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A supermassive black hole emits a plume of energetic particles in this image. Credit: NASA/JPL-Caltech

Near the center of the Milky Way sits a giant object that astronomers call Sagittarius A*. This “supermassive” black hole may have grown in tandem with our galaxy, and it’s not alone. Scientists suspect that similar stars lurk at the heart of nearly every large galaxy in the universe.

Some can get very large, said Joseph Simon, a postdoctoral fellow in the Department of Astrophysics and Planetary Sciences at the University of Colorado Boulder.

“The black hole at the center of our galaxy is millions of times the mass of the Sun, but we also see others that we think are billions of times the mass of the Sun,” he said.

The astrophysicist has devoted his career to studying the behavior of these hard-to-observe objects. In a recent study, he used computer models, or “models,” to predict the mass of the universe’s largest supermassive black holes—a mathematical concept called the black hole mass function.

In other words, Simon sought to determine what you might find if you could place each of these black holes one by one on a large scale.

His calculations suggest that billions of years ago, black holes may have been much larger on average than scientists once suspected. The findings could help researchers solve an even bigger mystery and clarify the forces that shaped objects like Sagittarius A* as they grew from small black holes into the giants they are today.

“We’re starting to see from various sources that there have been pretty big objects in the universe since quite early on,” Simon said.

He published his findings on May 30 in The Astrophysical Journal Letters.

Galactic symphony

For Simon, these “pretty big things” are his bread and butter.

The astrophysicist is part of another research effort called the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Through the project, Simon and hundreds of other scientists in the United States and Canada have spent 15 years searching for a phenomenon known as the “gravitational wave background.” The term refers to the constant flow of gravitational waves, or giant ripples in space and time, that oscillate through the universe on an almost constant basis.

This cosmic stream also owes its origin to supermassive black holes. Simon explained that if two galaxies collided in space, their central black holes could also collide and even merge. They spin around and around each other before they click together like two bells in an orchestra – only this bell crash produces gravitational waves, literally warping the fabric of the universe.

To understand the background of gravitational waves, however, scientists first need to know how massive the universe’s supermassive black holes really are. Bigger cymbals, Simon said, make more pops and produce much bigger gravity waves.

There’s just one problem.

“We have very good measurements of the mass of giant black holes for our own galaxy and for nearby galaxies,” he said. “We don’t have the same measurements for galaxies further away. We just have to guess.”

A black hole in the making

In his new research, Simon decided to guess in a completely new way.

First, he collected information on hundreds of thousands of galaxies, some billions of years old. (Light can only travel so fast, so when humans observe galaxies that are further away, they are looking back in time). Simon used this information to calculate the estimated black hole masses for the largest galaxies in the universe. He then used computer models to simulate the gravitational wave background that the galaxies might create and that now wash over Earth.

Simon’s findings reveal a smorgasbord of supermassive black hole mass in the universe dating back roughly 4 billion years. He also noticed something strange: There appeared to be many more large galaxies scattered around the universe billions of years ago than some previous studies have predicted. It didn’t make much sense.

“It has been expected that you would only see these very large systems in the nearby universe,” Simon said. “It takes time for a black hole to grow.”

However, his research adds to a growing body of evidence that suggests they may not take as much time as astrophysicists once thought. For example, the NANOGrav team has seen similar evidence of supermassive black holes lurking in the universe billions of years ago.

For now, Simon hopes to explore the entire spectrum of black holes stretching back even further in time – revealing clues about how the Milky Way, and ultimately our own solar system, came to be.

“Understanding the number of black holes is important for some of these fundamental questions like the background of gravitational waves, but also how galaxies grow and how our universe has evolved,” Simon said.

More information:
Joseph Simon, Exploring Proxies for the Supermassive Black Hole Mass Function: Implications for the Pulsar Timing Matrix, The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/acd18e

Diary information:
Astrophysical Journal Letters

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