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 K. Inoue, Yoshiki Matsuoka, Yoichi Tamura, Tom Bakx, Seiji Fujimoto, Kana Moriwaki, Yoshiaki Ono, Tohru Nagao, Ken Ichi Tadaki, Takashi Kojima, Takatoshi Shibuya, Eiichi Egami, Andrea Ferrara, Simona Gallerani, Takuya Hashimoto, Kotaro Kohno, Yuichi Matsuda, Hiroshi MatsuoAndrea Pallottini, Yuma Sugahara, Livia Vallini

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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, ∼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 C+ ionizing photons escape from galaxies, consistent with the [O iii]/[C ii]-Lyα EW correlation we find.

Original languageEnglish
Article number93
JournalAstrophysical Journal
Volume896
Issue number2
DOIs
Publication statusPublished - 2020 Jun 20

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Harikane, Y., Ouchi, M., Inoue, A. K., Matsuoka, Y., Tamura, Y., Bakx, T., Fujimoto, S., Moriwaki, K., Ono, Y., Nagao, T., Tadaki, K. I., Kojima, T., Shibuya, T., Egami, E., Ferrara, A., Gallerani, S., Hashimoto, T., Kohno, K., Matsuda, Y., ... Vallini, L. (2020). 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? Astrophysical Journal, 896(2), [93]. https://doi.org/10.3847/1538-4357/ab94bd