Gas conditions of a star-formation selected sample in the first billion years

We present Atacama Large Millimetre/submillimetre Array (ALMA) observations of the [O iii] 88 μ m emission of a sample of thirteen galaxies at $z = 6$ to 7.6 selected as [C ii]-emitting companion sources of quasars. To disentangle the origins of the luminous Oxygen line in the z\, >\, 6 Universe, we looked at emission-line galaxies that are selected through an excellent star-formation tracer [C ii] with star-formation rates between 9 and 162 M\,yr⁻¹. Direct observations reveal [O iii] emission in just a single galaxy (L_[O\, III]/L_[C\, II]\, = 2.3$), and a stacked image shows no [O iii] detection, providing deep upper limits on the $L_{[O\, { III}]}/L_{[C\, { II}]}$ ratios in the $z > 6$ Universe ($L_{[O\, { III}]}/L_{[C\, { II}]}\, < 1.2 at 3 σ). While the fidelity of this sample is high, no obvious optical/near-infrared counterpart is seen in the JWST imaging available for four galaxies. Additionally accounting for low-z CO emitters, line stacking shows that our sample-wide result remains robust: The enhanced L_[O\, III]/L_[C\, II] reported in the first billion years of the Universe is likely due to the selection towards bright, blue Lyman-break galaxies with high surface star-formation rates or young stellar populations. The deep upper limit on the rest-frame 90 μm continuum emission (< 141 μJy at 3 σ), implies a low average dust temperature (Tdust 30\, K) and high-dust mass (Mdust ~ 10⁸\, M_). As more normal galaxies are explored in the early Universe, synergy between JWST and ALMA is fundamental to further investigate the ISM properties of the a broad range of samples of high-z galaxies.