2026.01.05
Giant Spiral Galaxy” Discovered 11.5 Billion Years Ago
(This article is based on the press release from Nagoya University on March 28, 2025.)
In the early Universe, some galaxies formed stars at an explosive rate—hundreds to thousands of times faster than the Milky Way. Such starburst galaxies are called “monster galaxies.” In these galaxies, large amounts of dust created by intense star formation obscure starlight, making it difficult to understand their structure or the mechanisms driving such vigorous star formation.
An international research team led by Hideki Umehata, a YLC-designated assistant professor at Nagoya University, and including researchers from NAOJ’s ALMA Project, conducted high-resolution observations of a monster galaxy, ADF22.A1, located in a proto-cluster 11.5 billion years ago. These observations used the James Webb Space Telescope (JWST) and ALMA.
JWST captures emissions from stars in near- to mid-infrared wavelengths, which are relatively less affected by extinction caused by cosmic dust. ALMA, on the other hand, detects millimeter and submillimeter radio waves emitted by warm dust that absorbs ultraviolet light from young stars, revealing star formation activity hidden by dust. ALMA also investigates the distribution and motion of cold atomic and molecular gas. Together, these observations revealed the true nature of a monster galaxy previously concealed by dust, in terms of both its stellar and interstellar components.
As shown in Figure 1, JWST uncovered a spiral distribution of stars embedded in dust. ALMA revealed that gas in the galaxy forms a disk rotating at an exceptionally high speed of about 530 km per second (Figure 2). Combining these results, ADF22.A1 has been identified for the first time as a huge, rapidly rotating spiral galaxy, nearly twice the size of typical galaxies of that era.
The galaxy’s large angular momentum —a measure of rotational inertia—differs significantly from that of ordinary galaxies in the early Universe. The research team previously discovered massive filaments of gas connected to the “Cosmic Web” in the early Universe (Press Release, October 4, 2019). The inflow of gas from the Cosmic Web is thought to impart this large angular momentum to ADF22.A1 as it grows. This infalling gas, which also serves as raw material for star formation, likely drives its intense star-forming activity.
The galaxy’s large angular momentum —a measure of rotational inertia—differs significantly from that of ordinary galaxies in the early Universe. The research team previously discovered massive filaments of gas connected to the “Cosmic Web” in the early Universe (Press Release, October 4, 2019). The inflow of gas from the Cosmic Web is thought to impart this large angular momentum to ADF22.A1 as it grows. This infalling gas, which also serves as raw material for star formation, likely drives its intense star-forming activity.
Figure 1: Distributions of stars and dust in the monster galaxy ADF22.A1. (Left) An infrared image captured by JWST shows the distribution of stars embedded in dust. (Right) Dust distribution revealed by ALMA observations. The hidden structure of a large spiral galaxy emerges from the massive dust. (Credit: Nagoya University)
Figure 2: ALMA observations using the ionized carbon emission line show the distribution (left) and velocity field (right) of gas in ADF22.A1. In the right panel, red indicates gas moving away from us (redshift), while blue indicates gas moving toward us. These data reveal that the entire gas disk is rotating at an exceptionally high speed. (Credit: Nagoya University)
These observation results were published as Hideki Umehata et al “ADF22-WEB: A giant barred spiral starburst galaxy in the z = 3.1 SSA22 proto-cluster core” in the Publications of the Astronomical Society of Japan on March 17, 2025.
DOI: 10.1093/pasj/psaf010
This research was supported by NAOJ ALMA Scientific Research Grant (Number 2024-26A) and JSPS KAKENHI (Numbers JP 20H01953, JP 22KK0231, JP 23K20240, JP 22H0493, JP 23K20035, JP 24HH00004, JP 23K20870)
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organization 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.
