New Simulations Show How Black Holes Grow, Through Mergers and Accretion

One of the most pressing questions in astronomy concerns black holes. We know that massive stars that explode as supernovae can leave stellar mass black holes as remnants. And astrophysicists understand that process. But what about the supermassive black holes (SMBHs) like Sagittarius A-star (Sgr A*,) at the heart of the Milky Way?

SMBHs can have a billion solar masses. How do they get so big?

A group of scientists at the Harvard Center for Astrophysics are trying to shed some light on that question. They’ve created a simulation as part of the Black Hole Initiative (BHI), an interdisciplinary effort at Harvard to advance the understanding of black holes.

The key question is how do black holes grow? Is it through accretion or mergers? It looks like the answer is “Both. Sort of.”

“Black holes can grow in two ways.”

Dr. Fabio Pacucci, Harvard Center for Astrophysics

A pair of scientists have developed a new theoretical model to answer the black hole question. They are Dr. Avi Loeb, a Professor at Harvard, and Dr. Fabio Pacucci, an astrophysicist CfA. Both are involved with the BHI. Their model covers the local universe up to redshift 10, or roughly from the present day to about 13 billion years ago.

According to their simulation, two factors preside over the growth of black holes: their mass, and their redshift.

The new simulation shows that close to us, small black holes are growing mostly by accretion, while large black holes grow through mergers. But in the distant Universe, the reverse is true: small black holes grow through mergers, while large ones grow through accretion.

A new simulation of black hole growth shows the different ways that they grow in different scenarios. In the nearby Universe, small black holes grow by accretion, while large ones grow via mergers. In the distant Universe, the reverse is true: small one grow via mergers, while large ones grow through accretion. Image Credit: Credit: M. Weiss

In a press release, Pacucci explained it like this: “Black holes can grow in two ways. They can accrete mass from the space around them or they can merge with each other, forming one more massive black hole,” said Pacucci. “We currently believe that the first black holes started to form approximately with the first population of stars, over 13.5 billion years ago.”

“Understanding how black holes formed, grew and co-evolved with galaxies is fundamental to our understanding and knowledge of the universe, and with this study, we have one more piece of the puzzle.”

Dr. Fabio Pacucci, Harvard Center for Astrophysics

But that’s just the start of the answer. There’s more to it.

This answer doesn’t explain in detail how we get from these smaller “seed” black holes to the massive monstrosities at the heart of most—or all—galaxies: supermassive black holes.

Pacucci has more to say: “We can constrain their history not just by detecting light but also through gravitational waves, the ripples in spacetime that their mergers produce.”

A black hole is nothing to sneeze at. When two of these monsters merge, they send out gravitational waves, long-theorized curvatures in space time that were finally detected four years ago.

Pacucci and Loeb’s work also rests on previous studies of black holes, showing that black holes that grow through accretion are brighter than those that grow through mergers. That’s because as they accrete matter, the matter rotates around the hole in an accretion disk. That heats the material up, causing it to emit radiation.

Illustration of the supermassive black hole at the center of the Milky Way, surrounded by its rotating accretion disk. Credit: NRAO/AUI/NSF

“As the rate of rotation, or spin, fundamentally affects the way the region around a black hole shines, studying the main growth modality of black holes helps to provide us with a clearer picture of how bright these sources can be. We already know that matter falls toward the event horizon of black holes and, as it speeds up, it also heats up, and this gas starts to emit radiation,” said Pacucci.

“The more matter a black hole accretes, the brighter it is going to be; that’s why we’re able to observe far-away objects like supermassive black holes,” Pacucci continued. “They’re one billion times more massive than the sun, and they are able to emit enormous amounts of radiation so we can observe them from even billions of light years’ distance.”

But even if the black hole is in an area starved of matter that can feed its accretion disk, it can still grow through galaxy mergers. Rather than increased brightness, black hole mergers cause graviational waves.

Another central question when it comes to black holes concerns the SMBHs at the center of galaxies. “We believe that every galaxy contains a massive black hole at its center, which regulates the formation of stars in their host,” said Pacucci. “Understanding how black holes formed, grew and co-evolved with galaxies is fundamental to our understanding and knowledge of the universe, and with this study, we have one more piece of the puzzle.”

Artist’s impression of merging binary black holes. In February 2016, LIGO detected gravity waves for the first time. As this artist’s illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet.

It’s easy to be skeptical of models, and what their value is. But scientific modeling is critical in planning. In this case, it provides a theoretical framework that can be further investigated, and also provides some guidance into how to observe things in the future. And this model has already been partly tested, with good results.

“We already tested our model with data from close-by black holes, obtaining very encouraging results,” said Pacucci. “Our goal in this study was to provide the scientific community with a theory that describes how black holes may have grown during the evolution of the universe. This will inform decisions regarding observational strategies with future space telescopes, as well as lay the basis for models that describe other aspects of the evolution of the universe.”

One of those future space telescopes is LISA—the Laser Interferometer Space Antenna. LISA is a European Space Agency (ESA) mission designed to detect and measure gravitational waves. LISA will be a three-spacecraft system that form a triangle, with each side 2.5 million km (1.55 million miles) long. The distance between the three will be precisely controlled. Any gravitational waves would be detected by the lasers that form the sides of the triangle.

An illustration of the ESA’s LISA spacecraft, designed to detect gravitational waves from black hole mergers and other events. Image Credit: By NASA – NASA, Public Domain, https://commons.wikimedia.org/w/index.php?curid=10372273

The future is bright in the field of black holes. Scientists are detecting more and more mergers. The Event Horizon Telescope recently imaged one for the first time. Eventually, we’ll have a better understanding of what role SMBHs play in stellar formation in their galaxies. We may also come to a better understanding of how they merge, and how they grow to be so massive.

Loeb, too, is optimistic about the future. “We find surprisingly large ‘babies’ in the cosmic nurseries of black holes, but over the coming decades we will figure out who their parents were.”

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Evan Gough

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