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

TL;DR

This work presents the first systematic study of the [O III] 88 μm line in four z>8 galaxies, using deep ALMA Band 7 observations of JWST-selected, gravitationally lensed systems to measure ionized gas, dust content, and kinematics. Two galaxies, UNCOVER-10646 and DHZ1, are robustly detected in [O III] 88 μm with luminosities of about (1.1–1.6)×10^9 L⊙ and show that [O III] emission tracks the local SFR relation for dwarfs, while the other two targets yield deep upper limits for the line and for dust. A broad [O III] 88 μm component in UNCOVER-10646 indicates a powerful ionized outflow with M_out ≈ 1.4×10^8 M⊙, v_out ≈ 1160 km s⁻¹, and a mass-loading factor η ≈ 2.9, suggesting significant feedback capable of ejecting material and dust from the system. The results highlight the diagnostic power of [O III] 88 μm for early galaxies, the limited dust content in these systems, and the potential for strong feedback in shaping galaxy evolution during Cosmic Dawn, setting the stage for the PHOENIX ALMA program and JWST synergy to build a statistical view of ISM conditions and dust buildup at z>8.

Abstract

We present deep ALMA Band 7 observations of the [OIII] $88μ$m line and underlying dust continuum emission in four UV-bright, gravitationally lensed (magnification $μ= 1.4-3.8$), JWST-selected galaxies at $z = 8.5 - 10.3$, with observed magnitudes $-22.5 \lesssim M_\mathrm{UV} \lesssim -20.5$. [OIII] $88μ$m is confidently detected in UNCOVER-10646 at $z=8.5080 \pm 0.0011$ ($15σ$) and DHZ1 at $z=9.3113 \pm 0.0005$ ($6σ$), 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] $88μ$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 ($<3σ$). The high S/N [OIII] $88μ$m detection in UNCOVER-10646 uniquely reveals an additional broad component ($\mathrm{FWHM} = 1366_{-329}^{+473}$ $\mathrm{km/s}$; $Δ\mathrm{BIC}\approx20$) 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 $η= \dot{M}_\mathrm{out}/\mathrm{SFR} = 2.9_{-1.0}^{+1.8}$ 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] $88μ$m in characterizing galaxies in the early Universe.

Figures (14)

• Figure 1: Absolute UV magnitude versus redshift (bottom $x$-axis) and Cosmic time (top axis) for our targets (large, colored markers) and other $z>8$ galaxies targeted in [O iii]$_{88}$ in the literature hashimoto2018tamura2019fujimoto2024_s4590zavala2024_almacarniani2024_oiiischouws2024witstok2025. The larger, opaque markers show the apparent brightness, including the effects of gravitational lensing, while the smaller symbols -- connected via the dashed lines -- correspond to de-lensed values. We moreover show the compilation of $z>6.5$ galaxies spectroscopically confirmed with the JWST from tang2024tang2025 as well as the $z>8$ photometric candidates from hainline2024. Our targets are particularly UV-luminous galaxies at $z>8$.
• Figure 2: Naturally-weighted ALMA Band 7 dust continuum cutouts ($3"\times3"$) of our four targets, overplotted on JWST/NIRCam F150W imaging. Contours are drawn at $\pm2,3,4,\ldots\times\sigma$ intervals, where $\sigma$ is the continuum RMS. Negative contours are dashed. None of the sources are convincingly dust-detected, although UNCOVER-10646 shows a $\sim2\sigma$ continuum signal in tapered imaging (Figure \ref{['fig:continuum_UC10646_tapered']} in Appendix \ref{['app:dust_continuum_UC10646']}), tentatively suggesting the presence of an extended dust reservoir.
• Figure 3: ALMA [O iii]$_{88}$ cutouts ($4"\times4"$) of our four targets, overplotted on JWST/NIRCam F444W imaging. For UNCOVER-10646 and DHZ1, where [O iii]$_{88}$ emission is robustly detected, the moment-0 map spans all channels within $1.2\times$ the line FWHM. No line emission is detected in UHZ1 and UNCOVER-37126, and the moment-0 map is collapsed across $200\,\mathrm{km/s}$ centered on the fiducial redshifts of $z=10.073$ and $z=10.255$, respectively. Contours are drawn at $\pm2,3,4,\ldots, 10\sigma$ intervals, with negative contours being dashed and $\sigma$ being the RMS in the moment-0 map.
• Figure 4: Extracted 1D spectra showing the [O iii]$_{88}$ detections for UNCOVER-10646 at $z=8.51$ (left) and DHZ1 at $z=9.31$ (right). A Gaussian fit to the spectra is shown through the red lines. The vertical shading shows the channels across which the moment-0 maps in Figure \ref{['fig:momentZero']} are collapsed, while the gray shading represents the noise per channel. The best-fit parameters are presented in Table \ref{['tab:fluxes']}. Note that the spectrum of UNCOVER-10646 is better fit by a double-Gaussian, as detailed in Section \ref{['sec:discussion_outflow']}.
• Figure 5: Left: The [O iii]$_{88}$-SFR relation of $z>6$ galaxies. Our four $z>8.5$ targets are overplotted as the colored symbols, while $z>8$ ($6 \lesssim z < 8$) galaxies compiled from the literature by algera2025_oiii are shown as white (grey) squares. We show UNCOVER-37126 as a semi-transparent symbol, given that its [O iii]$_{88}$ line is possibly missed by our ALMA observations as a result of its uncertain, Lyman-break-based redshift. The local relations for starbursts (green), dwarf galaxies (blue) and AGN (purple) from delooze2014 are indicated through the shaded bands. All values are corrected for lensing, where necessary. Our sample shows [O iii]$_{88}$ luminosities consistent with the local dwarf galaxy relation, in agreement with previous studies of $z\gtrsim6$ galaxies with $\mathrm{SFR} \lesssim 100\,M_\odot\,\mathrm{yr}^{-1}$. Right: the [O iii]$_{88}$ luminosity of $z>6$ galaxies plotted against redshift. We highlight the individually [O iii]$_{88}$-detected galaxies at $z>8$hashimoto2018tamura2019fujimoto2024_s4590zavala2024_almacarniani2024_oiiischouws2024witstok2025. Our line-detected galaxies, UNCOVER-10646 and DHZ1, are the two most [O iii]$_{88}$-luminous sources known at $z>8$ and $z>9$, respectively.