We present new ALMA OIII₈₈ observations of eight previously CII₁₅₈-detected galaxies at 6. 8 z 7. 7. Six of our targets -- the primary sample -- are massive, UV-luminous galaxies drawn from the REBELS survey, while the remaining two are UV-fainter galaxies that were previously serendipitously detected through their luminous CII lines in the REBELS fields. We detect OIII₈₈ emission in all eight galaxies at 6. 2 - 17. 7σ significance, and find them to be consistent with the local dwarf galaxy relation between LOIII and star formation rate. Our sample spans OIII/CII 1. 9 - 9. 6, which is typical for the high-redshift galaxy population. Five of the primary targets benefit from JWST/NIRSpec observations, enabling a direct comparison of the OIII/CII ratio against rest-optical ISM diagnostics. We supplement our high-redshift sample with eleven z6-14 galaxies in the literature for which similar ALMA and JWST observations are available, and furthermore compare to the OIII/CII ratios measured for local dwarf galaxies. We find that, at fixed metallicity and ionization parameter, z>6 galaxies show elevated OIII/CII ratios compared to local dwarfs. Instead, we find that a large OIII₄₉₅₉, ₅₀₀₇+Hβ equivalent width -- a proxy for burstiness -- is the main driver of the high OIII/CII ratios seen in the early Universe, which is primarily due to CII being suppressed in bursty galaxies. Given the apparent validity of the OIII₈₈-SFR relation across most of cosmic time, as well as the abundance of young, bursty galaxies at high redshift, OIII₈₈ is set to remain a powerful ISM tracer at the cosmic dawn.
Algera et al. (2025) studied this question.