Did supermassive black holes collapse straight from giant clouds of gas? It could depend on magnetic fields

About half a century ago, astronomers realized that the powerful radio source emanating from the center of our galaxy (Sagitarrius A*) was a “monster” black hole. They have since found that supermassive black holes (SMBHs) reside at the center of most massive galaxies. This leads to so-called active galactic nuclei (AGN) or quasars, where the central region of a galaxy is so energetic that it outshines all the stars in its galactic disk. For all this time, astronomers have pondered how these giants (which play a crucial role in galactic evolution) came to be.

Astronomers suspect that the seeds that formed SMBHs arose from giant dust clouds that collapsed without first becoming stars — also known as Direct Collapse Black Holes (DCBHs). However, the role of magnetic fields in the formation of DCBHs remained unclear as none of the previous studies could simulate the full periods of accretion. To study this, an international team of astronomers performed a series of 3D cosmological magnetohydrodynamic (MHD) simulations that explained DCBH formation and showed that magnetic fields grow with the accretion disks, stabilizing them over time.

The research was led by Muhammad A. Latif, an assistant professor of physics at the University of the United Arab Emirates (UAEU) College of Science. He was joined by Associate Professor Dominik RG Schleicher from the Universidad de Concepcion in Chile and Sadegh Khochfar – the personal chair in Theoretical Astrophysics at the University of Edinburgh and the Royal Observatory. The paper describing their findings recently appeared online on the preprint server arXiv and is currently under consideration for publication in The Astrophysical Journal.

As they state in their paper, DCBHs are massive seeds of black holes (typically around 1 million solar masses) that existed in the early Universe – around 100 to 250 million years old. As the name suggests, DCBHs are formed directly from massive clouds of dust and gas (due to instabilities predicted by Einstein’s general theory of relativity). This distinguishes them from black holes, which descended from the earliest supermassive stars (SMSs), also known as Population III stars. like dr Latif told Universe Today via email, astrophysicists have long suspected that SMBHs may have formed in this way in the early Universe:

“DCBHs are about two orders of magnitude more massive (10⁵ solar mass) than black holes from other scenarios, such as stellar-mass black holes (about 100 solar masses) or black holes formed by stellar collisions (~1000 solar masses). This makes them leading candidates, particularly for the first SMBHs observed in the first post-Big Bang gyr.

The existence of SMBHs was originally proposed to explain the existence of high redshift primordial SMBHs that existed within 1 billion years of the Big Bang. But as Latif and his colleagues explain, there were contradictions between the astrophysicists’ theoretical predictions and the astronomers’ observations. In particular, the role that magnetic fields played in accumulating material with primordial dust clouds, eventually leading to gravitational collapse and the formation of DCBHs.

“The Standard Model of physics places no constraints on the initial magnetic field strength, and some models predict small B-fields on the order of 10^-20 G,” Latif said. “They are many orders of magnitude smaller than observed fields (about 1 G). Therefore, the scientific community thought that their role could only be secondary.”

This mystery remained as previous attempts to numerically simulate the formation of DCBHs were limited in scope. Previous simulations lacked the computational power to simulate the full length of the accretion process, which is believed to be comparable to the expected lifetime of SMSs – 1.6 million years. Thanks to advances in supercomputing over the past decade, various research groups have performed numerical simulations over the last decade showing that magnetic fields can be amplified in a short time.

Similarly, there is increasing evidence that magnetic fields were present around 13 billion years ago when DCBHs are thought to have formed. To unravel this mystery, Latif and his colleagues performed a series of 3D cosmological magneto-hydrodynamics (MHD) models that account for a lifetime of 1.6 million years:

“We model accretion onto the central clump formation in our simulation, which is a proxy for a protostar. We develop simulations for about 1.6 Myr, comparable to the expected lifetime of SMSs, and calculate how much mass accumulates on the clump, which tells us the rate of accretion. Previous work has only developed the simulation for short periods up to one kyr (1000 years), which is much shorter than the lifetime of SMSs (~2 million years). Therefore, it is important to know whether accretion can be sustained long enough that we show it is possible.”

Their findings are consistent with previous research by Latif and his colleagues (and other groups) showing how magnetic fields play a crucial role in the formation of massive stars and black holes. These studies have shown how magnetic fields are amplified (increasing Jean mass) by accumulating disks of gas and dust. These fields are responsible for reducing fragmentation and stabilizing the disks, eventually allowing these disks to reach the mass necessary (aka Jean mass) to undergo gravitational collapse and form supermassive stars and black holes .

“Such strong magnetic fields can even launch jets and outflows, and also help transport angular momentum, which is believed to be a barrier to star formation,” Latif explained. “Therefore, they will have important effects on the magnetization of interstellar and intergalactic media (similar to what we observe in the local Universe) and shape the formation of high redshift galaxies and the evolution of massive black holes.”

These results also show what future studies might reveal about magnetic fields and their role in the formation and evolution of early galaxies. In the coming decade and beyond, astronomers are expected to study the jets and outflows of the earliest black holes using powerful next-generation radio observatories such as the Square Kilometer Array (SKA) and the Very Large Array (ng-VLA), which are expected to they will do so by 2027 and 2029 respectively.