A first systematic study of [OIII] 88$\mu$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 [OIII] $88\mu$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 $-22.5\lesssim M_{\mathrm{U}\mathrm{V}}\lesssim -20.5$. [OIII] $88\mu$m is confidently detected in UNCOVER-10646 at $z=8.5080\pm 0.0011$ ($15\sigma$) and DHZ1 at $z=9.3113\pm 0.0005$ ($6\sigma$), with both being intrinsically luminous systems [$L_{[\mathrm{O}\mathrm{I}\mathrm{I}\mathrm{I}]}=(1.1-1.6)\times {10}^9$ $L_{\odot }$] that follow the local [OIII]-SFR relation. [OIII] $88\mu$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 $L_{[\mathrm{O}\mathrm{I}\mathrm{I}\mathrm{I}]}<6\times {10}^7$ $L_{\odot }$. Dust emission is not detected in any individual source nor in a stack ($<3\sigma$). The high S/N [OIII] $88\mu$m detection in UNCOVER-10646 uniquely reveals an additional broad component ($\mathrm{F}\mathrm{W}\mathrm{H}\mathrm{M}=1366_{-329}^{+473}$ $\mathrm{k}\mathrm{m}\mathrm{/}\mathrm{s}$; $\mathrm{\Delta }\mathrm{B}\mathrm{I}\mathrm{C}\approx 20$) indicative of an ionized outflow. We infer a high outflow rate of ${\dot{M}}_{\mathrm{o}\mathrm{u}\mathrm{t}}=128_{-46}^{+80}$ $M_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$, corresponding to a mass loading factor $\eta ={\dot{M}}_{\mathrm{o}\mathrm{u}\mathrm{t}}\mathrm{/}\mathrm{S}\mathrm{F}\mathrm{R}=2.9_{-1.0}^{+1.8}$ that is broadly consistent with theoretical predictions and JWST-based studies of ionized outflows at high redshift. While high-resolution ALMA follow-up is required to confirm the outflow, this first systematic study at $z>8$ highlights the unique diagnostic power of [OIII] $88\mu$m in characterizing galaxies in the early Universe.