round and round

New visual shows how a hypnotic swirl creates supermassive black holes

A computer simulation suggests a different origin for black holes.

A longstanding theory about black holes is this: As a large cosmic object nears the end of its existence, it will collapse under the weight of its own gravitational force. The remnants of that fateful collapse could give rise to a supermassive black hole, which is what happens when a large amount of matter compressed into a small space from which even light cannot escape.

At least that's the long-standing theory, anyway. Scientists believe this cosmic collapse of a space object can lead to the formation of supermassive black holes.

Enter a team of astrophysicists from Tohoku University in Japan, who are proposing a new origin story for supermassive black holes.

First, let's give you the backstory — There are different theories as to how supermassive black holes form. Some black holes are thought to have formed shortly after the Big Bang — amidst the chaos of an early universe, while other black holes are believed to form at the same time that their host galaxies form, as there is quite often a correlation between the size of a black hole and its galaxy

Supermassive black holes lurk at the center of galaxies, with sizes that reach up to 10 billion times the mass of the Sun. For example, the supermassive black hole at the center of the Milky Way, Sagittarius A*, is 4 million times the mass of the Sun.

Another theory is that black holes form when primordial clouds of interstellar gas collapse under the weight of their own gravity and form supermassive stars, which then evolve into supermassive black holes.

However, that particular origin story didn't quite add up.

Previous observations have shown that heavy elements such as carbon and oxygen would change the dynamics of the interstellar gas, and instead cause a cosmic object to break up into fragments of smaller clouds, which would then result in the birth of smaller stars rather than a few large stars.

The team of researchers behind the new study wanted to find out whether interstellar gas with heavy elements could still produce massive stars.

Their research was published this week in the journal Monthly Notices of the Royal Astronomical Society

In order to test their theory, they used the National Astronomical Observatory of Japan's supercomputer ATERUI II to perform long-term, 3D high-resolution simulations.

The simulations showed that while the break up of gas does result in smaller stars, these stars are essentially swallowed up by larger, more massive stars.

In the visualization above, you can see the interstellar gas flowing towards the center of the cloud, dragging the smaller stars along with it. The smaller stars are then swallowed up by the larger stars in the center of the cloud, resulting in the formation of a massive star that is 10,000 times more massive than the Sun.

These massive stars will then continue to grow over time and eventually become a supermassive black hole.

"This is the first time that we have shown the formation of such a large black hole precursor in clouds enriched in heavy-elements," Sunmyon Chon, a postdoctoral fellow at the Japan Society for the Promotion of Science and Tohoku University, and lead author of the study, said in a statement. "We believe that the giant star thus formed will continue to grow and evolve into a giant black hole."

The results resolve a longstanding mystery surrounding one of the more popular theories behind the origin of supermassive black holes as scientists continue their quest to try and understand these cosmic monsters a little better.

Abstract: Direct collapse black hole (DCBH) formation with mass ≳105 M⊙ is a promising scenario for the origin of high-redshift supermassive black holes. It has usually been supposed that the DCBH can only form in the primordial gas since the metal enrichment enhances the cooling ability and causes the fragmentation into smaller pieces. What actually happens in such an environment, however, has not been explored in detail. Here, we study the impact of the metal enrichment on the clouds, conducting hydrodynamical simulations to follow the cloud evolution in cases with different degree of the metal enrichment Z/Z⊙ = 10−6 to 10−3. Below Z/Z⊙ = 10−6, metallicity has no effect and supermassive stars form along with a small number of low-mass stars. With more metallicity Z/Z⊙≳5×10−6⁠, although the dust cooling indeed promotes fragmentation of the cloud core and produces about a few thousand low-mass stars, the accreting flow preferentially feeds the gas to the central massive stars, which grows supermassive as in the primordial case. We term this formation mode as the super competitive accretion, where only the central few stars grow supermassive while a large number of other stars are competing for the gas reservoir. Once the metallicity exceeds 10−3 Z⊙ and metal-line cooling becomes operative, the central star cannot grow supermassive due to lowered accretion rate. Supermassive star formation by the super competitive accretion opens up a new window for seed BHs, which relaxes the condition on metallicity and enhances the seed BH abundance.
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