First direct detection of star-forming gas in early galaxies

In the early Universe, the first galaxies began to take shape roughly million years after the Big Bang. Within these young systems, stars formed from vast reservoirs of cold gas, gradually building the structures we see in the cosmos today. Understanding this star-forming gas is key to explaining how galaxies grew, but directly tracing its neutral component has remained challenging, especially at great distances. In this study, the researchers overcome this challenge by detecting the [O I] 145 µm emission line, a direct tracer of neutral gas, enabling detailed investigation of star-forming conditions in early galaxies.

Despite its importance, neutral gas has remained difficult to study. Modern telescopes, such as the James Webb Space Telescope (JWST) and the Hubble Space Telescope (HST), can observe stars and hot gas in distant galaxies with remarkable clarity. However, they cannot directly detect the neutral gas that feeds star formation. To overcome this challenge, the researchers targeted the [O I] 145 µm emission line, a direct tracer of neutral gas that provides a clearer view of star-forming material within galaxies. In contrast, commonly used signals, such as the [C II] emission line, can originate from both neutral and ionized regions, making them harder to interpret. By also analyzing the [N II] emission line, which traces only ionized gas, the team was able to disentangle these contributions and isolate the neutral gas component.

Now, an international research team led by Assistant Professor Yoshinobu Fudamoto and Professor Masamune Oguri from the Center for Frontier Science, Chiba University, Japan, has addressed this challenge. Their study, which will be published in the Astrophysical Journal on June 15, 2026, reports new observations of distant galaxies using the Atacama Large Millimeter/submillimeter Array (ALMA). Members of the team included Dr. Akio K. Inoue from the Waseda Research Institute for Science and Engineering, Waseda University, Japan, Dr. Hanae Inami from Hiroshima Astrophysical Science Center, Hiroshima University, Japan, and Dr. Takuya Hashimoto from Tomonaga Center for the History of the Universe (TCHoU), University of Tsukuba, Japan.

The team targeted four typical star-forming galaxies seen as they were about 700 to 800 million years after the Big Bang. Using ALMA, the researchers detected the [O I] 145 µm emission line in all four galaxies. This signal, emitted by neutral oxygen atoms, serves as a clear tracer of the neutral gas. By combining these observations with data from JWST, the team was able to analyze the physical and chemical conditions of this star-forming material in unprecedented detail for such distant galaxies.

To support this analysis, the team also examined the [N II] 205 µm emission line, which traces only ionized gas. Its weak or absent signal indicates that most of the emission in these galaxies arises from neutral gas. This comparison further strengthens the interpretation of the [O I] detection and helps clarify the origin of previously observed signals such as [C II], placing them in the context of the galaxies' star-forming reservoirs. "Our results represent the most distant direct detection of neutral gas in typical star-forming galaxies to date," remarks Dr. Fudamoto. "This analysis unlocks the wealth of existing [C II] observations as a probe of neutral gas in the early Universe."

The team also used the [O I] and [C II] detections together to model the physical conditions in the neutral gas. They found that gas densities were very high, even comparable to those in starburst galaxies, which are among the most vigorously star-forming systems known. However, the intensity of the radiation field was moderately lower than in starburst galaxies. This paints a picture of early galaxies as compact and dense sites of star formation.

Overall, the researchers showcased how observations made using instruments like ALMA can shed light on key details about the history of the Universe. "Our work establishes the [O I] emission line as an effective tool for studying an elusive gas component in the early Universe, opening a new window onto the 'fuel' behind star formation," remarks Dr. Inoue.

Looking ahead, Dr. Fudamoto adds: "We plan to extend these observations to a larger sample of galaxies and, by combining ALMA with JWST and other facilities, build a comprehensive picture of how galaxies formed and evolved from the cosmic dawn to the present day. Basic research of this kind addresses one of humanity's most fundamental questions, namely how the Universe and our own Milky Way came to be what it is today."

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About Assistant Professor Yoshinobu Fudamoto from Chiba University, Japan

Dr. Yoshinobu Fudamoto obtained a Doctor of Science degree from the University of Geneva in 2020. He currently serves as a Project Appointed Assistant Professor at the Center for Frontier Science at Chiba University. His research focuses on elucidating the formation and evolution of stars and galaxies in the early Universe, leveraging facilities such as ALMA and the James Webb Space Telescope. He has 129 major publications to his name on these topics. He is a member of the Astronomical Society of Japan.

