Elusive medium-size black holes may form in dense 'birthing nests'

Researchers have discovered that elusive intermediate-mass black holes could form in dense star clusters containing anywhere between tens of thousands to millions of tightly packed stars called "globular clusters."

An intermediate-mass black hole has a mass between 100 and 10,000 suns. They're heftier than solar-mass black holes, which have a mass range between 10 and 100 solar masses, yet lighter than supermassive black holes, which have masses equivalent to millions or even billions of suns.

These cosmic inbetweeners have proved elusive for astronomers to discover, with the first example being found in 2012. Designated GCIRS 13E, it has a mass 1,300 times that of the sun and is located 26,000 light-years away, toward the galactic center of the Milky Way.

One of the mysteries surrounding intermediate-mass black holes concerns their formation. Stellar-mass black holes are born when massive stars collapse, and supermassive black holes grow from subsequent mergers of larger and larger black holes. Yet a star massive enough to die and create a black hole with thousands of solar masses should be incredibly rare and should struggle to retain that mass when it "dies."

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To investigate the mystery of how these intermediate-mass black holes come to be, a team of researchers performed the first-ever star-by-star simulation of massive clusters. This showed that a dense enough molecular cloud "birthing nest" of globular clusters could create stars massive enough to collapse and spawn an intermediate-mass black hole.

"Previous observations have suggested that some massive star clusters, globular clusters, host an intermediate-mass black hole," team leader and University of Tokyo scientist Michiko Fujii said in a statement. "So far, there has been no strong theoretical evidence to show the existence of intermediate-mass black hole with 1,000 to 10,000 solar masses compared to less massive (stellar mass) and more massive (supermassive) ones."

A chaotic birthplace for black holes

The term "birthing nest" may well summon images and feelings of warmth, comfort, and tranquility, but this couldn't be less appropriate for star formation in globular clusters. 

These densely packed conglomerations of stars live in chaos and turmoil, with differences in density causing stars to collide and merge. That process results in stars piling on mass, thus increasing their gravitational influences, dragging more stars into their vicinity, and thus driving more and more mergers.

The runaway collision and merger process occurring at the hearts of globular clusters can lead to the creation of stars with masses equivalent to around 1,000 suns. That's enough mass to create an intermediate-mass black hole, but there is a hurdle.

Astrophysicists know that when stars collapse to create black holes, a great deal of their masses gets blown away in supernova explosions or by stellar winds. Previous simulations of intermediate-mass black hole creation have confirmed this, further suggesting that even massive stars with 1,000 solar masses would end up too small to create an intermediate-mass black hole.

To discover if a massive star could "survive" with enough mass to birth an intermediate-mass black hole, Fujii and team simulated a globular cluster as it formed.

"We, for the first time, successfully performed numerical simulations of globular cluster formation, modeling individual stars," Fujii said. "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."

In the simulated globular cluster, runaway collisions and mergers led to the formation of extremely massive stars that could retain enough mass to collapse and birth an intermediate-mass black hole.

The team also found the simulation predicted a mass ratio between the intermediate-mass black hole and the globular cluster within which it is formed. That ratio, as it turned out, matches actual astronomical observations.

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"Our final goal is to simulate entire galaxies by resolving individual stars," Fujii explained. "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."

Fujii and her team also intend to target the star clusters formed in the early universe. "The first clusters are also places where intermediate-mass black holes can be born," she said.

The team's research was published on Thursday (May 30) in the journal Science.