Scientists are trying to understand how supermassive black holes (SMBHs) formed early in the universe. One idea is that the first stars, called Population III (Pop III) stars, may have helped. These stars were made of hydrogen and helium and were very massive but short-lived. When they exploded, they left behind black holes. For a black hole to grow into an SMBH, it needs a lot of gas. Researchers studied whether Pop III stars could create special conditions to form even bigger stars, called supermassive stars, which could later become SMBHs. They focused on atomic-cooling halos (ACHs), large gas clouds where stars could form. The first Pop III stars produced radiation that could affect nearby gas and slow down star formation. However, computer simulations showed that this radiation wasn’t strong enough to stop cooling and create a supermassive star. Instead, smaller stars formed, leading to smaller black holes. This means forming SMBHs through this process alone is unlikely. Scientists think other factors, like nearby galaxies, might help. Understanding how SMBHs formed can teach us more about the early universe and galaxy formation.
Supermassive black holes (SMBHs) are found at the center of most galaxies, including our Milky Way. Scientists are still trying to understand how these enormous black holes formed so early in the universe. Some ideas suggest that the first stars, known as Population III (Pop III) stars, may have played a key role in their formation.
Pop III stars were the first generation of stars, made from pure hydrogen and helium left over from the Big Bang. These stars were very massive and short-lived, exploding as supernovae after just a few million years. When they died, they could leave behind black holes. However, for a black hole to grow into a supermassive black hole, it needs to gather huge amounts of gas quickly. Scientists have been studying how this process could have happened.
One idea is that Pop III stars might have formed in special regions called atomic-cooling halos (ACHs). These halos were large clouds of gas that could cool and collapse to form stars. Normally, small halos needed molecular hydrogen (H₂) to cool down enough for star formation, but radiation from earlier stars could destroy H₂, making cooling more difficult. This process, called Lyman-Werner (LW) radiation, could prevent star formation in smaller halos and allow larger halos to form massive stars instead.
A group of researchers, led by Sullivan and colleagues, studied whether the first Pop III stars in an atomic-cooling halo could create enough LW radiation to affect other nearby stars forming in the same region. They used computer simulations to see if the radiation from one star could heat up and remove the cooling ability of the surrounding gas, allowing a second, even bigger star (potentially a supermassive star) to form. Supermassive stars are important because they could collapse directly into massive black holes, which could later grow into SMBHs.
In their simulations, they found that while the first star did produce LW radiation, it was not strong enough to completely stop the cooling of nearby gas. The gas could not heat up to the high temperatures needed to form a supermassive star. Instead, normal Pop III stars formed, which were much smaller and would create smaller black holes.
This result suggests that forming supermassive black holes in this way is more difficult than previously thought. To create the necessary conditions, the first star would need to be much more massive, or the gas in the halo would need to collapse much faster. The researchers concluded that it is unlikely for SMBHs to form through this process alone.
However, they noted that other factors, such as nearby galaxies or merging halos, could provide additional radiation to help the process. Future research will continue to explore whether different conditions might allow these massive stars to form and lead to the creation of SMBHs.
Understanding how the first black holes formed is important for understanding the evolution of the universe. If scientists can figure out how SMBHs grew so quickly, it could provide new insights into the early universe and the formation of galaxies.
Reference: James Sullivan, Zoltan Haiman, Mihir Kulkarni, Eli Visbal, "Can supermassive stars form in protogalaxies due to internal Lyman-Werner feedback?", Arxiv, 2024. https://arxiv.org/abs/2501.12986
Technical terms
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Population III (Pop III) Stars – The first stars that formed in the universe, made only of hydrogen and helium, without any heavier elements.
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Supermassive Black Holes (SMBHs) – Extremely large black holes found at the center of galaxies, with masses millions or billions of times bigger than the Sun.
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Minihalos – Small clouds of gas and dark matter where the first stars formed in the early universe.
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Atomic-Cooling Halos (ACHs) – Bigger gas clouds that can cool down using atomic hydrogen, helping in the formation of massive stars.
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Lyman Werner (LW) Radiation – A type of ultraviolet light from stars that can break apart hydrogen molecules, stopping them from cooling and forming stars.
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Molecular Hydrogen (H2) Cooling – A process where hydrogen molecules help gas cool down, allowing stars to form.
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Accretion – The process of gas and dust falling into a black hole or star, making it grow bigger.
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Fragmentation – When a cloud of gas breaks into smaller parts, leading to the formation of multiple stars instead of one big star.
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Protostar – A very young star that is still forming by collecting gas from its surroundings.
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Virial Temperature – The temperature a gas cloud needs to reach for it to collapse and form stars.