After two ALMA observing cycles, only a handful of [C II] 158 µm emission line searches in z > 6 galaxies have reported a positive detection, questioning the applicability of the local [C II]-SFR relation to high-z systems. To investigate this issue we use the Vallini et al. (2013, V13) model, based on high-resolution, radiative transfer cosmological simulations to predict the [C II] emission from the interstellar medium of a z ≈ 7 (halo mass M h = 1.17 × 10 11 M ) galaxy. We improve the V13 model by including (a) a physically-motivated metallicity (Z) distribution of the gas, (b) the contribution of Photo-Dissociation Regions (PDRs), (c) the effects of Cosmic Microwave Background on the [C II] line luminosity. We study the relative contribution of diffuse neutral gas to the total [C II] emission (F diff /F tot ) for different SFR and Z values. We find that the [C II] emission arises predominantly from PDRs: regardless of the galaxy properties, F diff /F tot ≤ 10% since, at these early epochs, the CMB temperature approaches the spin temperature of the [C II] transition in the cold neutral medium (T CMB ∼ T CNM s ∼ 20 K). Our model predicts a high-z [C II]-SFR relation consistent with observations of local dwarf galaxies (0.02 < Z/Z < 0.5). The [C II] deficit suggested by actual data (L CII < 2.0 × 10 7 L in BDF3299 at z ≈ 7.1) if confirmed by deeper ALMA observations, can be ascribed to negative stellar feedback disrupting molecular clouds around star formation sites. The deviation from the local [C II]-SFR would then imply a modified Kennicutt-Schmidt relation in z > 6 galaxies. Alternatively/in addition, the deficit might be explained by low gas metallicities (Z < 0.1 Z ).

We present our new Atacama Large Millimeter/Submillimeter Array (ALMA) observations targeting [O iii]88 μm, [C ii]158 μm, [N ii]122 μm, and dust-continuum emission for three Lyman break galaxies at z = 6.0293–6.2037, identified in the Subaru/Hyper Suprime-Cam survey. We clearly detect [O iii] and [C ii] lines from all of the galaxies at 4.3–11.8σ levels, and identify multi-band dust-continuum emission in two of the three galaxies, allowing us to estimate infrared luminosities and dust temperatures simultaneously. In conjunction with previous ALMA observations for six galaxies at z > 6, we confirm that all the nine z = 6–9 galaxies have high [O iii]/[C ii] ratios of , ∼10 times higher than z ∼ 0 galaxies. We also find a positive correlation between the [O iii]/[C ii] ratio and the Lyα equivalent width (EW) at the ∼90% significance level. We carefully investigate physical origins of the high [O iii]/[C ii] ratios at z = 6–9 using Cloudy, and find that high density of the interstellar medium, low C/O abundance ratio, and the cosmic microwave background attenuation are responsible to only a part of the z = 6–9 galaxies. Instead, the observed high [O iii]/[C ii] ratios are explained by 10–100 times higher ionization parameters or low photodissociation region (PDR) covering fractions of 0%–10%, both of which are consistent with our [N ii] observations. The latter scenario can be reproduced with a density-bounded nebula with PDR deficit, which would enhance the Lyα, Lyman continuum, and ionizing photons escape from galaxies, consistent with the [O iii]/[C ii]-Lyα EW correlation we find.

We investigate the morphology of the [Cii] emission in a sample of "normal" star-forming galaxies at 5 < z < 7.2 in relation to their UV (rest-frame) counterpart. We use new ALMA observations of galaxies at z ∼ 6−7, as well as a careful re-analysis of archival ALMA data. In total 29 galaxies were analysed, 21 of which are detected in [Cii]. For several of the latter the [Cii] emission breaks into multiple components. Only a fraction of these [Cii] components, if any, is associated with the primary UV systems, while the bulk of the [Cii] emission is associated either with fainter UV components, or not associated with any UV counterpart at the current limits. By taking into account the presence of all these components, we find that the L [CII] -SFR relation at early epochs is fully consistent with the local relation, but it has a dispersion of 0.48±0.07 dex, which is about two times larger than observed locally. We also find that the deviation from the local L [CII] -SFR relation has a weak anti-correlation with the EW(Lyα). The morphological analysis also reveals that [Cii] emission is generally much more extended than the UV emission. As a consequence, these primordial galaxies are characterised by a [Cii] surface brightness generally much lower than expected from the local Σ [CII] − Σ SFR relation. These properties are likely a consequence of a combination of different effects, namely: gas metallicity, [Cii] emission from obscured star-forming regions, strong variations of the ionisation parameter, and circumgalactic gas in accretion or ejected by these primeval galaxies.