Large Population of ALMA Galaxies at z > 6 with Very High [O iii] 88 μm to [C ii] 158 μm Flux Ratios: Evidence of Extremely High Ionization Parameter or PDR Deficit?

We present our new Atacama Large Millimeter/Submillimeter Array (ALMA) observations targeting [O iii ]88 μ m, [C ii ]158 μ m, [N ii ]122 μ m, 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 4.3–11.8 σ 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 Ly α 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 Ly α , Lyman continuum, and ionizing photons escape from galaxies, consistent with the [O iii ]/[C ii ]-Ly α EW correlation we find.