Black holes vary widely in size. Those known as stellar mass black holes arise when huge stars collapses, usually having a mass ranging from 5 to 100 solar masses, and they can be found dispersed across galaxies. On the contrary, supermassive black holes reside at the core of most galaxies, such as ours—the Milky Way—representing an entirely different scale.
Unlike stellar mass black holes, supermassive black holes have masses ranging from millions to billions of times that of our Sun. These colossal entities influence the development of galaxies and can drive quasar activity during active accretion phases. However, nestled between these extremes lies an even more mysterious classification: intermediate mass black holes (IMBHs). Weighing in at 100 to 100,000 solar masses, IMBHs remain far less understood.
Researchers have found it challenging to conclusively categorize this particular group, despite growing proof of its presence derived from peculiar star movements, gravitational wave readings, and extraordinarily bright x-ray emissions. Gaining insight into these intermediate-mass black holes might provide essential understanding regarding the progression of small black holes into massive ones and bridge a significant gap in our comprehension.
Present hypotheses propose that globular clusters might host intermediate-mass black holes due to their incredibly crowded stellar conditions. These intermediate-mass black holes could develop quickly from star collisions forming supermassive stars which then collapse, or gradually through sequential mergers of smaller stellar mass black holes. Observations made with the Hubble Space Telescope of cluster M15 indicated it possibly contains an intermediate-mass black hole ranging between 1,700 and 3,200 times the sun’s mass, inferred from velocity dispersion data.
Nevertheless, this concept is still debated since the readings were obtained from a point where numerous compact stars might have affected the outcomes without definitively pointing towards a black hole.
A new study conducted by Associate Professor Yang Huang from the University of the Chinese Academy of Sciences has uncovered a potential technique for identifying intermediate-mass black holes (IMBHs) through monitoring fast-moving stars expelled from globular clusters. This research involved examining orbit details of around 1,000 high-speed stars along with more than 100 globular clusters. As part of their findings, they detected that star J0731+3717 was thrown out of globular cluster M15 approximately 20 million years ago traveling at an astounding velocity of 550 kilometers per second.
The work is published in the journal National Science Review .
This high speed indicates that the star was likely propelled by the gravitational slingshot effect (known as the Hills mechanism), due to an encounter with an intermediate-mass black hole at the core of M15. The researchers propose that a close-binary system must have come within one astronomical unit of an intermediate-mass black hole weighing thousands of times more than our Sun.
It’s anticipated that the binary system should have been disrupted because of gravitational tidal forces, resulting in one black hole being captured while the other was expelled. These discoveries offer strong proof for these hard-to-find medium-sized black holes that fill the size gap between those formed from stars and extremely massive ones at galaxy centers.
More information: Yang Huang and colleagues reported about a high-speed star that was recently expelled from the globular cluster M15 due to an intermediate-mass black hole. National Science Review (2024). DOI: 10.1093/nsr/nwae347
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