RGC-funded SES research proves existence of dark matter surrounding black holes
A research team from SES has proven that there is a substantial amount of dark matter surrounding black holes.
The research paper co-written by Dr Chan Man-ho and Mr Lee Chak-man, both from SES, was published in Volume 943, Number 2 of The Astrophysical Journal Letters in January 2023.
A recent study conducted by a research team from the Department of Science and Environmental Studies (SES) has proven that there is a substantial amount of dark matter surrounding black holes. The study was supported by the Research Grants Council, with the related paper recently published in the peer-reviewed scientific journal The Astrophysical Journal Letters.
The team selected two nearby black holes (A0620-00 and XTE J1118+480) as research subjects. Both are considered binary systems, which means that each black hole has a companion star orbiting it. Based on the orbits of the companion stars, observations indicate that their rates of orbital decay are approximately one millisecond (1ms) per year, which is about 50 times greater than the theoretical estimation of about 0.02ms annually.
Dark matter, as its name suggests, does not emit or reflect light, nor does it interact with electromagnetic forces, making it exceptionally difficult to detect. To examine whether dark matter exists around black holes, the SES team applied the “dark matter dynamical friction model” – a theory widely held in academia – to the two chosen binary systems, through computer simulations. The team found that the companion stars’ fast rate of orbital decay precisely matches the data observed.
Notably, this is indirect evidence that dark matter around black holes can generate significant dynamical friction, slowing down the companion stars’ orbital speed.
The findings, which verified a theoretical hypothesis formulated in the late 20th century, represent a breakthrough in dark matter research. According to the hypothesis, dark matter close enough to black holes gets swallowed, leaving the remnants to be redistributed. The process ends up forming a “density spike” around the black holes.
Dr Chan Man-ho is the principal investigator of the research project.
“Such a high density of dark matter would create dynamical friction to the companion star, in a way similar to drag force,” Dr Chan Man-ho, Associate Professor from SES and principal investigator of the research, explained. Dr Lee Chak-man, a senior research assistant at SES, assisted Dr Chan’s research project.
This is the first-ever study to apply the ‘dynamical friction model’ in an effort to validate and prove the existence of dark matter surrounding black holes.
“This is the first-ever study to apply the ‘dynamical friction model’ in an effort to validate and prove the existence of dark matter surrounding black holes,” Dr Chan said. “The study provides an important new direction for future dark matter research.”
The SES scholar further mentioned that previous studies have relied mostly on gamma rays and gravitational wave detection to examine the presence of dark matter, and have depended on the occurrence of rare events, such as two black holes merging. According to him, the dependence on rare events can result in prolonged waiting times for astronomers.
The novel approach adopted by the EdUHK team, however, is not confined by such limitations. Dr Chan added, “In the Milky Way Galaxy alone, there are at least 18 binary systems akin to our research subjects, which can provide rich information to help unravel the mystery of dark matter.”
The original research paper entitled “Indirect Evidence for Dark Matter Density Spikes around Stellar-mass Black Holes”, written by the SES researchers, was published in Volume 943, Number 2 of Astrophysical Journal Letters in January 2023.
