RIOJA. JWST and ALMA Unveil the Inhomogeneous and Complex Interstellar Medium Structure in a Star-forming Galaxy at z = 6.81

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 (AV ≈ 0.14 mag), moderate electron density (ne ≈ 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 (Te ≈ 26,000 K; ne ≈ 600 cm−3) and an extended, cooler, and lower-density component (Te ≈ 8000 K; ne ≈ 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.