First Detection of Neutral Oxygen Emission from Galaxies in the Early Universe — Opening a New Window on Observing the 'Materials for Stars' with ALMA

An international research team led by Project Assistant Professor Yoshinobu Fushimoto and Professor Shinobu Oguri of Chiba University’s Advanced Science Research Center, Professor Akihiko Inoue of Waseda University, Assistant Professor Takuya Hashimoto of the University of Tsukuba, and Associate Professor Kanae Inomata of Hiroshima University, has successfully detected the '[O I] 145㎛ (micrometers)' emission line from neutral oxygen in four galaxies approximately 700 to 800 million years after the birth of the universe, using the Atacama Large Millimeter/submillimeter Array (ALMA) (Note 1). This represents the most distant direct detection to date of emission from cold neutral gas (Note 3) in typical star-forming galaxies. Neutral gas is the direct material for star formation, but it is difficult to observe directly with visible and near-infrared telescopes such as the James Webb Space Telescope (JWST) (Note 4), making this an observation uniquely enabled by ALMA. This research result opens a new observational window for understanding how stars were born and evolved in galaxies of the early universe.

The findings were published on June 15, 2026 (Eastern Time, USA) in the academic journal Astrophysical Journal. (Paper: 10.3847/1538-4357/ae5bad)

Figure: Galaxy A1689-zD1, existing about 700 million years after the Big Bang (background), and the emission line from neutral oxygen detected via ALMA observations (contours and spectrum)

Background of the Research

Stars are born when 'neutral gas'—composed of hydrogen atoms and molecules—cools and accumulates within galaxies. Therefore, to understand how galaxies formed in the early universe, it is essential to investigate the properties of neutral gas, the direct material for stars. However, visible and near-infrared telescopes such as JWST, which have recently achieved significant results, can observe ionized gas (Note 5) and stars themselves, but cannot directly capture cold neutral gas. Radio telescopes like ALMA are required for such observations, yet direct observations of galaxies in the early universe have been extremely rare.

Key Points of the Research Results

- First direct observation of the 'material for stars' from early galaxies: Using ALMA, the team detected the '[O I] 145μm' emission line, indicating the presence of neutral gas—the direct material for stars—from four star-forming galaxies (REBELS-38, A1689-zD1, REBELS-25, REBELS-18) approximately 700 to 800 million years after the Big Bang. This emission line has not previously been observed in star-forming galaxies in the early universe and represents the most distant detection from typical star-forming galaxies. - Early galaxies were rich in 'materials for stars': Combined analysis of multiple emission line observations revealed that these galaxies possess high-density neutral gas comparable to present-day starburst galaxies. Furthermore, by combining with oxygen abundance ratio measurements from JWST, the team successfully directly estimated the mass of neutral gas. - Confirming the origin of the widely used '[C II] 158μm' emission line (Note 6): In this study, the team also observed the [N II] 205μm (Note 7) emission line originating from ionized gas and conducted comparative analysis of multiple far-infrared emission lines. The widely observed '[C II] 158㎛' emission line could originate from both neutral and ionized gas, leaving ambiguity about its primary source. By comparing with the current observations, the team successfully verified for the first time that it is primarily emitted from neutral gas. This opens the way to utilizing accumulated [C II] observational data for neutral gas research.

Future Outlook

These observations have revealed for the first time that the neutral oxygen emission line can be effectively observed even from galaxies in the early universe. By confirming its utility for studying the 'materials for stars,' this research has opened a new door in cosmic studies. In the future, the team plans to conduct observations on a larger number of galaxies to clarify the overall picture of star formation in galaxies immediately after the birth of the universe. Additionally, by combining with other telescopes such as JWST, they aim to comprehensively understand how galaxies formed and evolved from the beginning of the universe to the present day.

Glossary

- Note 1) ALMA Telescope: The world’s largest radio telescope, located in the Atacama Desert in northern Chile at an altitude of about 5,000 meters, built through international collaboration involving Japan, Europe, North America, and other regions. - Note 2) [O I] 145㎛: Light emitted at a specific wavelength by atoms or molecules is called an 'emission line,' from which the type and state of atoms or molecules can be determined. [O I] 145㎛ refers to a far-infrared emission line emitted by neutral oxygen atoms. - Note 3) Neutral Gas: Cold gas in which electrons remain bound to atomic nuclei, serving as the direct material for star formation. - Note 4) James Webb Space Telescope (JWST): A space telescope jointly developed by NASA, the European Space Agency, and the Canadian Space Agency. Launched in 2021, it enables high-precision observations of stars and ionized gas in early galaxies through near- to mid-infrared wavelengths. - Note 5) Ionized Gas: Hot gas from which electrons have been stripped away by strong ultraviolet radiation emitted by young, hot stars. - Note 6) [C II] 158㎛: Refers to a far-infrared emission line emitted by ionized carbon. - Note 7) [N II] 205μm: Refers to a far-infrared emission line emitted by ionized nitrogen.

Paper Information

- Title: ALMA Observations of [OⅠ]145µm and [NⅡ]205µm Emission lines from Star-Forming Galaxies at z ∼ 7 - Authors: Yoshinobu Fushimoto, Akihiko Inoue, Rychard Bouwens, Kanae Inomata, Renske Smit, Dan Stark, Manuel Aravena, Andrea Pallottini, Takuya Hashimoto, Shinobu Oguri, et al. (15 others) - Journal: Astrophysical Journal - DOI: 10.3847/1538-4357/ae5bad

About the Research Project

This research was conducted with support from the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grants JP22K21349, JP23K13149).