| Summary: | 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 μ m, as well as H α and H β . 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 μ m 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 ≈ 8000 K; n _e ≈ 50 cm ^−3 )—successfully reproduces the observed line ratios of [O iii ] 88 μ m/[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.
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