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

We present Atacama Large Millimetre/submillimetre Array (ALMA) observations of the [OIII] 88 μm emission of a sample of thirteen galaxies at z = 6 to 7.6 selected as [CII]-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 [CII] with star-formation rates between 9 and 162 M_/yr. Direct observations reveal [OIII] emission in just a single galaxy (L_ [OIII]/L_ [CII] = 2.3), and a stacked image shows no [OIII] detection, providing deep upper limits on the L_ [OIII]/L_ [CII] ratios in the $z > 6$ Universe (L_ [OIII]/L_ [CII] < 1.2 at 3σ). 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-redshift CO emitters, line stacking shows that our sample-wide result remains robust: The enhanced L_ [OIII]/L_ [CII] 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 3σ), implies a low average dust temperature (Tdust < 30K) 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.