Globular Clusters Should Contain More Intermediate-mass Black Holes

We live in a Universe studded with black holes. Countless stellar mass and supermassive ones exist in our galaxy and most others. It’s likely they existed as so-called “primordial” black holes in the earliest epochs of cosmic history. Yet, there seems to be a missing link category: intermediate-mass black holes (IMBH). Astronomers have searched for these rare beasts for years and there’s only one possible observation thanks to gravitational-wave data. So, where are they?

IMBH might be hidden away in the hearts of globular clusters. But, given the tightly packed nature of those compact collections of stars, how would we know if they contained any IMBH? Teams of researchers in Japan and China came up with a couple of ways to search them out. One is to look for fast-moving stars ejected from globular clusters. The other is to do simulations of collisions of stars in the hearts of newly forming clusters. Both methods may point the way to more IMBH discoveries.

What Are Intermediate-mass Black Holes?

These rare objects are pretty much what their name says: black holes with masses somewhere between their stellar-mass cousins and the supermassive behemoths at the hearts of galaxies. They can contain as little as a thousand times the mass of the Sun, which would be fairly “small”, up to maybe a million solar masses. Beyond that are the supermassive monsters with millions or billions of times the mass of the Sun. The IMBH don’t come from supernova explosions, since there’s no massive star big enough to collapse to produce an IMBH. The birth of an IMBH should involve multiple massive objects coalescing together. This makes them more like their big supermassive black hole siblings.

So, where would such a collisional event happen? It would help if you had a dense agglomeration of stars tightly packed together. That describes globular clusters to a T. They’re crowded with stars, and likely have a good collection of very massive ones. Those are the stars that explode as supernovae and collapse down to produce a stellar-mass black hole. If enough of them exist in the cluster, they could merge and create an IMBH. Another suggestion to create an IMBH is for massive stars to collide to create a single more-massive object.

Many globular clusters orbit the core of the Milky Way Galaxy. Some of the densest ones have millions of stars pulled together by gravity. The cluster Messier 15 (M15) is a good example. It contains more than 100,000 stars crammed into an area of space about 175 light-years across. If runaway star collisions or stellar-mass black hole mergers occurred in M15, that could be enough to create an IMBH.

Simulating Globular Clusters and Intermediate-Mass Black Hole Growth

Another idea is to explore the formation of globulars to see if it produces any clues to the origins and existence of IMBH. That’s what a team of scientists at the University of Tokyo did. They created advanced simulations of star cluster formation to see if massive-star collisions could occur and lead to the birth of IMBH. It’s not an easy task. Previous simulations suggested stellar winds would blow away the needed masses to create these missing black holes.

“Star cluster formation simulations were challenging because of the simulation cost,” said team leader Michiko Fujii. “We, for the first time, successfully performed numerical simulations of globular cluster formation, modeling individual stars. By resolving individual stars with a realistic mass for each, we could reconstruct the collisions of stars in a tightly packed environment. For these simulations, we have developed a novel simulation code, in which we could integrate millions of stars with high accuracy.”

The resulting simulation run showed that runaway collisions brought very massive stars together. These are perfect candidates to end up as IMBH candidates. “Our final goal is to simulate entire galaxies by resolving individual stars,” Fujii points to future research. “It is still difficult to simulate Milky Way-size galaxies by resolving individual stars using currently available supercomputers. However, it would be possible to simulate smaller galaxies such as dwarf galaxies. We also want to target the first clusters, star clusters formed in the early universe. First clusters are also places where IMBHs can be born.”

Runaway Stars and IMBH

Okay, so simulations show that such IMBH could be possible in the globular cluster environment, but what’s the physical proof they actually exist? No one has actually detected the collisions of stellar-mass black holes inside a cluster to create an IMBH. Nor have they seen stellar collisions that might create a monster object — although the Japanese simulations proved they can happen. The trick now is to observe both types of event. Until that happens, astronomers can figure out if IMBH exist through indirect means.

A Chinese research team, led by Yang Huang of the University of the Chinese Academy of Sciences, recently posted a paper about a high-velocity star fleeing the scene of a collision in the heart of Messier 15. The star, called J0731+3717, was ejected by an encounter with an intermediate-mass black hole embedded very close to the center of the cluster.

J0731+3717 got tossed out on its high-speed journey about 21 million years ago. The team examined its metallicity (that is, its ratios of hydrogen and heavier elements (called “metals” by astronomers)) and found that it matches the stars in M15. The rogue star moves away from the cluster at a velocity of about 550 kilometers per second and once “lived” at a distance of about 1 AU from the cluster’s core. The team analyzed those measurements and did reverse orbital calculations of that star (and others within 5 kpc of the Sun). Based on their calculations, they concluded the star had a too-close encounter with an intermediate-mass black hole containing about 100 solar masses.

The team suggests that this method be used to prove the existence of other IMBH in similar environments. They conclude their paper with a look at future observations to prove the concept. “With the increasing power of ongoing Gaia and large-scale spectroscopic surveys, we expect to discover dozens of cases within the 5kpc volume and ten times more within a 10kpc volume, which should shed light on the understanding of the evolutionary path from stellar-mass BHs to SMBHs.”

For More Information

Simulations Yield New Intermediate Mass Black Holes Recipe
Medium and Mighty: Intermediate-mass Black Holes Can Survive in Globular Clusters
A High-velocity Star Recently Ejected by an Intermediate-mass Black Hole in M15

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