There should be more average-mass black holes in globular clusters

We live in a Universe littered with black holes. There are countless supermassive black holes and circumstellar-mass black holes in our galaxy and most other galaxies. It is likely that they appeared as so-called “primordial” black holes in the earliest eras of cosmic history. However, there appears to be a missing link in this classification: intermediate mass black holes (IMBHs). Astronomers have been searching for these rare monsters for years, but there is only one possible sighting thanks to gravitational wave data. So where are they?

BSSMs may be hiding in the hearts of globular clusters. But given the tight packing of these compact clusters of stars, how do we know if they contain BSSM? Teams of researchers from Japan and China have come up with several ways to find them. One of them is to look for fast-moving stars ejected from globular clusters. Another is to simulate collisions of stars in the hearts of newly formed clusters. Both methods can point the way to new discoveries of BSSM.

What are intermediate mass black holes?

These rare objects are much like their name: black holes with masses somewhere between stellar masses and supermassive giants in the hearts of galaxies. They can have a mass of up to a thousand times the mass of the Sun, which is considered quite “small”, and up to a million solar masses. Then supermassive monsters begin with a mass millions or billions of times greater than the mass of the Sun. BSSMs do not arise as a result of supernova explosions, since there are no stars massive enough that could collapse and give rise to BSSMs. The birth of BSSM should occur during the merger of several massive objects. This makes them more similar to their larger cousins, supermassive black holes.

So where might such a collision occur? In a dense cluster of stars, closely packed next to each other. This is precisely the definition of globular clusters. They are full of stars, and most likely some of them are very massive. It is these stars that explode as supernovae and are destroyed, forming a black hole of stellar mass. If there are enough such stars in a cluster, they can merge and create a black hole. Another option for creating a BSSM is the collision of massive stars to form one more massive object.

Many globular clusters orbit the core of the Milky Way galaxy. In some of the densest clusters, millions of stars are attracted to each other by gravity. A good example is the Messier 15 (M15) cluster. It contains more than 100,000 stars crammed into a region of space about 175 light years across. If stellar collisions or mergers of black holes with stellar mass occurred in M15, this could be enough to create a BHSM.

Modeling globular clusters and the growth of intermediate mass black holes

Another idea is to study the formation of globular clusters to see if there are any clues to the origin and existence of BSSMs. This is exactly what a group of scientists from the University of Tokyo did. They created advanced simulations of star cluster formation to find out whether collisions of massive stars could lead to the birth of BSSMs. This is not an easy task. Previous simulations suggested that stellar winds would blow away the mass needed to create these missing black holes.

“Modeling the formation of star clusters has been difficult due to the cost of modeling,” says team leader Michiko Fujii. “We successfully performed numerical simulations of globular cluster formation for the first time by simulating individual stars. By resolving individual stars with realistic masses for each, we were able to reconstruct stellar collisions in a densely packed environment. For these simulations, we developed new simulation code in which we could integrate millions of stars with high precision.”

The simulations revealed that collisions cause very massive stars to come together. These are ideal candidates for the role of CHDS. “Our ultimate goal is to simulate entire galaxies by resolving individual stars,” says Fujii, pointing to future research. “It is still difficult to simulate galaxies the size of the Milky Way by resolving individual stars with currently available supercomputers. However, it would be possible to simulate smaller galaxies, such as dwarf galaxies. We also want to study the first clusters – star clusters that formed in the early Universe. The first clusters are also places where CDS can be born.”

Runaway stars and ChDSM

So, simulations show that such BSSMs are possible in the environment of globular clusters, but what is the physical evidence that they actually exist? No one has discovered stellar-mass black holes colliding within a cluster to create a BHSM. There have been no observed stellar collisions that could create a monster object, although Japanese simulations have proven that such a thing can happen. The challenge now is to observe both types of events. Until this happens, astronomers can find out whether BDSM exist indirectly.

A Chinese research team led by Yan Huang of the University of the Chinese Academy of Sciences recently published a paper on a high-velocity star that escaped from a collision at the center of the Messier 15 cluster. The star, named J0731+3717, was ejected by a collision with an intermediate-mass black hole located very nearby to the center of the cluster.

J0731+3717 began its high-speed journey about 21 million years ago. The team studied its metallicity (that is, the ratio of hydrogen to heavier elements (astronomers call them “metals”)) and found that it matches the stars in M15. The rogue star is moving away from the cluster at a speed of about 550 kilometers per second and once lived at a distance of about 1 AU from the cluster core. The team analyzed these measurements and back-calculated the orbit of this star (and others within 5 kpc of the Sun). Based on their calculations, they concluded that the star collided too closely with an intermediate-mass black hole containing about 100 solar masses.

The team proposes to use this method to prove the existence of other BSSMs in similar environments. The authors conclude their paper by looking to future observations that will prove this concept. “With the increasing power of ongoing Gaia observations and large-scale spectroscopic surveys, we expect to find dozens of cases at 5 kpc and ten times more at 10 kpc, which should shed light on understanding the evolutionary path from stellar-mass black holes to supermassive black holes.”