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?

Journal
Date Issued
2020
Author(s)
Harikane, Yuichi
•
Ouchi, Masami
•
Inoue, Akio K.
•
Matsuoka, Yoshiki
•
Tamura, Yoichi
•
Bakx, Tom
•
Fujimoto, Seiji
•
Moriwaki, Kana
•
Ono, Yoshiaki
•
Nagao, Tohru
•
Tadaki, Ken-ichi
•
Kojima, Takashi
•
Shibuya, Takatoshi
•
Egami, Eiichi
•
Ferrara, Andrea
•
Gallerani, Simona
•
Hashimoto, Takuya
•
Kohno, Kotaro
•
Matsuda, Yuichi
•
Matsuo, Hiroshi
•
Pallottini, Andrea
•
Sugahara, Yuma
•
DOI
Abstract
We present our new Atacama Large Millimeter/Submillimeter Array (ALMA) observations targeting [O III]88 μm, [C II]158 μm, [N II]122 μm, 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 4.3-11.8σ 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 [O III]/[C II] ratios of ${L}_{[{\rm{O}}{\rm{III}}]}/{L}_{[{\rm{C}}{\rm{II}}]}\sim 3\mbox{--}20$, ∼10 times higher than z ∼ 0 galaxies. We also find a positive correlation between the [O III]/[C II] ratio and the Lyα 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 Lyα, Lyman continuum, and ${{\rm{C}}}^{+}$ ionizing photons escape from galaxies, consistent with the [O III]/[C II]-Lyα EW correlation we find.
Volume
896
Issue
2
Start page
93
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2020ApJ...896...93H
Rights
open.access
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