RIOJA. JWST and ALMA Unveil the Inhomogeneous and Complex Interstellar Medium Structure in a Star-forming Galaxy at z = 6.81
Journal article, 2025
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.
Galaxy evolution (594)
Galaxy formation (595)
Interstellar medium (847)
Author
Mitsutaka Usui
University of Tsukuba
Ken Mawatari
Waseda University
University of Tsukuba
J. Álvarez-Márquez
Centro de Astrobiologia (CAB)
Takuya Hashimoto
University of Tsukuba
Yuma Sugahara
Waseda University
Rui Marques-Chaves
University of Geneva
Akio K. Inoue
Waseda University
L. Colina
Centro de Astrobiologia (CAB)
S. Arribas
Centro de Astrobiologia (CAB)
Carmen Blanco-Prieto
Centro de Astrobiologia (CAB)
Yurina Nakazato
University of Tokyo
Naoki Yoshida
Research Center for the Early Universe
University of Tokyo
Tom Bakx
National Astronomical Observatory of Japan
Nagoya University
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Daniel Ceverino
Universidad Autonoma de Madrid (UAM)
Luca Constantin
Centro de Astrobiologia (CAB)
A. Gomez
Space Telescope Science Institute (STScI)
Masato Hagimoto
Nagoya University
Hiroshi Matsuo
National Astronomical Observatory of Japan
The Graduate University for Advanced Studies (SOKENDAI)
Wataru Osone
University of Tsukuba
Yi W. Ren
Waseda University
Yoshinobu Fudamoto
Chiba University
Takeshi Hashigaya
Kyoto University
M. Pereira-Santaella
Spanish National Research Council (CSIC)
Y. Tamura
Nagoya University
Astrophysical Journal Letters
2041-8205 (ISSN) 2041-8213 (eISSN)
Vol. 991 2 L38Subject Categories (SSIF 2025)
Atom and Molecular Physics and Optics
Astronomy, Astrophysics, and Cosmology
DOI
10.3847/2041-8213/ae0574