2025/07/02 by Usui, Mitsutaka, Mawatari, Ken, Álvarez-Márquez, Javier +21
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2507.02054
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 JWST/NIRSpec IFS and ALMA observations. These observations detect key emission lines, including \oii~λλ~3727, 3730, \oiii~4364, \oiii~λλ~4960, 5008, \oiii 88 \micron, 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\rm V ≈ 0.14 mag), moderate electron density (n\rm e ≈ 500~cm-3), and low gas-phase metallicity (∼10%). However, the strong far-infrared \oiii 88 \micron 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, high-density gas components (Te ≈ 26,000 K; ne ≈ 600~cm-3) and an extended, cooler, lower-density component (Te ≈ 8,000 K; ne ≈ 50~cm-3), successfully reproduces the observed line ratios of \oiii~88~\micron/\oiii~5008~Å and \oiii~4364/\oiii~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.