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How did black holes get so big and so fast? The answer lies in dark matter

New findings suggest that dark matter played a critical role in the formation of supermassive black holes in the early universe

A view of the black hole at the center of the Milky Way, Sagittarius A* in polarized light. Credit: EHT Collaboration
A view of the black hole at the center of the Milky Way, Sagittarius A* in polarized light. Credit: EHT Collaboration

A supermassive black hole, like the one at the center of our Milky Way galaxy, takes a long time to form. Normally, for a black hole to form, a giant star with a mass of at least 50 solar masses has to shut down - a process that takes a billion years - and its core has to collapse in on itself.

Even so, with a size of about ten solar masses, the resulting black hole is a far cry from Sagittarius A*, the 4 million solar mass black hole found in the Milky Way, or the billion solar mass supermassive black holes found in other galaxies. Such supermassive black holes can form from smaller black holes by absorbing gas and stars, and by merging with other black holes, which takes billions of years.

Why, then, is the James Webb Space Telescope discovering supermassive black holes near the beginning of time, eons before they could have formed? UCLA astrophysicists have an answer as mysterious as the black holes themselves: Dark matter kept the hydrogen from cooling long enough for gravity to condense it into clouds that were big and dense enough to become black holes instead of stars.

"We were very surprised to discover a supermassive black hole the size of a billion solar masses when the universe itself is only half a billion years old," said senior author Alexander Kusenko. "It's like finding a modern car among dinosaur bones and wondering who made the car in prehistoric times."

Web image showing the quasar J0148 circled in red. The upper small image shows the central black hole and the lower the stellar emission from the host galaxy.
Web image showing the quasar J0148 circled in red. The upper small image shows the central black hole and the lower the stellar emission from the host galaxy

Some astrophysicists have proposed that a large cloud of gas collapsed to form a supermassive black hole directly, bypassing the long process of star formation, accretion and mergers. But there is a problem: gravity will indeed pull and collect a large cloud of gas, but not into one large cloud. Instead, it collects parts of the gas into small haloes that float close to each other but do not form a black hole.

The reason is that the gas cloud cools too quickly. As long as the gas is hot, its pressure can resist gravity. But if the gas cools, the pressure is small and gravity will prevail in many small regions, which collapse into compressed objects before gravity has a chance to pull the entire cloud into a single black hole.

"The cooling speed of the gas is highly dependent on the amount of molecular hydrogen," said first author Yifan Lu. "Hydrogen atoms bound together in a molecule dissipate energy when they encounter a free hydrogen atom. The hydrogen molecules become a cooling agent when they absorb thermal energy and radiate it out. The hydrogen clouds in the early universe had too much molecular hydrogen, and the gas cooled quickly and formed small halos instead of large clouds."

Lu and postdoctoral researcher Zachary Picker wrote code to calculate all the possible processes in this scenario and discovered that additional radiation can heat the gas and separate the hydrogen molecules, changing the way the gas cools.

"If you add radiation in a certain energy range, it destroys the molecular hydrogen and creates conditions that prevent the dispersal of large clouds," he told him.

But what is the source of the radiation?

Only a very small fraction of the matter in the universe is of the kind that makes up our bodies, the earth, the stars, and everything else we can discern. The vast majority of matter, revealed by its gravitational effects on interstellar objects and by the bending of light rays from distant sources, is made of some new particles that scientists have not yet identified.

The properties and forms of dark matter are still an unsolved mystery. We don't know what dark matter is, but particle theorists have long speculated that it could contain unstable particles that can decay into photons, the particles of light. Including this dark matter in the simulations provided the radiation needed for gas to remain in a large cloud as it collapses into a black hole.

The product of the decay can be the radiation of photons, which break up molecular hydrogen and prevent the hydrogen clouds from cooling too quickly. Even a very mild decay of dark matter produced enough radiation to prevent cooling and form large clouds, and eventually supermassive black holes.

for the scientific article

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