Weight Gain: Growing Little Black Holes in the Early Universe

Title: Growth of Light-Seed Black Holes in Gas-Rich Galaxies at High Redshift

Authors: Daxal Mehta, John A. Regan, Lewis Prole

First Author’s Institution: Department of Physics and Centre for Astrophysics and Space Science Maynooth, Maynooth University, Maynooth, Ireland

Status: published on arXiv [open access]

How Big are Baby Black Holes?

Astronomers believe that nearly every galaxy in the universe contains a supermassive black hole (SMBH) – a black hole having a mass at least a million times more than the sun – at its center. Recent observations from the James Webb Space Telescope (JWST) have peered back into the early universe, and observers have found SMBHs in the extremely young galaxies in this era. But, at just a few hundred million years after the Big Bang, how did these black holes form?

In the modern universe, black holes form from the deaths of massive stars, with masses between ~5-50 solar masses. Astronomers think black holes in the early universe may have formed in the same manner from the first generation of stars, called Population III stars. Pop. III stars would be metal-free, meaning that they only contain hydrogen and helium (and perhaps trace amounts of lithium leftover from Big Bang nucleosynthesis). With this chemical makeup, astronomers believe Pop. III stars could have been much more massive than modern stars, leaving behind black holes up to ~100 solar masses.

This theory runs into one major roadblock. In order for these “light seed” black holes to become SMBHs, they need to get way more massive, primarily by accreting material. However, there is a theoretical limit to how quickly a black hole can accrete called the Eddington limit. When a black hole accretes matter, this matter loses gravitational potential energy as it falls in, and some of that energy is converted into radiation. This exerts radiation pressure that slows further accretion. The Eddington limit is where the outward radiation pressure balances with the gravitational infall. Astronomers believe that black holes may be able to accrete above the Eddington limit (called hyper-Eddington accretion) in certain situations, but this isn’t sustainable is only expected to occur for short periods of time. Due to the Eddington limit, astronomers question how likely it is that light seeds could grow to supermassive scales in just a few hundred million years. Today’s authors investigate this question by simulating the formation and growth of light seed black holes in gas-rich galaxies like those observed in the early universe.

A Recipe for Black Holes and Accretion

Today’s authors begin with a simulated galaxy roughly 20 kiloparsecs wide, initialized to be representative of high-redshift galaxies observed in the early universe. This galaxy is simulated with particles representing dark matter, gas, and Population III stars. The state of the galaxy at the start of the simulation is shown in Figure 1.

As the galaxy evolves, gas collects into high-density areas. Once a region exceeds a critical density, this gas is converted into a Population III star. When a star dies, one of three things happens, depending on its mass:

• the star explodes into a supernova and the core collapses into a black hole;
• the star explodes into a supernova without forming a black hole; or
• the star collapses directly into a black hole without a supernova.

These black holes are then free to accrete surrounding material.

The authors perform two simulations of this galaxy: one with supernova feedback and one without. This allows them to study how energy and momentum from a supernova released into the surrounding gas affects how the remnant black hole accretes.

Insatiable Appetites

In the no-feedback simulation, most of the black holes do not experience substantial growth. However, a few undergo a prolonged period of hyper-Eddington accretion lasting roughly 100,000 years. In these cases, the black holes can grow to about 1000 times larger than their seed mass, highlighted in Figure 2. In this simulation, the hyper-Eddington accretion ends when a black hole has accreted most of the surrounding material, starving itself and lowering the accretion rate.

When supernova feedback is turned on, things begin to look a little different. First, the authors note that significantly fewer black holes grow to such large scales. Interestingly, a majority of these formed with a supernova explosion (see Figure 3). This seems counterintuitive because the supernova is expected to push gas away from the new black hole, leaving it with little material to accrete. However, upon closer inspection, the explosion may not push all the gas away, allowing it to recollapse; furthermore, this recollapse can make accretion more efficient, leading to the larger fraction of cases of hyper-Eddington accretion for black holes formed with supernovae. The authors also note that in this simulation, hyper-Eddington accretion typically ends when gas in the galaxy is disrupted by supernovae and black hole formation.

Today’s authors show that although most light seed black holes are not expected to undergo significant accretion, a few could experience hyper-Eddington accretion and have the potential to become SMBHs in the early universe. This is true even in the event of a supernova explosion – in fact, if the supernova explosion does not fully disperse the gas surrounding the black hole, the subsequent recollapse could lead to more efficient accretion than without a supernova. The authors note that there are a few methods to improve the simulation that they plan to address in future work:

1. They do not allow for black hole mergers, which is another method for growth but could disrupt accretion.
2. They do not account for any elements heavier than helium (what astronomers refer to as metals), which can affect the chemistry of the gas being accreted by black holes. This is particularly important for supernovae because they enrich the gas in the galaxy with metals.
3. They do not account for radiative feedback from the black holes or from stars, which would further push gas away from the black hole.

Astrobite edited by Catherine Slaughter

Featured image credit: Wikimedia Commons

• About the Author

About Brandon Pries

I am a graduate student in physics at Georgia Institute of Technology (Georgia Tech). I do research in computational astrophysics with John Wise, using machine learning to study the formation and evolution of supermassive black holes in the early universe. I've also done extensive research with the IceCube Collaboration as an undergraduate at Michigan State University, studying applications of neural networks to event reconstructions and searching for signals of neutrinos from dark matter annihilation.

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