RIOJA. JWST and ALMA unveil the inhomogeneous and complex ISM structure in a star-forming galaxy at $z=6.81$

Abstract

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~$\hat{I}»\hat{I}»$~3727, 3730, \oiii~4364, \oiii~$\hat{I}»\hat{I}»$~4960, 5008, \oiii\ 88 \micron, as well as H$\hat{I}\pm$ and H${\hat{I}}^2$. JWST spectroscopy alone indicates ISM properties that are typical for galaxies at $z\sim 7$. These include low dust extinction ($A_{\mathrm{V}}\approx 0.14$ mag), moderate electron density ($n_{\mathrm{e}}\approx 500$~cm${}^{-3}$), and low gas-phase metallicity ($\sim 10\%$). 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 ($T_e\approx 26,000$\,K; $n_e\approx 600{\mathrm{cm}}^{-3}$) and an extended, cooler, lower-density component ($T_e\approx 8,000$\,K; $n_e\approx 50{\mathrm{cm}}^{-3}$), successfully reproduces the observed line ratios of \oiii~88~\micron/\oiii~5008~Ã and \oiii~4364/\oiii~5008~Ã , 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.

1 pages, 4 figures, 1 table. Resubmitted to ApJ Letter. Comments welcome