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

ABSTRACT We present Atacama Large Millimetre/submillimetre Array (ALMA) observations of the O iii 88 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 (LO\, III/LC\, II\, = 2. 3), and a stacked image shows no O iii detection, providing deep upper limits on the LO\, III/LC\, II ratios in the z 6 Universe (LO\, III/LC\, II\, 1. 2 at 3). 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 LO\, III/LC\, 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 Jy at 3), 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.