Intermediate-mass black holes are notoriously hard to find but a new study indicates there may be some at the centre of dense star clusters located throughout the universe. For decades, astronomers have detected smaller black holes equal in mass either to a few suns or giant black holes with mass similar to millions of suns but the missing-link of black holes in between those sizes have eluded discovery. The existence of these intermediate-sized or massive black holes has long been theorised but finding them has proven difficult as the light emitted by objects falling into them is not easy to detect. To address this challenge, the research team at the Washington State University in the US, used the Chandra X-Ray Observatory - the world's most powerful X-ray telescope - to look for X-ray signatures of black holes in nuclear star clusters in 108 different galaxies.
"One of the biggest open questions in black hole astrophysics right now is how do black holes form that are between the size of a stellar mass black hole and a supermassive black hole," said lead author Vivienne Baldassare, Assistant Professor of physics and astronomy at Washington State University. "Most of the theories for their formation rely on conditions that are found only in the very early universe. We wanted to test another theory that says they can form throughout cosmic time in these really dense star clusters," Baldassare added. The study, published in the Astrophysical Journal, sheds new light on when and where black holes of about 100-100,000 solar masses could form and how they came into being. Nuclear star clusters are found at the centre of most small or low-mass galaxies and are the densest known stellar environments.
Previous research has identified the presence of black holes in nuclear star clusters but little is known about the specific properties that make these regions conducive for the formation of black holes. Baldassare and colleagues' analysis showed that nuclear star clusters that were above a certain mass and density threshold emitted the X-ray signatures indicative of a black hole at twice the rate of those below the threshold. The study provides the first observational evidence supporting the theory that intermediate-sized black holes can form in nuclear star clusters. "Basically, it means that star clusters that are sufficiently massive and compact should be able to form a blackhole," Baldassare said. "It is exciting because we expect many of these black holes to be in the intermediate mass regime between supermassive black holes and stellar mass black holes where there is very little evidence for their existence." The findings not only suggest that intermediate-sized black holes can form in nuclear star clusters but also provides a mechanism by which they could potentially form throughout cosmic time rather than just during the first few billion years of the universe.