JWST and ALMA Multiple-Line Study in and around a Galaxy at $z=8.496$: Optical to FIR Line Ratios and the Onset of an Outflow Promoting Ionizing Photon Escape

Abstract

We present ALMA deep spectroscopy for a lensed galaxy at $z_{\mathrm{spec}}=8.496$ with $\log (M_{\mathrm{star}}/M_{\odot })\sim 7.8$ whose optical nebular lines and stellar continuum are detected by JWST/NIRSpec and NIRCam Early Release Observations in SMACS0723. Our ALMA spectrum shows [OIII]88$\hat{I}¼$m and [CII]158$\hat{I}¼$m line detections at $4.0\ddot{I}ƒ$ and $4.5\ddot{I}ƒ$, respectively. The redshift and position of the [OIII] line coincide with those of the JWST source, while the [CII] line is blue-shifted by 90 km s${}^{-1}$ with a spatial offset of $0{.}^{\prime \prime }5$ ($\approx 0.5$ kpc in source plane) from the JWST source. The NIRCam F444W image, including [OIII]$\hat{I}»$5007 and H${\hat{I}}^2$ 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 whose oxygen and carbon produce the extended [OIII]$\hat{I}»$5007 and the offset [CII] emission, which would promote ionizing photon escape and facilitate reionization. With careful slit-loss corrections and removals of emission spatially outside the galaxy, we evaluate the [OIII]88$\hat{I}¼$m/$\hat{I}»$5007 line ratio, and derive the electron density $n_{\mathrm{e}}$ by photoionization modeling to be $220_{-100}^{+170}$ cm${}^{-3}$, which is comparable with those of $z\sim 2-3$ galaxies. We estimate an [OIII]88$\hat{I}¼$m/[CII]158$\hat{I}¼$m line ratio in the galaxy of $>4$, as high as those of known $z\sim 6-9$ galaxies. This high [OIII]88$\hat{I}¼$m/[CII]158$\hat{I}¼$m line ratio is generally explained by the high $n_{\mathrm{e}}$ as well as the low metallicity ($Z_{\mathrm{gas}}/Z_{\odot }=0.04_{-0.02}^{+0.02}$), high ionization parameter ($\log U>-2.27$), and low carbon-to-oxygen abundance ratio ($\log$(C/O) $=[-0.52:-0.24]$) obtained from the JWST/NIRSpec data; further [CII] follow-up observations will constrain the covering fraction of photodissociation regions.

27 pages, 11 figures, 5 tables. ApJ in press