An international team of astronomers and physicists has made a groundbreaking discovery about the role of dark matter in the evolution of galaxies. Their research, published in The Astrophysical Journal, reveals that dark matter dominates the halos of supermassive black holes in galaxies that are approximately 13 billion light years away. This study provides crucial new insights into how dark matter and supermassive black holes interacted in the early universe and how these cosmic elements played a significant role in the development of galaxies over cosmic time.
The findings are a significant advancement in our understanding of the universe’s formative years, when galaxies were still in their infancy. The researchers, led by Qinyue Fei, a graduate student from Peking University, and Professor John Silverman from The University of Tokyo’s Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), have uncovered key details about the dark matter content in galaxies during a critical time, around redshift 6. This period corresponds to a time when the universe was only about 900 million years old, or roughly 7% of its current age.
The Discovery of Dark Matter’s Role in Galaxies
The study of dark matter’s relationship with galaxies goes back decades, but the first major breakthrough came in the 1970s with astronomer Vera Rubin. Rubin’s work on the rotation curves of local galaxies marked the beginning of our understanding of dark matter’s importance in galactic formation. She noticed that the outer parts of galaxies, especially their spiral arms, were rotating much faster than expected, contrary to what Newtonian physics would predict. According to the standard laws of motion, the outer regions of a galaxy should rotate more slowly compared to the inner regions, where most of the visible mass is concentrated.
However, Rubin’s observations showed that galaxies were behaving differently. Instead of slowing down at the outskirts, the outer stars were moving at the same high velocity as those near the center. This anomaly could only be explained if an unseen form of mass was present, one that didn’t emit light or energy, but still exerted a gravitational pull. This invisible mass was later identified as dark matter.
Rubin’s discovery led to a paradigm shift in astronomy, with dark matter now understood to make up a significant portion of a galaxy’s mass. She also proposed that galaxies were surrounded by a halo of dark matter that extended far beyond the reach of the visible stars and gas. These halos were responsible for the gravitational forces that kept galaxies from flying apart as they rotated. The idea of dark matter—once considered a speculative hypothesis—was now firmly rooted in our understanding of the cosmos.
The New Study and its Implications
Despite its foundational role in modern cosmology, the early evolution of dark matter—especially in distant galaxies—has remained largely uncharted. The research team, utilizing cutting-edge technology and methods, was able to investigate the dark matter content of supermassive black holes in early-universe galaxies. Supermassive black holes are thought to reside at the centers of most galaxies, and understanding their relationship with dark matter can offer insights into both galaxy formation and the growth of these black holes.
The team focused on two quasar host galaxies at redshift 6, meaning that these galaxies are 13 billion light-years away—essentially providing a glimpse into the universe when it was still in its infancy. These distant galaxies, filled with quasars—ultra-bright centers of galaxies powered by supermassive black holes—offer a unique opportunity to study the interplay between dark matter and supermassive black holes in the early universe.
By using data from the Atacama Large Millimeter/submillimeter Array (ALMA), the researchers observed the ionized carbon (C+) emission line. This emission line allowed them to probe the gas dynamics within these galaxies and assess the amount of dark matter present. They measured the rotation curves of the galaxies, which reflect how the velocity of the gas changes at different distances from the galaxy’s center. Rotation curves are a crucial tool for determining the distribution of both visible and invisible mass in galaxies.
Flat Rotation Curves and the Role of Dark Matter
The results of the study revealed that dark matter made up approximately 60% of the total mass in these distant galaxies. This finding was significant because it showed that even in the early universe, dark matter played a dominant role in shaping the structure of galaxies and their rotational behavior. The researchers found flat rotation curves, a characteristic typically seen in nearby galaxies, especially those with large disk structures. This was in stark contrast to earlier studies of rotation curves in the distant universe, which showed a decline in the velocities at the outskirts of galaxies, suggesting a low fraction of dark matter.
The flat rotation curve observed in this study implies that these early galaxies required a significant amount of dark matter to explain the high velocities of the gas, much like the spiral galaxies we observe today. This is an important discovery because it suggests that, even in the early stages of galaxy formation, dark matter was already a major component of galaxy mass. It also indicates that the process by which galaxies form and accumulate dark matter has been ongoing since the universe’s earliest times.
The relationship between dark matter and supermassive black holes is also central to this study. The findings suggest that dark matter may have played an integral role in the formation and growth of black holes in the early universe. Since supermassive black holes are thought to be closely connected to their host galaxies, understanding the distribution of dark matter can provide crucial insights into how these enigmatic objects grew to their massive sizes.
A Closer Look at the Methodology
The research team used an advanced technique known as “kinematic modeling” to analyze the gas velocity within the two distant quasar host galaxies. This technique involves studying the way gas moves within the galaxies, looking for blue- and red-shifted gas. Gas that is moving toward the observer will appear blue-shifted (its light is compressed), while gas moving away will appear red-shifted (its light is stretched). By mapping the velocity of gas across the galaxy, the team was able to calculate the amount of mass required to explain the observed motion.
The team’s analysis revealed that the observed galaxies had flat rotation curves, similar to those found in nearby spiral galaxies, which typically feature large dark matter halos. This new evidence suggests that dark matter has been an essential part of galaxies since their formation, providing the gravitational framework necessary for their structure and the growth of supermassive black holes.
What This Means for Understanding Galaxy Evolution
The research has significant implications for our understanding of galaxy evolution. It suggests that dark matter, far from being an incidental component, was an essential driver of galaxy formation from the very beginning of the universe. By studying the relationship between dark matter and supermassive black holes in these ancient galaxies, the team has provided a new lens through which we can understand the process of galaxy formation.
The findings also challenge earlier assumptions about the relationship between dark matter and galaxies in the distant universe. For many years, scientists believed that the outskirts of early galaxies would show little to no dark matter. However, this study provides strong evidence that dark matter was present in significant quantities in the early universe, likely contributing to the formation of both galaxies and the supermassive black holes at their centers.
Looking Ahead: Dark Matter in the Early Universe
The study marks a major milestone in the exploration of dark matter, especially as it relates to the early universe. Despite being one of the most pervasive substances in the universe, dark matter remains mysterious, as it has yet to be directly observed. By studying its influence on distant galaxies and black holes, researchers are slowly piecing together the role it played in the universe’s evolution.
The findings also open the door for further studies of galaxies and black holes at even higher redshifts, potentially providing even deeper insights into the early universe and the origins of dark matter. As technology advances and more powerful telescopes come online, such as the James Webb Space Telescope (JWST), astronomers will be able to probe the distant past of the universe in even greater detail, potentially uncovering more secrets about the mysterious relationship between dark matter, galaxies, and supermassive black holes.
Conclusion
The study of dark matter’s influence on supermassive black holes in galaxies 13 billion light-years away is a groundbreaking achievement in astrophysics. By revealing that dark matter made up around 60% of the mass in these early galaxies, the researchers have provided new insights into the evolution of galaxies and black holes. This discovery not only sheds light on the role of dark matter in the cosmos but also marks an important step toward understanding the broader processes that have shaped the universe since its earliest days.
Reference: Qinyue Fei et al, Assessing the Dark Matter Content of Two Quasar Host Galaxies at z ∼ 6 through Gas Kinematics, The Astrophysical Journal (2025). DOI: 10.3847/1538-4357/ada145