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We present our new ALMA observations targeting [OIII]88um, [CII]158um,[NII]122um, and dust continuum emission for three Lyman break galaxies atz=6.0293-6.2037 identified in the Subaru/Hyper Suprime-Cam survey. We clearlydetect [OIII] and [CII] lines from all of the galaxies at 4.3-11.8sigma levels,and identify multi-band dust continuum emission in two of the three galaxies,allowing us to estimate infrared luminosities and dust temperaturessimultaneously. In conjunction with previous ALMA observations for six galaxiesat 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 finda positive correlation between the [OIII]/[CII] ratio and the Lya equivalentwidth (EW) at the ~90% confidence level. We carefully investigate physicalorigins of the high [OIII]/[CII] ratios at z=6-9 using Cloudy, and find thathigh density of the interstellar medium, low C/O abundance ratio, and thecosmic microwave background attenuation are responsible to only a part of thez=6-9 galaxies. Instead, the observed high [OIII]/[CII] ratios are explained by10-100 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 boundednebula with PDR deficit, which would enhance the Lya, Lyman continuum, and C+ionizing photons escape from galaxies, consistent with the [OIII]/[CII]-Lya EWcorrelation we find.

astrophysical journal/the astrophysical journal

Large Population of ALMA Galaxies at z>6 with Very High [OIII]88um to  [CII]158um Flux Ratios: Evidence of Extremely High Ionization Parameter or  PDR Deficit?