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?
Yuichi Harikane, Masami Ouchi, Akio Inoue, Yoshiki Matsuoka, Yoichi Tamura, Tom J. L. C. Bakx, Seiji Fujimoto, Kana Moriwaki, Yoshiaki Ono, Tohru Nagao, Ken-ichi Tadaki, Takashi Kojima, Takatoshi Shibuya, Eiichi Egami, Andrea Ferrara, S. Gallerani, Takuya Hashimoto, Kotaro Kohno, Yuichi Matsuda, Hiroshi Matsuo, A. Pallottini, Yuma Sugahara, L. Vallini
Abstract
Abstract 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.