China Focus: Chinese scientists find evidence for existence of intermediate-mass black holes

BEIJING -- Chinese astronomers have identified a runaway star that was ejected from the M15 globular cluster, providing strong evidence for the existence of intermediate-mass black holes (IMBHs), a long-missing link in our understanding of black hole evolution.

The discovery was led by researchers from the Chinese Academy of Sciences' National Astronomical Observatories (NAOC) in collaboration with multiple institutions, and was published as a cover story in the National Science Review journal.

Black holes with masses between those of stellar-mass black holes, which are formed from the collapse of massive stars, and those of supermassive black holes, which reside at the centers of large galaxies, are known as IMBHs, said Huang Yang, an associate professor at NAOC and the University of the Chinese Academy of Sciences who co-led the study.

IMBHs are often considered a crucial missing link in our understanding of the evolution of seed black holes into supermassive black holes. To date, only a few controversial candidates have been found, leaving the existence of IMBHs an open question in astrophysics.

The high-velocity runaway star, J0731+3717, was ejected from M15 approximately 20 million years ago at a staggering speed of nearly 550 kilometers per second, according to the study.

An analysis of data from the European Space Agency's Gaia spacecraft and China's Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), as well as data from other large-scale spectroscopic surveys, has also revealed that the star has a near-identical chemical composition and age to M15, said Liu Jifeng, director of NAOC and co-lead author of the study.

Prior observations hinted that M15's center might host an IMBH weighing 1,700 to 3,200 solar masses. However, uncertainties persisted as the gravitational signals observed could also stem from dense clusters of neutron stars.

The extreme velocity of J0731+3717 resolves this debate.

"Such a high-speed ejection requires a tight binary star to pass within 1 astronomical unit of an IMBH," Huang said. "The black hole's tidal forces would rip the binary apart, capturing one star and hurling the other outward."

This phenomenon, known as the Hills mechanism, confines thousands of solar masses within a region just a few astronomical units wide, ruling out alternative explanations such as the presence of clustered compact stars.

"With the continuous accumulation of data from Gaia and large-scale spectroscopic surveys such as those of LAMOST, we expect to discover several more stars like J0731+3717 in the near future, which will greatly advance our understanding of the elusive IMBHs," said Zhang Huawei, director of the Department of Astronomy at Peking University's School of Physics and co-lead author of the study.