2025MNRAS.542.1388V

We present the [O III]$_{88\mu \text{m}}$ spectral scan results from the ALMA Large Program REBELS (Reionization Era Bright Emission Line Survey). The generally high luminosity of [O III]$_{88\mu \text{m}}$ and ALMA's Band 7 efficiency motivated its use for line scans of REBELS targets at $z>8$. Spectral scans of four sources covered 326.4-373.0 GHz ($z=8.10$-9.39), reaching [O III]$_{88\mu \text{m}}$ luminosities of $\mathrm{\sim 7.6\times 10^8\ L_{\odot }}$ ($5\sigma$) for a full width at half maximum (FWHM) of 400 km s$^{-1}$. No credible lines are detected for the four targets. For REBELS-04, the non-detection is unexpected given the $\ge 92~{{\ \rm per\ cent}}$ coverage of the redshift likelihood distribution and its estimated star formation rate of 40 $\text{M}_{\odot }\ \text{yr}^{-1}$. Possible explanations for the faint [O III]$_{88\mu \text{m}}$ emission (assuming an FWHM of 100 km s$^{-1}$) include high interstellar medium densities ($>n_{\text{crit}} \approx 510\ \text{cm}^{-3}$) and low ionization parameters ($\mathrm{log_{10}\ U_{ion}\lesssim -2.5}$). For REBELS-37, a subsequent detection of [C II]$_{158\mu \text{m}}$ ($z=7.643$) confirmed it lay outside our scan range. For REBELS-11 and REBELS-13, it remains unclear if the non-detection is due to the depth of the line scan or redshift coverage. REBELS-04 and REBELS-37 show significant ($\ge 3.8\sigma$) dust continuum emission in Band 7. If the photometric redshift of REBELS-04 is accurate, i.e. $z_{\mathrm{phot}}=8.57^{+0.10}_{-0.09}$ or $z_{\mathrm{phot}}=8.43^{+0.10}_{-0.10}$ accounting for additional neutral hydrogen in the circumgalactic medium, REBELS-04 would constitute the most distant dust-detected galaxy identified with ALMA to date. Additional Band 6 dust observations of REBELS-37 constrain the shape of the far-IR spectral energy distribution, ruling out cold dust temperatures ($\lesssim 28$ K) at $3\sigma$. Further insight into these galaxies will require spectroscopic redshifts and deeper multiband dust observations.