To understand how stars and galaxies formed shortly after the birth of the Universe approximately 13.8 billion years ago, it is crucial to observe distant galaxies. For instance, light and radio waves emitted by celestial bodies located 13 billion light-years away take 13 billion years to reach Earth, allowing us to observe the appearance of those celestial bodies from 13 billion years ago.

In 2012, the research team embarked on the exploration of ultra-distant galaxies using the Atacama Large Millimeter/submillimeter Array (ALMA). Subsequently, in 2016, they detected radio waves emitted by oxygen, setting a world record for the farthest oxygen detection at that time. They continued to break records and successfully identified the farthest galaxy ever known by detecting radio waves emitted by oxygen 13.28 billion light-years away in 2018. Furthermore, in 2019, the team successfully detected radio waves emitted by both oxygen and dust from another galaxy called MACS0416_Y1, located 13.2 billion light-years away. Dust originates from the remnants scattered around when stars reach the end of their lives. The existence of dust in the early Universe, where the cycle of the reincarnation of stars had not yet repeated extensively, was a remarkable discovery.

Clouds composed of cold dust and gas appear dark because the dust obscures the light of stars, and they are called dark nebulae. Dark nebulae, which are accumulations of stellar remnants, are known as the birthplaces of new stars. Therefore, detailed observations of the interior of dark nebulae are crucial in understanding how stars are born, die, and give rise to the birth of the next generation of stars within galaxies. When hot, massive stars are born within dark nebulae, they ionize the surrounding gas by stripping electrons due to intense ultraviolet light emitted by the hot stars. These ionized gas nebulae are called emission nebulae. The dust and oxygen found in the distant galaxy MACS0416_Y1 in 2019 are believed to be emitted from dark and emission nebulae, respectively. Therefore, detailed observations of the distribution of dust and oxygen provide clues about how stars are born within dark nebulae and how emission nebulae are formed.

The research team led by Yoichi Tamura, an astronomer at Nagoya University, attempted high-resolution observations of MACS0416_Y1. By configuring the ALMA antennas with a resolution equivalent to a 3.4 kilometer telescope and conducting a 28-hour-long exposure, they succeeded in obtaining observation images with unprecedentedly high resolution and sensitivity for the distant galaxy. As a result, they were able to distinguish the locations of radio waves emitted by dust and oxygen. The images obtained in 2019 lacked sufficient resolution, and dust and oxygen, namely dark and emission nebulae, appeared as a single entity. By examining the images obtained in this study, we can see that emission and dark nebulae are intricately intertwined, avoiding each other’s regions. This suggests a process where newly formed stars within dark nebulae ionize the surrounding gas, transforming it into emission nebulae, similar to how villages and fields spread through the valleys.

Figure: The ALMA image of the galaxy MACS0416_Y1 located 13.2 billion light-years away, harboring the farthest dark nebula ever seen. The image spans approximately 15,000 light-years on each side.
(Left) Radio images captured by ALMA depicting the dark nebula (emitting radio waves from dust, shown in red) and the emission nebula (emitting radio waves from oxygen, green), along with the image of stars captured by the Hubble Space Telescope (blue). Credit: ALMA (ESO/NAOJ/NRAO), Y. Tamura et al., NASA/ESA Hubble Space Telescope. Download only observational image
(Right) Image captured by ALMA, only showing the radio waves emitted by the dust within the dark nebula. A vertically elongated elliptical cavity, a candidate for a superbubble, is visible in the central region.” Credit: ALMA (ESO/NAOJ/NRAO), Y. Tamura et al.

Furthermore, the image of dark nebulae, represented in red, reveals the presence of a massive cavity spanning approximately 1,000 light-years at the center of the image. Previous studies revealed that MACS0416_Y1 has been producing stars at a rate approximately 100 times higher than the Milky Way Galaxy over the past several million years. These stars are born at about the same time and undergo successive supernova explosions, resulting in the creation of enormous “superbubbles.” The discovered cavity may indeed be such a superbubble. Eventually, this gigantic bubble is expected to burst, scattering the remnants of stars (various elements and dust) into the interstellar space within the galaxy and the vast outer space of the galaxy. These elements and dust, not only serve as materials for the next generation of stars and planets by being incorporated into dark nebulae, but are also thought to be the driving force behind the “cosmic ecocycle,” which transforms the chemical composition of galaxies and their surrounding environment.

Moreover, the research team was able to determine the motion of the gas that constitutes the nebula. The gas was found to be turbulent, with speeds reaching up to 200,000 kilometers per hour. “Under such turbulent conditions, it is suggested that stars may form as massive clusters,” said Yoichi Tamura at Nagoya University. These giant star clusters are a characteristic feature observed in the early stages of galaxy formation. “In the future, more detailed information can be obtained by conducting high-resolution observations of these star clusters themselves, using instruments such as the James Webb Space Telescope and the planned extremely large telescopes.”

The high-resolution and high-sensitivity observations were made possible by ALMA’s performance, enabling this breakthrough. Typically, increasing resolution within the same observation time sacrifices sensitivity. However, in the case of the target galaxy located 13.2 billion light-years away, which is extremely faint, high sensitivity was crucial. Takuya Hashimoto from the University of Tsukuba described the observation performance as follows: “It corresponds to capturing the extremely weak light emitted by two fireflies located 3 centimeters apart on the summit of Mount Fuji as seen from Tokyo, and being able to distinguish between those two fireflies. The significance of this study lies in bringing out the ultimate performance of ALMA, leading to an understanding of the formation of early galaxies, the life and death of stars, and the ecocycle of matter in the Universe.”

These observation results were published as Yoichi Tamura et al. “The 300 parsec resolution imaging of a z = 8.31 galaxy: Turbulent ionized gas and potential stellar feedback 600 million years after the Big Bang” in the Astrophysical Journal on July 13, 2023 (DOI: 10.3847/1538-4357/acd637).

This work is supported by a Grant-in-Aid from the Japan Society for the Promotion of Science (17H06130, 19H01931, 20H01951, 20H05861, 20K22358, 21H01128, 21H04496, 22J21948, 22H01258, 22H04939), NAOJ ALMA Scientific Research Grant (2018-09B, 2020-16B), and Leading Initiative for Excellent Young Researchers, MEXT, Japan (HJH02007).

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.