Funding

Yoshinobu Fudamoto, Yuma Sugahara, and Akio K. Inoue acknowledge support from NAOJ ALMA Scientific Research Grant number 2020-16B. This work was supported by JSPS KAKENHI Grant Numbers JP22K21349 and JP23K13149. Pratika Dayal acknowledges support from the NWO grant 016.VIDI.189.162 ("ODIN") and from the European Commission's and University of Groningen's CO-FUND Rosalind Franklin program. Hanae Inami and HSBA acknowledge support from the NAOJ ALMA Scientific Research Grant Code 2021-19A. Hanae Inami acknowledges support from JSPS KAKENHI Grant Number JP19K23462. Rebecca A. A. Bowler acknowledges support from an STFC Ernest Rutherford Fellowship [grant number ST/T003596/1]. Seiji Fujimoto acknowledges the funding from NASA through the NASA Hubble Fellowship grant #HST-HF2-51505.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Manuel Aravena is supported by FONDECYT grant number 1252054 and gratefully acknowledges support from ANID Basal Project FB210003 and ANID MILENIO NCN2024 112.

Reference

Title of original paper: ALMA Observations of [O I] 145 µm and [N II] 205 µm Emission lines from Star-Forming Galaxies at z ∼ 7

Journal: The Astrophysical Journal

DOI: https://doi.org/10.3847/1538-4357/ae5bad  

Authors: Yoshinobu Fudamoto^1,2,3, Akio K. Inoue^2,4, Rychard Bouwens⁵, Hanae Inami⁶, Renske Smit⁷, Dan Stark⁸, Manuel Aravena^9,10, Andrea Pallottini¹¹, Takuya Hashimoto^12,13, Masamune Oguri¹, Hiddo Algera¹⁴, Rebecca A. A. Bowler¹⁵, Elisabete da Cunha^16,17, Pratika Dayal¹⁸, Andrea Ferrara¹¹, Seiji Fujimoto¹⁹, Kasper E. Heintz^20,21, Alexander P. S. Hygate⁵, Ivana F. van Leeuwen⁵, Ilse De Looze²², Lucie E. Rowland²³, Mauro Stefanon^24,25, Yuma Sugahara^2,3, Joris Witstok^26,27, and Paul P. van der Werf²³

Affiliations: ¹Center for Frontier Science, Chiba University, Japan

²Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, Japan

³National Astronomical Observatory of Japan, Japan

⁴Department of Physics, School of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University, Japan

⁵Leiden Observatory, Leiden University, Netherlands

⁶Hiroshima Astrophysical Science Center, Hiroshima University, Japan

⁷Astrophysics Research Institute, Liverpool John Moores University, United Kingdom

⁸Steward Observatory, University of Arizona, United States

⁹Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Chile

¹⁰Millenium Nucleus for Galaxies (MINGAL)

¹¹Scuola Normale Superiore, Italy

¹²Division of Physics, Faculty of Pure and Applied Sciences, University of Tsukuba, Japan

¹³Tomonaga Center for the History of the Universe (TCHoU), Faculty of Pure and Applied Sciences, University of Tsukuba, Japan

¹⁴Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan, R.O.C

¹⁵Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, School of Natural Sciences, The University of Manchester, UK

¹⁶International Centre for Radio Astronomy Research, University of Western Australia, Australia

¹⁷ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia

¹⁸Kapteyn Astronomical Institute, University of Groningen, The Netherlands

¹⁹Department of Astronomy, The University of Texas at Austin, USA

²⁰Cosmic Dawn Center (DAWN), Denmark

²¹Niels Bohr Institute, University of Copenhagen, Denmark

²²Sterrenkundig Observatorium, Ghent University, Belgium

²³Leiden Observatory, Leiden University, The Netherlands

²⁴Departament d’Astronomia i Astrofísica, Universitat de València, Spain

²⁵Unidad Asociada CSIC “Grupo de Astrofísica Extragaláctica y Cosmología” (Instituto de Física de Cantabria - Universitat de València), Spain

²⁶Kavli Institute for Cosmology, University of Cambridge, UK

²⁷Cavendish Laboratory, University of Cambridge, UK

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