The Formation of Supermassive Black Holes in Galaxies: Theories and Evidence

Supermassive black holes are among the most intriguing and mysterious objects in our universe, and their formation has puzzled astronomers for decades. Unraveling the mystery of how these behemoths came to be is a key driver for advanced scientific missions like the James Webb Space Telescope (JWST).

Two Main Hypotheses: Starting Small or Starting Big

Currently, there are two primary hypotheses about the origin of supermassive black holes (SMBHs).

The First Hypothesis: Starting Small

According to this view, SMBHs might have started as smaller entities, formed either by the collapse of the first stars or through the collapse of density fluctuations in the early universe (primordial black holes). These smaller black holes would then grow over time through a process known as super-Eddington accretion, a rapid rate of matter absorption. However, achieving these high rates of accretion is highly challenging, leading some astronomers to favor the alternative hypothesis.

The Second Hypothesis: Starting Big

The "born large" hypothesis suggests that SMBHs were formed already in a very large size. Multiple theories have been proposed to support this notion, including:

Direct collapse of a massive gas cloud Formation of a supermassive star, which then implodes into a black hole Presence of large primordial black holes, which can vary in size depending on the parameters of the early universe

While the "starting small" hypothesis is considered more appealing for its simplicity, the "born large" scenario is currently more supported by available evidence.

Accretion Disk and Growth Mechanisms

The formation and growth of SMBHs are driven by accretion disks. These disks stabilize the matter around the black hole, allowing normal matter to lose energy through electromagnetic (EM) means and spiral inward. Some of the matter is also ejected from the poles, contributing to cosmic jets.

However, as the black hole reaches the ultramassive end of the scale, its ability to feed diminishes. Nevertheless, it can still grow, particularly through direct hits and mergers with other black holes.

The rate of growth is a significant factor in early universe models. Dark matter, due to its inability to lose energy through EM means, poses a unique challenge for black holes. Recent evidence suggests that dark matter can orbit a black hole. This makes it difficult for black holes to assimilate dark matter through conventional means like accretion, except through direct collisions where gravitational wave decay can occur. Low-mass dark matter particles would struggle to lose energy and would be in stable orbits.

For further exploration on this topic, further study into Planck mass black holes as potential dark matter candidates is suggested.

Conclusion

The formation of supermassive black holes remains one of the most challenging questions in astrophysics. While there are strong motivations supporting both hypotheses, the current evidence suggests that the "born large" scenario is more plausible. Continued research and new technological advancements, such as the James Webb Space Telescope, will undoubtedly provide more insights into this fascinating and complex phenomenon.