How Did Supermassive Black Holes Get So Big, So Early? They Might Have Had a Head Start

Supermassive Black Holes (SMBHs) can have billions of solar masses, and observational evidence suggests that all large galaxies have one at their centres. However, the JWST has revealed a foundational cosmic mystery. The powerful space telescope, with its ability to observe ancient galaxies in the first billion years after the Big Bang, has shown us that SMBHs were extremely massive even then. This contradicts our scientific models explaining how these behemoths became so huge.

How did they get so massive so early?

Black holes of all masses are somewhat mysterious. We know that massive stars can collapse and form stellar-mass black holes late in their lives. We also know that pairs of stellar-mass black holes can merge, and we’ve detected the gravitational waves from those mergers. So, it’s tempting to think that SMBHs also grow through mergers when galaxies merge together.

The problem is, in the early Universe, there wasn’t enough time for black holes to grow large enough and merge often enough to produce the SMBHs. The JWST has shown us the errors in our models of black hole growth by finding quasars powered by black holes of 1-10 billion solar masses less than 700 million years after the Big Bang.

Astrophysicists are busy trying to understand how SMBHs became so massive so soon in the Universe. New research titled “Primordial black holes as supermassive black holes seeds” attempts to fill in the gap in our understanding. The lead author is Francesco Ziparo from the Scuola Normale Superiore di Pisa, a public university in Italy.

This artist’s conception illustrates a supermassive black hole (central black dot) at the core of a young, star-rich galaxy. Observational evidence suggests all large galaxies have one. Image credit: NASA/JPL-Caltech

There are three types of black holes: Stellar-mass black holes, intermediate-mass black holes (IMBHs), and SMBHs. Stellar-mass black holes have masses ranging from about five solar masses up to several tens of solar masses. SMBHs have masses ranging from hundreds of thousands of solar masses up to millions or billions of solar masses. IMBHs are in between, with masses ranging from about one hundred to one hundred thousand solar masses. Researchers have wondered if IMBHs could be the missing link between stellar-mass black holes and SMBHs. However, we only have indirect evidence that they exist.

This is Omega Centauri, the largest and brightest globular cluster that we know of in the Milky Way. An international team of astronomers used more than 500 images from the NASA/ESA Hubble Space Telescope spanning two decades to detect seven fast-moving stars in the innermost region of Omega Centauri. These stars provide compelling new evidence for the presence of an intermediate-mass black hole. Image Credit: ESA/Hubble & NASA, M. Häberle (MPIA)

There’s a fourth type of black hole that is largely theoretical, and some researchers think they can help explain how the early SMBHs were so massive. They’re called primordial black holes (PBHs.) Conditions in the very early Universe were much different than they are now, and astrophysicists think that PBHs could’ve formed by the direct collapse of dense pockets of subatomic matter. PBHs formed before there were any stars, so aren’t limited to the rather narrow mass range of stellar-mass black holes.

Artist illustration of primordial black holes. NASA’s Goddard Space Flight Center

“The presence of supermassive black holes in the first cosmic Gyr (gigayear) challenges current models of BH formation and evolution,” the researchers write. “We propose a novel mechanism for the formation of early SMBH seeds based on primordial black holes (PBHs).”

Ziparo and his co-authors explain that in the early Universe, PBHs would’ve clustered and formed in high-density regions, the same regions where dark matter halos originated. Their model takes into account PBH accretion and feedback, the growth of dark matter halos, and dynamical gas friction.

In this model, the PBHs are about 30 solar masses and are in the central region of dark matter (DM) halos. As the halos grow, baryonic matter settles in their wells as cooled gas. “PBHs both accrete baryons and lose angular momentum as a consequence of the dynamical friction on the gas, thus gathering in the central region of the potential well and forming a dense core,” the authors explain. Once clustered together, a runaway collapse occurs that ends up as a massive black hole. Its mass depends on the initial conditions.

Planted soon enough, these seeds can explain the early SMBHs the JWST has observed.

This figure from the research illustrates how PBHs could form the seeds for SMBHs. (Left) As the gas cools, it settles into the center of the dark matter gravitational potential, and the PBHs become embedded at the center. (Middle) The PBHs lose angular momentum due to the gas’s dynamic friction and concentrate in the core of the DM halo. (Right) PBH binaries form and merge rapidly because of their high density. The end result is a runaway merger process that creates the seeds of SMBHs. Image Credit: Ziparo et al. 2024.

There’s a way to test this model, according to the authors.

“During the runaway phase of the proposed seed formation process, PBH-PBH mergers are expected to copiously emit gravitational waves. These predictions can be tested through future Einstein Telescope observations and used to constrain inflationary models,” they explain.

The Einstein Telescope or Einstein Observatory is a proposal from several European research agencies and institutions for an underground gravitational wave (GW) observatory that would build on the success of the laser-interferometric detectors Advanced Virgo and Advanced LIGO. The Einstein Telescope would also be a laser interferometer but with much longer arms. While LIGO has arms four km long, Einstein would have arms 10 km long. Those longer arms, combined with new technologies, would make the Telescope much more sensitive to GWs.

The Einstein Telescope should open up a GW window into the entire population of stellar and intermediate-mass black holes over the entire history of the Universe. “The Einstein Telescope will make it possible, for the first time, to explore the Universe through gravitational waves along its cosmic history up to the cosmological dark ages, shedding light on open questions of fundamental physics and cosmology,” the Einstein website says.

A thorough understanding of SMBHs is a ways away, but it’s important to understand them because of their role in the Universe. They help explain the universe’s large-scale structure by influencing the distribution of matter on large scales. The fact that they appeared so much earlier in the Universe than we thought possible shows that we have a lot to learn about SMBHs and how the Universe has evolved to the state it’s in now.

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