In a recent study published in The Astrophysical Journal, a team of scientists from the University of Tokyo, Peking University, and the University of Texas discovered that dark matter predominates in the halos of two supermassive black holes in galaxies located about 13 billion light years away.
Left: Gas distribution of ionized carbon (C+) on the halo scale of P009-10 as shown by the color image and black contours. Nuclear gas distribution, centered on the quasar (large black cross), is shown by the magenta contours. Right: Velocity field of the C+ emission from -200 (in blue; moving towards us) to +200 (in red; moving away from us) km/s indicating coherent rotation in a massive dark matter halo. Image Credit: Fei et al.
Their research sheds new light on how galaxies have changed over time and the connection between dark matter and supermassive black holes when the universe was still very young.
Astronomer Vera Rubin was the first to identify the role of dark matter in galaxies when she observed that the outer regions of nearby galaxies were rotating faster than anticipated in the 1970s, creating what was subsequently referred to as a flat rotation curve.
The periphery of a galaxy would move more slowly than the peak velocity near the galaxy's center if galaxies were composed solely of stars and gas and followed Newton's laws. Rubin's observations would only make sense if a substantial amount of invisible mass—later called dark matter—surrounded the galaxy like a halo, allowing stars and gas far from the galaxy's center to move at higher speeds.
Moreover, despite its fundamental importance to understanding the Universe, the formation of dark matter in the early Universe has never been directly observed and remains mysterious.
Vera Rubin provided the first evidence for dark matter using the rotation curves of nearby local galaxies. We are using the same technique but now in the early universe.
John Silverman, Professor, Kavli Institute for the Physics and Mathematics of the Universe
The researchers discovered the gas dynamics of two quasar host galaxies at redshift 6, thanks to information from the ionized carbon (C+) emission line and the Atacama Large Millimeter/submillimeter Array (ALMA).
They discovered that dark matter accounted for roughly 60% of each galaxy's total mass by examining their rotation curves.
Blue-shifted gas (moving in the direction of the researchers) and red-shifted gas (moving away) capture velocity changes with galaxy radius.
It is interesting to note that previous studies' rotation curves in the distant universe show a decline in the galaxy outskirts, indicating a low fraction of dark matter. However, the team's data reveals a flat rotation curve that resembles the massive disk galaxies near Earth, suggesting that more dark matter is needed to account for the high velocities.
The team's findings clarify the complex connection between dark matter and supermassive black holes. They provide an essential component in comprehending the evolution of galaxies from the early universe to the structures seen today.
Journal Reference:
Fei, Q., et al. (2025) Assessing the dark matter content of two quasar host galaxies at z~6 through gas kinematics. The Astrophysical Journal. doi.org/10.3847/1538-4357/ada145.