An international team of researchers has found dark matter dominating the halos of two supermassive black holes in galaxies roughly 13 billion light years away, reports a new study published in The Astrophysical Journal on February 5.

Their study gives new insight into the relationship between dark matter and supermassive black holes when the universe was still very young, and how galaxies have evolved until today.

The first person to discover that dark matter played an important role in galaxies was astronomer Vera Rubin, who in the 1970s, noticed that the outer parts of local galaxies were rotating at higher speeds than expected, forming what was later named a flat rotation curve. If galaxies were only made up of stars and gas, and obeyed Newton's laws, the outskirts of a galaxy would move slower than the peak velocity closer to the galaxy’s center. Rubin’s observations could only make sense if there were a large amount of invisible mass, later called dark matter, that surrounded the galaxy like a halo, allowing stars and gas anywhere far from the galaxy center to move at higher velocity. Furthermore, dark matter assembly so far back in the universe has never been observationally constrained and remains unknown, despite its fundamental importance to our understanding of the Universe.

A research team led by The University of Tokyo Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI) visiting researcher and Peking University graduate student Qinyue Fei, and including Kavli IPMU Professor John Silverman, the University of Texas Austin’s Dr. Seiji Fujimoto, and Peking University Kavli Institute for Astronomy and Astrophysics Associate Professor Ran Wang, have studied the dark matter content of supermassive black holes about 13 billion light years away.

“Vera Rubin provided the first evidence for dark matter using the rotation curves of nearby local galaxies. We’re using the same technique but now in the early universe,” said Silverman.

Made possible using data from the Atacama Large Millimeter/submillimeter Array (ALMA) and the ionized carbon (C+) emission line, the researchers were able to uncover the gas dynamics of two quasar host galaxies at redshift 6. By studying the rotation curves of each galaxy (Figure 2), they found dark matter made up about 60 per cent of its total mass.

Velocity changes with radius in the galaxy are captured by blue-shifted gas (moving towards the researchers), and the red-shifted gas (moving away) (Figure 1).

Interestingly, the rotation curves in the distant universe from past studies reveal a decrease in the galaxy outskirts, meaning a low fraction of dark matter. But the data taken by Fei and Silverman’s team shows a flat rotation curve, similar to the massive disk galaxies close to Earth, which indicates more dark matter is required to explain the high velocities (Figure 2).

The team's findings shed light on the intricate relationship between dark matter and supermassive black holes. They offer a crucial piece of the puzzle in understanding how galaxies evolved from the early universe to the structures we observe today.