A first systematic study of [OIII] 88\(μ\)m at \(z>8\): two luminous oxygen lines and a powerful ionized outflow in the first 600 million years

We present deep ALMA Band 7 observations of the \(\mathrm{[O\,III]}88\,\mu\mathrm{m}\) line and underlying dust continuum emission in four UV-bright, gravitationally lensed (magnification \(\mu = 1.4-3.8\)), JWST-selected galaxies at \(z = 8.5 - 10.3\), with observed magnitudes \(-22.5 \lesssim M_\mathrm{UV} \lesssim -20.5\). \(\mathrm{[O\,III]}\) is confidently detected in UNCOVER-10646 at \(z=8.5080 \pm 0.0011\) (\(15\sigma\)) and DHZ1 at \(z=9.3113 \pm 0.0006\) (\(6\sigma\)), with both being intrinsically luminous systems [\(L_\mathrm{[O\,III]} = (1.1 - 1.6) \times 10^9\,L_\odot\)] that follow the local \(\mathrm{[O\,III]}\)-SFR relation. \(\mathrm{[O\,III]}88\,\mu\mathrm{m}\) remains undetected in the two \(z>10\) targets, including in the \(z=10.07\) X-ray AGN UHZ1, where we obtain a deep limit of \(L_\mathrm{[O\,III]} < 6 \times 10^7\,L_\odot\). Dust emission is not detected in any individual source nor in a stack (\(<3\sigma\)). The high S/N \(\mathrm{[O\,III]}88\,\mu\mathrm{m}\) detection in UNCOVER-10646 uniquely reveals an additional broad component (\(\mathrm{FWHM} = 1366_{-329}^{+473}\,\mathrm{km/s}\); \(\Delta\mathrm{BIC}\approx20\)) indicative of an ionized outflow. We infer a high outflow rate of \(\dot{M}_\mathrm{out} = 128_{-46}^{+80}\,M_\odot\,\mathrm{yr}^{-1}\), corresponding to a mass loading factor \(\eta = \dot{M}_\mathrm{out}/\mathrm{SFR} = 2.9_{-1.0}^{+1.8}\) that matches or exceeds theoretical predictions and JWST-based studies of ionized outflows at high redshift. While high-resolution ALMA follow-up is required to confirm and spatially resolve the outflow, this first systematic study at \(z>8\) highlights the unique diagnostic power of \(\mathrm{[O\,III]}88\,\mu\mathrm{m}\) in characterizing galaxies in the early Universe.