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Mathematical and physical sciences

June 13, 2023

Secrets of the ancient universe revealed: James Webb Space Telescope captures ionization of intergalactic gas by the first galaxies

An international team of researchers from ETH Zürich, Massachusetts Institute of Technology, North Carolina State University, and Nagoya University has achieved a groundbreaking discovery in the field of astronomy. Using the James Webb Space Telescope (JWST), they were able to witness the process of cosmic reionization, taking place about 12.9 billion years ago. The team observed young star-forming galaxies ionizing the surrounding intergalactic gas, marking the first time this phenomenon has been directly observed.


Cosmic reionization is the last major event in the evolution of our Universe. After the Big Bang, a period existed in which most cosmic gas was neutral, and the Universe was opaque. However, around 400 million years after the Big Bang, when the first stars and galaxies formed, this started to change. Over the course of the next 600 million years, the hydrogen atoms in our Universe started getting ionized and formed hot plasma. The change in state of these hydrogen atoms from neutral to ionized, combined with their increase in temperature, meant that the conditions in our Universe changed drastically and it became transparent. Several theories for the cause of this reionization have been proposed. The most commonly accepted theory is that reionization was caused by ultraviolet radiation from star-forming young galaxies, but other possibilities, such as radiation from black holes and the collapse of exotic particles, have also been suggested. However, no direct observations to support any of these theories had been reported. Until now.


A study led by Nagoya University’s Dr. Daichi Kashino used the JWST to detect star-forming galaxies around 1 billion years after the Big Bang, in combination with ground-based observations of distant quasars. Quasars are among the brightest and most energetic objects in the Universe, often outshining entire galaxies. In fact, they are so bright that we can observe quasars that existed billions of years ago, during the reionization period.


In their study, Dr. Kashino and his team members studied the light emitted by a quasar at the end of the reionization period, about 900 million years after the Big Bang. Using the JWST, the team then identified over 100 galaxies that existed between 800 million and 1.1 billion years after the Big Bang and were located in (relatively) close proximity of the so-called “line of sight” towards this quasar. By studying these galaxies and the quasar spectrum, the team were able to measure how much light was transmitted by intergalactic gas as a function of distance from those galaxies.


The team found that when the Universe was only partially ionized, galaxies were in bubbles of transparent, ionized gas, that allowed light from the quasar to pass through. The surroundings of the bubbles were made up of opaque, neutral gas. However, with time these ionization bubbles expanded and merged, and eventually all opaque, neutral gas between the galaxy bubbles became ionized. This means that the transmitted quasar light observed around the earlier galaxies could only be observed because those regions were already ionized. Therefore, the researchers concluded reionization must have originated from a local source: the galaxies themselves. This observation confirms that it is normal galaxies that caused cosmic reionization and not rare quasars or exotic possibilities.


In a related study led by Dr. Jorryt Matthee from ETH Zürich, the properties of the aforementioned galaxies were investigated. The team found that the galaxies produced ionizing photons efficiently, while producing relatively low amounts of heavy elements and dust. This makes them an effective source for reionization, supporting the abovementioned results. Only about 1% of present-day galaxies exhibit similar properties, indicating how strongly the properties of galaxies have evolved over cosmic time.  


Gaining a deeper understanding of the cosmic reionization process is one of the major scientific objectives for the James Webb Space Telescope mission. The insights obtained from this study provide a foundation for further observational research on the early stages of reionization and the period before, generally referred to as the cosmic dark ages. Furthermore, this research accomplishment represents a significant step toward the ultimate goal of astronomists: to understand the entire history of our Universe.



The celestial area that JWST observed in this study (only the central 8% of the entire survey field is shown).  The quasar J0100+2802 that was used as a “beacon” to extract the information about the intervening gas in this work appears to be a bright “star” at the center of the image. 

Credit: (c) The EIGER collaboration



Schematic representation of how the team looked back in cosmic time to study the process of reionization.

By using the James Webb Space Telescope to identify galaxies that were in the line of sight of the quasar, the team was able to study the degree of reionization and its origin at multiple points in time during the reionization period.

Credit: (c) Kashino


The study EIGER I. a large sample of [OIII]-emitting galaxies at 5.3 < z < 6.9 and direct evidence for local reionization by galaxies was published in the Astrophysical Journal, on June 12th, 2023. DOI: 10.3847/1538-4357/acc588
Authors: Daichi Kashino, Simon J. Lilly, Jorryt Matthee, Anna-Christina Eilers, Ruari Mackenzie, Rongmon Bordoloi, and Robert A. Simcoe.



The team’s study led by Dr. Matthee entitled “EIGER II. first spectroscopic characterization of the young stars and ionized gas associated with strong Hβ and [OIII] line-emission in galaxies at z=5-7 with JWST” and “EIGER III. JWST/NIRCam observations of the ultra-luminous high-redshift quasar J0100+2802”, led by Dr. Anna-Christina Eilers, were published in the Astrophysical Journal on June 12th, 2023. DOI: 10.3847/1538-4357/acc846, 10.3847/1538-4357/acd776, respectively.


More information on these papers can be found on the website of the Space Telescope Science Institute:


In other news, on May 10th, 2023, Dr. Kashino reported in Nature on the detection of previously unknown galaxies through radio emission around gas clouds containing highly ionized metal ions. This discovery helps understand cosmic reionization and the enrichment of heavy elements in intergalactic gas in the early Universe. DOI: 10.1038/s41586-023-05901-3



Media Contact:

Azusa Minamizaki, Katinka Wondergem

International Communications Office, Nagoya University


Related information:

Interview article by the Institute for Advanced Research, Nagoya University (Japanese only)
Interview article by the Academic Research & Industry-Academia-Government Collaboration, Nagoya University (Japanese only)