Â鶹ÒùÔº

Black holes come in a range of sizes. Stellar mass black holes form from the collapse of massive stars, typically weighing between 5 and 100 times the mass of our sun, and are scattered throughout galaxies. At the other extreme are the supermassive black holes that lurk at the center of most galaxies, including our own Milky Way.

In stark contrast with stellar mass black holes, the masses of supermassive black holes range from millions to billions of times that of the sun. They shape galactic evolution and can power quasars when actively feeding, but between the two is another, rather more elusive category known as intermediate mass black holes (IMBHs), which weigh between 100 and 100,000 solar masses.

Scientists have struggled to definitively identify this latter category, although evidence of its existence is mounting from observations of unusual stellar motions, gravity wave detections and extremely luminous X-ray sources. Understanding these intermediate mass black holes (IMBHs) could reveal crucial insights about how stellar mass black holes grow into supermassive ones and fill an important gap in our knowledge.

Current theories suggest that globular clusters may harbor IMBHs thanks to their extremely dense stellar environments. These IMBHs could form either rapidly through stellar mergers creating massive stars that collapse, or slowly via successive mergers of stellar mass black holes. Hubble Space Telescope observations of M15 suggested it might contain an IMBH of 1,700–3,200 solar masses, based on velocity dispersion measurements.

However, this idea remains controversial because the measurements were taken at a distance where thousands of compact stars could influence the results without necessarily indicating a black hole.

A recent study led by Associate Professor Yang Huang from the University of the Chinese Academy of Sciences has identified a promising method for detecting IMBHs by tracking high-velocity stars ejected from globular clusters. The team analyzed orbital data from nearly 1,000 high-velocity stars and over 100 globular clusters, and as a result, discovered that star J0731+3717 had been ejected from globular cluster M15 about 20 million years ago at an extraordinary speed of 550 km/s.

The work is in the journal National Science Review.

This extreme velocity strongly suggests the star was accelerated by the gravitational slingshot effect (Hills mechanism) from an interaction with an IMBH at M15's center. The team suggests that a tight binary system must have passed within one astronomical unit of an IMBH with a mass of several thousand solar masses.

It's expected the binary pair would have been torn apart due to gravitational tidal interactions, capturing one while ejecting the other. The findings provide compelling evidence for these elusive mid-sized black holes that bridge the gap between stellar mass and supermassive varieties.