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

Abstract

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 $-22.5 \lesssim M_\mathrm{UV} \lesssim -20.5$. [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 [$L_\mathrm{[OIII]} = (1.1 - 1.6) \times 10^9$ $L_\odot$] 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 $L_\mathrm{[OIII]} < 6 \times 10^7$ $L_\odot$. 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 ($\mathrm{FWHM} = 1366_{-329}^{+473}$ $\mathrm{km/s}$; $\Delta\mathrm{BIC}\approx20$) 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 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.