Research team finds indirect evidence for existence of dark matter surrounding black holes

It is very challenging to identify dark matter because it does not produce or reflect light or interact with electromagnetic forces. However, a study team from The Education University of Hong Kong (EdUHK) has demonstrated that black holes are surrounded by a significant quantity of dark matter. The Astrophysical Journal Letters publishes the study's findings.

The crew chose two neighboring binary systems (A0620-00 and XTE J1118+480) black holes as their study subjects. In other words, a partner star orbits each black hole. Based on the partner stars' paths, measurements show that their orbital decay rates are about one millisecond (1ms) per year, which is about 50 times higher than the theoretical estimate of 0.02ms per year.

The EdUHK team used computer models to apply the "dark matter dynamical friction model"—a widely accepted academic theory—to the two selected binary systems in order to determine whether dark matter is present around black holes. The group discovered that the measured data perfectly fits the companion stars' rapid orbital decay.

Notably, this provides indirect proof that the partner stars' orbital speeds can be slowed down by substantial dynamical friction produced by dark matter around black holes.

The results reflect an advance in the study of dark matter because they supported a theoretical theory put forth in the late 20th century. The idea is that dark matter would be consumed if it were near enough to black holes, with the leftovers being dispersed. A "density spike" eventually forms around the black holes as a result of the process.

According to Dr. Chan Man-ho, principal investigator and associate professor in the department of science and environmental studies, such a high abundance of dark matter would cause the partner star to experience dynamical friction in a manner akin to drag force.

According to him, this is the first research to use the "dynamical friction model" in an attempt to support and establish the reality of dark matter surrounding black holes. "The study offers a crucial new tack for ongoing dark matter research,"

Dr. Chan also pointed out that earlier investigations into the existence of dark matter, which primarily relied on gamma ray and gravitational wave detection, counted on the incidence of extremely unlikely occurrences like the merger of two black holes. He said that might mean scientists would have to wait a while.

But these restrictions will no longer apply to the innovative strategy used by the EdUHK squad. "There are at least 18 binary systems similar to our research subjects in the Milky Way Galaxy alone, which can provide rich information to help unravel the mystery of dark matter," he continued.

Journal information: Astrophysical Journal Letters