JWST and ALMA Multiple-line Study in and around a Galaxy at z = 8.496: Optical to Far-Infrared Line Ratios and the Onset of an Outflow Promoting Ionizing Photon Escape
Abstract
We present Atacama Large Millimeter/submillimeter Array (ALMA) deep spectroscopy for a lensed galaxy at z spec = 8.496 with $\mathrm{log}({M}_{\mathrm{star}}/{M}_{\odot })\sim 7.8$ whose optical nebular lines and stellar continuum are detected by JWST/NIRSpec and NIRCam Early Release Observations in the field of SMACS J0723.3–7327. Our ALMA spectrum shows [O III] 88 μm and [C II] 158 μm line detections at 4.0σ and 4.5σ, respectively. The redshift and position of the [O III] line coincide with those of the JWST source, while the [C II] line is blueshifted by 90 km s‑1 with a spatial offset of 0.″5 (≈0.5 kpc in the source plane) from the centroid of the JWST source. The NIRCam F444W image, including [O III] λ5007 and Hβ line emission, spatially extends beyond the stellar components by a factor of >8. This indicates that the z = 8.5 galaxy has already experienced strong outflows as traced by extended [O III] λ5007 and offset [C II] emission, which would promote ionizing photon escape and facilitate reionization. With careful slit-loss corrections and the removal of emission spatially outside the galaxy, we evaluate the [O III] 88 μm/λ5007 line ratio, and derive the electron density n e by photoionization modeling to be ${220}_{-130}^{+230}$ cm‑3, which is comparable with those of z ∼ 2–3 galaxies. We estimate an [O III] 88 μm/[C II] 158 μm line ratio in the galaxy of >4, as high as those of known z ∼ 6–9 galaxies. This high [O III] 88 μm/[C II] 158 μm line ratio is generally explained by the high n e as well as the low metallicity ( ${Z}_{\mathrm{gas}}/{Z}_{\odot }={0.04}_{-0.02}^{+0.02}$ ), high ionization parameter ( $\mathrm{log}U -2.27$ ), and low carbon-to-oxygen abundance ratio (log(C/O) = [‑0.52: ‑0.24]) obtained from the JWST/NIRSpec data; further [C II] follow-up observations will constrain the covering fraction of photodissociation regions.
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