Abstract

We present new [ O III] 88- μ m observations of five bright z ∼7 Lyman-break galaxies spectroscopically confirmed by ALMA through [ C II] 158 μ m, unlike recent [ O III] detections where Lyman α was used. This nearly doubles the sample of Epoch of Reionization galaxies with robust (5σ) [ C II] and [ O III] detections. We perform a multiwavelength comparison with new deep HST images of the rest-frame UV, whose compact morphology aligns well with [ O III] tracing ionized gas. In contrast, we find more spatially extended [ C II] emission likely produced in neutral gas, as indicated by an [ N II] 205- μm non-detection in one source. We find a correlation between the optical [O III]+ H β equivalent width and [ O III]/[ C II], as seen in local metal-poor dwarf galaxies. cloudy models of a nebula of typical density harbouring a young stellar population with a high-ionization parameter adequately reproduce the observed lines. Surprisingly, however, our models fail to reproduce the strength of [ O III] 88- μ m, unless we assume an α/Fe enhancement and near-solar nebular oxygen abundance. On spatially resolved scales, we find [ O III]/[ C II] shows a tentative anticorrelation with infrared excess, LIR/LUV, also seen on global scales in the local Universe. Finally, we introduce the far-infrared spectral energy distribution fitting code mercurius to show that dust-continuum measurements of one source appear to favour a low dust temperature and correspondingly high dust mass. This implies a high stellar metallicity yield and may point towards the need of dust production or grain-growth mechanisms beyond supernovae.