Dual constraints with ALMA: new [OII] 88um and dust-continuum observations reveal the ISM conditions of luminous LBGs at z~7

We present new [OIII] 88-mu m observations of five bright z similar to 7 Lyman-break galaxies spectroscopically confirmed by ALMA through [CII] 158 mu m, unlike recent [OIII] detections where Lyman alpha was used. This nearly doubles the sample of Epoch of Reionization galaxies with robust (5 sigma) [CII] and [OIII] detections. We perform a multiwavelength comparison with new deep HST images of the rest-frame UV, whose compact morphology aligns well with [OIII] tracing ionized gas. In contrast, we find more spatially extended [CII] emission likely produced in neutral gas, as indicated by an [NII] 205-mu m non-detection in one source. We find a correlation between the optical [OIII]+H beta equivalent width and [OIII]/[CII], 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 [OIII] 88-mu m, unless we assume an alpha/Fe enhancement and near-solar nebular oxygen abundance. On spatially resolved scales, we find [OIII]/[CII] shows a tentative anticorrelation with infrared excess, L-IR/L-UV, 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.