Star cluster 13.3 billion light years away discovered by the James Webb Space Telescope: The most distant galaxy ever seen provides new perspectives on the evolution of galaxies

An international research team consisting of Professor Akio Inoue of the Faculty of Science and Engineering at Waseda University; Professor Masamune Oguri of the Center for Frontier Sciences at Chiba University; Professor Yoichi Tamura of the Graduate School of Science at Nagoya University; Assistant Professor Takuya Hashimoto of the Institute of Pure and Applied Sciences at the University of Tsukuba; and Dr. Angela Adamo of Stockholm University (Sweden), as well as the Space Telescope Science Institute (USA), conducted research using the James Webb Space Telescope. The team announced the discovery of a cluster of stars in a galaxy 13.3 billion light years away, which is the most distant cluster ever found, with a cosmic age of 460 million years. It is expected to lead to shed light on the mechanism behind the birth of globular clusters and provide a new perspective on the formation of massive stars and black hole seeds, which are important for the evolution of galaxies. The results were published in the international journal Nature on June 24.

In the Milky Way galaxy, to which Earth belongs, there are star clusters called globular clusters that have been maintained in groups by their own gravity for billions of years. While globular clusters are thought to be objects that originated in the early universe, it is not known when or where they formed.

The James Webb Space Telescope is a huge space telescope with a primary mirror aperture of 6.5 meters launched by NASA (US) in 2021. It started full-scale operation in 2022, enabling the observation of distant space with a sensitivity higher than conventional ones. In this observation, five very compact star clusters were discovered in SPT0615-JD1 (also known as Cosmic James Ark), a galaxy with a cosmic age of about 460 million years. The observation was made possible by the gravitational lensing effect of a foreground galaxy cluster, SPT-CL J0615-5746, which magnified the long side of the image by approximately one hundred times. The gravitational lensing effect is a phenomenon wherein the substantial gravity of a cluster of galaxies bends light rays from distant galaxies in the background, causing them to appear magnified and brighter or splitting them into multiple images. The discovered cluster is the most distant to date and has a much higher mass and stellar number density than those of the globular clusters in the Milky Way galaxy.

In the universe immediately after the Big Bang, the resulting gas was in an ionized state due to high temperatures; however, as the universe expanded, the temperature decreased, and the gas became temporarily neutralized. Subsequently, the formation of primary objects and galaxies emitted intense ultraviolet radiation, which is thought to have reionized the gas by around 1 billion years of cosmic age, a phenomenon known as cosmic reionization. The newly discovered star cluster may be the source of the ultraviolet radiation that caused the cosmic reionization.

Inoue said, "We plan to make detailed spectroscopic observations of this object using the James Webb Space Telescope. We would like to further explore the formation of star clusters and galaxies in the early universe."

Journal Information
Publication: Nature
Title: Bound star clusters observed in a lensed galaxy 460 Myr after the Big Bang
DOI: 10.1038/s41586-024-07703-7