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?

Abstract

We present our new Atacama Large Millimeter/Submillimeter Array (ALMA) observations targeting [Oiii]88 mu m, [Cii]158 mu m, [Nii]122 mu m, and dust-continuum emission for three Lyman break galaxies atz = 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 4.3-11.8 sigma 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 atz > 6, we confirm that all the ninez = 6-9 galaxies have high [Oiii]/[Cii] ratios of<CDATA<i, similar to 10 times higher thanz similar to 0 galaxies. We also find a positive correlation between the [Oiii]/[Cii] ratio and the Ly alpha equivalent width (EW) at the similar to 90% significance level. We carefully investigate physical origins of the high [Oiii]/[Cii] ratios atz = 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 thez = 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 Ly alpha, Lyman continuum, and<CDATA<iionizing photons escape from galaxies, consistent with the [Oiii]/[Cii]-Ly alpha EW correlation we find.