Astronomers have solid evidence for the existence of stellar-mass black holes and supermassive black holes. However, evidence for Intermediate Black Holes (IMBHs) is more elusive. Their existence remains hypothetical.

However, study by study, evidence is accumulating for IMBHs. The latest comes from the globular cluster M15, where a fast-moving star suggests the presence of something massive. Could it be an elusive IMBH?

IMBHs bridge the gap between stellar-mass black holes, which have up to about 100 solar masses, and supermassive black holes (SMBHs), which have millions to billions of solar masses. Though their existence still isn’t proven, many astronomers expect they’ll be found one day. Scientists think they can form in three different ways: the merger of multiple stellar-mass black holes, the direct collapse of huge gas clouds in the early Universe, or through collisions in dense stellar environments.

Globular clusters are prime locations where IMBHs could form because the stellar density is so high in their cores. In Omega Centauri, the largest Globular Cluster (GC) in the Milky Way, estimates show there may be several thousand stars per cubic parsec, an incredible density of stars. In our solar neighbourhood, the stellar density is only 0.004 stars per cubic parsec.

Several studies pointed to the existence of an IMBH in Omega Centauri, and in the summer of 2024, astronomers found more evidence with the Hubble Space Telescope.

New research shows that M15, another of the Milky Way’s GCs, may also host an IMBH. It’s based on observations of a runaway high-velocity star.

The research, titled “A high-velocity star recently ejected by an intermediate-mass black hole in M15,” has been accepted for publication in the National Science Review. Yang Huang, from the School of Astronomy and Space Science, University of Chinese Academy of Sciences, is the lead author.

“The existence of intermediate-mass black holes (IMBHs) is crucial for understanding various astrophysical phenomena, yet their existence remains elusive, except for the LIGO-Virgo detection,” the authors write. They’re referring to GW190521, the most massive gravitational wave binary observed. It was in 2020 and created a black hole remnant of 142 solar masses. Some call this the first detection of an IMBH.

“We report the discovery of a high-velocity star J0731+3717, whose backward trajectory about 21 Myr ago intersects that of globular cluster M15 within the cluster tidal radius,” the researchers write. They hypothesize that gravitational interactions with an IMBH in M15 are responsible for the star’s ejection.

The cluster tidal radius is the distance from the center of a GC where the GC’s gravitational influence gives way to that of the surrounding galaxy. This is strong evidence that the star may have originated in M15. However, it’s not the researchers’ only evidence; the star’s metallicity also suggests it came from M15. “Both its metallicity [Fe/H] and its alpha-to-iron abundance ratio [?/Fe] are consistent with those of M15,” Huang and his colleagues write.

The researchers compared the metallicity and alpha-to-iron abundance of M15 and the runaway star with stars from APOGEE (Apache Point Observatory Galactic Evolution Experiment). APOGEE is a large-scale spectroscopic survey of stars in the Milky Way. Its data are used to measure stellar populations, star chemistry, and the history of star formation in our galaxy, so it makes a useful comparison for M15 and the runaway star.

“It is extremely unlikely for the association of J0731+3717 and M15 to be by pure chance, given the probability for random association, chemical and age similarities,” the authors explain.

This is good evidence that the runaway star originated in M15. However, metallicity can’t tell us whether an IMBH is responsible for ejecting the star. For that, the researchers looked at the star’s speed and trajectory. They started by examining known hypervelocity stars in the Milky Way.

“To discover high-velocity stars ejected from globular clusters, backward orbital integrations are carried out for 934 high-velocity (VGSR ? 400 km s^?1) halo stars in the searching volume within 5 kpc from the Sun and 145 Galactic globular clusters,” the authors explain. They traced the backward trajectories of the stars and the clusters to 250 million years ago. Then, they calculated the closest distance for each pair of stars and GCs.

“Amongst the hundred thousand pairs, only J0731+3717 has the closest distance smaller than the tidal radius of M15, making it a rare candidate of cluster ejected high-velocity star,” the authors write.

There are other ways that a GC can eject a star. Interactions with other stars, a supernova explosion, interactions with a massive gas cloud, or even passing too close to the Milky Way’s galactic disk are all potential causes. But none of those fit, according to the researchers. “In summary, the above alternative ejection mechanisms are not viable to kick-off J0731+3717 from M15,” the authors conclude.

By rejecting alternative explanations for the star’s ejection, they were left with an IMBH as the only viable cause.

Like other GCs, M15 has an extremely high stellar density in its core, one of the highest of any known GCs. Astronomers think that M15 underwent a process called core collapse, which created its extremely compact center packed full of stars. M15’s core is about one million times more dense than our stellar neighbourhood. It’s so dense that even our most powerful telescopes struggle to resolve individual stars. In this crowded environment, stars are expected to collide and merge frequently, and interactions between all types of stellar objects are more likely. That makes it a prime area for the mergers of stellar-mass black holes into IMBHs.

The evidence for IMBHs is mounting, but there’s still no widespread agreement that their existence is proven. However, we may not have to wait long for the scientific community to gather enough convincing evidence. “With the increasing power of ongoing Gaia and large-scale spectroscopic surveys, we expect to discover dozens of cases within the 5 kpc volume and ten times more within a 10 kpc volume, which should shed light on the understanding of the evolutionary path from stellar-mass BHs to SMBHs,” the authors explain.

It’s even possible that the Event Horizon Telescope or something similar that succeeds it will be able to image an IMBH. A lot would have to go right for that to happen, but it’s a possibility.

For now, we can watch as researchers gather incremental evidence of IMBH’s existence and watch as the story unfolds.