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${}_{\mathrm{III}}$] 88 $\mu$m emission of a sample of thirteen galaxies at $z$ = 6 to 7.6 selected as [C${}_{\mathrm{II}}$]-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${}_{\mathrm{II}}$] with star-formation rates between 9 and 162 M${}_{\odot }$/yr. Direct observations reveal [O${}_{\mathrm{III}}$] emission in just a single galaxy (L${}_{[{\mathrm{O}}_{\mathrm{III}}]}$/L${}_{[{\mathrm{C}}_{\mathrm{II}}]}$ = 2.3), and a stacked image shows no [O${}_{\mathrm{III}}$] detection, providing deep upper limits on the L${}_{[{\mathrm{O}}_{\mathrm{III}}]}$/L${}_{[{\mathrm{C}}_{\mathrm{II}}]}$ ratios in the $z>6$ Universe (L${}_{[{\mathrm{O}}_{\mathrm{III}}]}$/L${}_{[{\mathrm{C}}_{\mathrm{II}}]}$ < 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-redshift CO emitters, line stacking shows that our sample-wide result remains robust: The enhanced L${}_{[{\mathrm{O}}_{\mathrm{III}}]}$/L${}_{[{\mathrm{C}}_{\mathrm{II}}]}$ 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 $\mu$m continuum emission (< 141 $\mu$Jy at 3$\sigma$), implies a low average dust temperature (T${}_{\mathrm{dust}}$ < 30K) and high dust mass (M${}_{\mathrm{dust}}$ ~ 10${}^8$ 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.

20 pages; 13 figures; accepted for publication in MNRAS