results demonstrate that JWST alone probes only a fraction of the ISM and highlight the critical importance of combining JWST and ALMA to reveal the density-stratified ISM of early galaxies.