(ORDO NEWS) — Intermediate-mass black holes are notoriously hard to find, but a new study shows they could exist at the center of dense star clusters located throughout the universe.
A study published today in the Astrophysical Journal sheds new light on when and where black holes of about 100 to 100,000 solar masses may have formed and how they came about.
“One of the biggest open questions in black hole astrophysics right now is how black holes form that are between the sizes of a stellar-mass black hole and a supermassive black hole,” said Vivien Baldassare, lead author of the study and assistant professor of physics and astronomy at the State University.
Washington. “Most of the theories for their formation rely on conditions that only occur in the very early universe. We wanted to test another theory that they can form throughout cosmic time in these really dense star clusters.”
For decades, astronomers have been discovering small black holes as massive as either a few suns or giant black holes with the mass of millions of suns, but the missing black hole link between these sizes has eluded detection.
The existence of these intermediate or massive black holes has long been theorized, but finding them has proven difficult because the light emitted by objects falling into them is not easy to detect.
To solve this problem, the research team used the Chandra X-ray Observatory, the world’s most powerful X-ray telescope, to look for X-ray signatures of black holes in star clusters in 108 different galaxies.
Chandra is practically the only instrument in the world capable of doing this kind of work,” said Baldassare. “It is able to very accurately locate X-ray sources, which is very important when looking for signs of black holes in these very compact nuclear star clusters.
” clusters are at the center of most small or low-mass galaxies and are the densest known stellar environment.Previous studies have identified the presence of black holes in nuclear star clusters, but little is known about the specific properties that make these regions favorable for black hole formation.
An analysis by Baldassare and colleagues found that nuclear star clusters above a certain mass and density threshold emit X-ray signatures indicative of a black hole twice as often as clusters below that threshold.
Their work represents the first observational evidence supporting the theory that intermediate-sized black holes can form in nuclear star clusters.
“Essentially, this means that sufficiently massive and compact star clusters should be able to form black holes,” Baldassare said.
“This is interesting because we expect many of these black holes to be in an intermediate mass regime between supermassive black holes and stellar-mass black holes, where there is very little evidence for their existence.”
The research team’s work not only suggests that intermediate-sized black holes can form in nuclear star clusters, but also suggests a mechanism by which they could potentially form throughout cosmic time, not just in the first few billion years of the universe’s existence.
“One of the prevailing theories is that massive black holes could only form in the early universe, when everything was denser,” Baldassare said.
“Our study is more in line with the picture that massive black holes do not necessarily have to form in the very early universe, but rather may continue to form throughout cosmic time under these specific conditions.”
Going forward, the researchers plan to continue using Chandra to collect X-ray measurements of nuclear star clusters, with the ultimate goal of learning more about the specific conditions under which massive black holes can form.
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