Large population of ALMA Galaxies at z > 6 with Very High [Oiii]88µm to [Cii]158µm flux ratios: Evidence of extremely high ionization parameter or PDR deficit?

We present our new ALMA observations targeting [Oiii]88µm, [Cii]158µm, [Nii]122µm, 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 [Oiii] and [Cii] 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 [Oiii]/[Cii] ratios of L_[OIII]/L_[CII] ∼ 3 − 20, ∼ 10 times higher than z ∼ 0 galaxies. We also find a positive correlation between the [Oiii]/[Cii] ratio and the Lyα equivalent width (EW) at the > 91% confidence level. We carefully investigate physical origins of the high [Oiii]/[Cii] 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 [Oiii]/[Cii] 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 [Nii] observations. The latter scenario can be reproduced with a density bounded nebula with PDR deficit, which would enhance the Lyα, Lyman continuum, and C⁺ ionizing photons escape from galaxies, consistent with the [Oiii]/[Cii]-Lyα EW correlation we find.