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  |$\mu {\rm m}$| 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 |$z\, \gt\, 6$| 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  |$\rm {\rm M}_{\odot }\,yr^{-1}$|⁠. Direct observations reveal [O  iii ] emission in just a single galaxy (⁠|$L_\mathrm{[O\, {\small III}]}/L_\mathrm{[C\, {\small II}]}$| |$\, = 2.3$|⁠), and a stacked image shows no [O  iii ] detection, providing deep upper limits on the |$L_\mathrm{[O\, {\small III}]}/L_\mathrm{[C\, {\small II}]}$| ratios in the |$z \gt 6$| Universe (⁠|$L_\mathrm{[O\, {\small III}]}/L_\mathrm{[C\, {\small II}]}$| |$\, \lt 1.2$| at |$3 \sigma$|⁠). 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 |$L_\mathrm{[O\, {\small III}]}/L_\mathrm{[C\, {\small II}]}$| 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  μ m continuum emission (⁠|$\lt 141 \mu$| Jy at |$3 \sigma$|⁠), implies a low average dust temperature (⁠|$T_\mathrm{dust} \lesssim 30\,$|  K) and high-dust mass (⁠|$M_\mathrm{dust} \sim 10^8\, \mathrm{M}_\odot$|⁠). 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. [ABSTRACT FROM AUTHOR]