The discovery of black holes was the first collision of quantum gravity with general relativity. In 2019, astrophysicists at Ontario’s Western University found evidence for the direct formation of black holes that do not need to emerge from a star remnant. The production of black holes in the early universe, formed from massive seeds aided by gravitational fields soon after the Big Bang, provide scientists with an explanation for what appeared to be the anomaly of extremely massive black holes at a very early stage in the history of our universe.
Supermassive black holes formed very, very quickly in the early Universe over very, very short periods of time and then suddenly, they stop
Shantanu Basu and Arpan Das from Western’s Department of Physics & Astronomy developed an explanation for the observed distribution of supermassive black hole masses and luminosities, for which there was previously no scientific explanation. They concluded that supermassive black holes form very, very quickly in the early Universe over very, very short periods of time and then suddenly, they stop.
No Supernova Required
In a study, published June 28, 2019 in The Astrophysical Journal Letters, the researchers ran a computer model to show that certain supermassive black holes in the very early universe could have formed by simply accumulating a gargantuan amount of gas into one gravitationally bound cloud. The researchers found that, in a few hundred million years, a sufficiently large cloud could collapse under its own mass and create a small black hole — no supernova required.
This explanation contrasts with the current understanding of how stellar-mass black holes are formed, which is that they emerge when the center of a very massive star collapses in upon itself.
Perhaps they started off unusually massive, through some physical process we don’t fully understand”
“This is indirect observational evidence that black holes originate from direct-collapses and not from stellar remnants,” says Basu, an astronomy professor at Western who is internationally recognized as an expert in the early stages of star formation and protoplanetary disk evolution.
“Our model for the quasar luminosity function provides indirect observational evidence that black holes originate from direct-collapses and not from stellar remnants,” Shantanu Basu wrote in an email to The Daily Galaxy. “It implies that there was a brief period of rapid growth of the numbers of these objects and their masses within about two and four hundred million years after the Big Bang. New instruments like the Nancy Roman Telescope are expected to significantly increase the observed sample of quasars at redshifts z ~ 6 – 7, providing much better statistics of the quasar luminosity function and allowing us to constrain our models.”
Basu and Das developed the new mathematical model by calculating the mass function of supermassive black holes that form over a limited time period and undergo a rapid exponential growth of mass. The mass growth can be regulated by the Eddington limit that is set by a balance of radiation and gravitation forces or can even exceed it by a modest factor.
“Supermassive black holes only had a short time period where they were able to grow fast and then at some point, because of all the radiation in the universe created by other black holes and stars, their production came to a halt,” explains Basu. “That’s the direct-collapse scenario.”
During the last decade, many supermassive black holes that are a billion times more massive than the Sun have been discovered at high ‘redshifts,’ meaning they were in place in our universe within 800 million years after the Big Bang. The presence of these young and very massive black holes question our understanding of black hole formation and growth.
“The formation of very massive initial seeds could have been enhanced by the presence of magnetic fields, which can remove angular momentum from collapsing gas clouds, allowing a large amount of mass to gather into a central object,” Basu told The Daily Galaxy.
The direct-collapse scenario allows for initial masses that are much greater than implied by the standard stellar remnant scenario, and can go a long way to explaining the observations. This new result provides evidence that such direct-collapse black holes were indeed produced in the early universe.
The Last Word–Shantanu Basu in an email to The Daily Galaxy
“There is growing evidence that the supermassive black holes of over a billion solar masses that are observed to exist less than a billion years after the Big Bang required very large initial seed masses of about 100,000 solar masses, much greater than possible from the end of a pop III star’s life.
The direct collapse black hole scenario can explain such very large seed masses if the early universe contained atomic cooling (AC) halos in which there is no molecular hydrogen or metals.
“Gas in such halos can contract gravitationally while maintaining a high temperature of nearly 10,000 K. In this case the mass accretion rate to the center is very high and consequently a black hole of mass about 100,000 solar masses can form after a brief supermassive star phase . These seed black holes will continue to accrete from their host halos.
“Black holes generate ultraviolet radiation that can dissociate molecular hydrogen in nearby halos and convert them into AC halos. This leads to a chain-reaction effect: the formation of an AC halo and embedded DCBH leads to the formation of other DCBHs in nearby halos. The scene is set for a rapid exponential growth of numbers of DCBH. But the DCBH formation era cannot last very long as the growing radiation field will eventually photoevaporate the gas from the gravitational well of most of the halos, disabling the halos from forming DCBHs. So the DCBH formation is a short-lived boom-bust phenomenon.
Will the Halo Collapse to the Center of a Galaxy?
“Another outstanding question is whether an AC halo will really collapse to the center rather than fragmenting into many smaller mass objects. Fragmentation can easily occur if matter falls onto a rotationally supported disk rather than to the center.
“Our recent work shows that even a very weak seed magnetic field in the AC halo can transport enough angular momentum away from the central collapse zone and allow mass to efficiently move to the center without settling into a circumstellar disk.”
(Note: The Last Word second paragraph is based on results in the paper Hirano, Machida, & Basu, 2021, ApJ, 917, 34.)
Maxwell Moeastrophysicist, NASA Einstein Fellow, University of Arizona, via Shantanu Basu and University of Western Ontario
Image at the top of the page: shows the inner 30 light-years of a dark matter halo in a cluster of young galaxies. The rotating gaseous disk breaks apart into three clumps that collapse under their own gravity to form supermassive stars. Credits: John Wise, Georgia Institute of Technology.
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Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona. Max can be found two nights a week probing the mysteries of the Universe at the Kitt Peak National Observatory. Max received his Ph.D in astronomy from Harvard University in 2015.