A research team led by Professor Ji Myung-kook of Yonsei University's Department of Astronomy has successfully reconstructed the dark matter distribution of a massive galaxy cluster that existed approximately 10.5 billion years ago, during the period known as "cosmic noon," using data from the James Webb Space Telescope (JWST).
Cosmic noon refers to the period around 10 billion years ago when star formation and galaxy growth were most active in cosmic history. Just as the sun is at its highest point at noon, this was the peak era when cosmic evolution was most vigorous. The research team analyzed the massive galaxy cluster "XLSSC 122," which existed during this period.
The analysis confirmed that XLSSC 122 was a massive structure weighing approximately 160 trillion times the mass of the sun. The cluster showed mass highly concentrated at its center. This indicates that large-scale structures were already considerably mature when the universe was only about 3 billion years old, contrary to expectations. The finding raises the need to reexamine theories about the formation and evolution of early galaxy clusters.
The study also confirmed "separation between matter components" through multi-wavelength observations. The mass center, presumed to be dark matter, largely coincided with the hot gas center identified through X-ray observations and the position of the brightest galaxy. However, SZ observations tracking gas pressure showed the center offset by approximately 320,000 light-years.
This phenomenon, where dark matter and gas components show different centers, is a characteristic that can appear during galaxy cluster mergers. Based on this evidence, the researchers suggested that massive structures may have grown through active merger processes even 10.5 billion years ago, during the early stages of cosmic evolution.
The three-dimensional understanding of the galaxy cluster's structure was possible by combining weak gravitational lensing analysis with JWST's high-resolution, high-sensitivity data. The team captured large quantities of background galaxy signals that were difficult to obtain with the Hubble Space Telescope, analyzed the shapes of background galaxies subtly distorted by gravity, and reconstructed a precise dark matter map.
Another achievement of this research was confirming that even during cosmic noon, the distribution of "intracluster light" showed spatial similarity to mass distribution. Intracluster light refers to starlight that does not belong to any specific galaxy but drifts throughout the galaxy cluster. This suggests that intracluster light could serve as an important supplementary indicator when studying dark matter distribution in distant objects where gravitational lensing observations are difficult.
The research was conducted through collaboration between domestic and international researchers, including Dr. Kyle Finner of the California Institute of Technology (Caltech) and doctoral candidate Jack Scofield of Yonsei University, under Professor Ji's guidance. The team plans to systematically study the formation and evolution of large-scale structures in the early universe by combining observations from NASA's Nancy Grace Roman Space Telescope, scheduled for launch this year.