(CTN News) – Astronomers used X-ray data and complex supercomputer models to study the evolution of supermassive black holes over 12 billion years of cosmic history.
Scientists discovered that the black hole at the center of the Milky Way galaxy reached its four million solar masses relatively late in its lifetime.
Astronomers have struggled to understand the beginnings of supermassive black holes, which can be millions or billions of times more massive than our sun.
Astronomers Fan Zou and W. Niel Brandt, both from Penn State University, have now headed a team that has used observations and simulations to connect the two pathways of black hole formation. The results may finally provide some answers.
Zou presented their study at the 244th meeting of the American Astronomical Society in Wisconsin, raising the topic of how supermassive black holes evolve to such large sizes. “To address that, we need to track the overall growth history of these supermassive black holes.”
As previously stated, black holes grow via two basic mechanisms. The accretion of cold gas from their home galaxy is what causes one. This gas creates an accretion disc around the black hole, and stuff from it spirals towards its core.
The accretion disc can become so dense that friction between gas molecules heats it to millions of degrees, emitting X-rays. The other mechanism occurs when galaxies collide. When galaxies merge, their supermassive black holes fuse, causing a surge of gravitational waves.
The study team analyzed archival data from NASA’s Chandra X-ray Observatory, the European Space Agency’s XMM-Newton mission, and the eROSITA X-ray instrument on the joint German-Russian Spektr-RG spacecraft to determine the role of gas accretion in supermassive black hole growth.
The researchers were able to spot X-ray emissions coming from approximately 8,000 rapidly developing supermassive black holes.
“When supermassive black holes accrete the surrounding gas they emit strong X-rays, so by detecting them in the X-ray bands we can measure their accretion power,” Zou said.
Astronomers Supermassive black hole stories
They used the IllustrisTNG cosmological supercomputer simulation to model galaxy mergers throughout cosmic history. The scientists used X-ray data and simulated mergers to study the evolution of supermassive black holes across 12 billion years, from 1.8 billion years after the Big Bang to now.
These computer models “capture the overall large-scale structure [of the universe] but are also able to probe individual galaxies,” Zou stated.
According to Zou and Brandt, X-ray data reveal that accretion has been the primary driver of black hole expansion throughout cosmic history.
Furthermore, the more massive the galaxy, the faster its supermassive black hole develops through accretion. Simulations indicate mergers may impact black hole growth but are not the primary cause.
“Accretion dominates the supermassive black-hole growth in most cases, and mergers make some notable secondary contributions,” Zou stated.
These findings also indicate that supermassive black holes expanded rapidly earlier in the universe, with new ones often forming.
However, by roughly 7 billion years ago, the total number of supermassive black holes had remained rather stable, with few new ones developing. Mergers had a greater impact in later history, peaking in relevance to black-hole growth 4 billion years ago.
“We found that once the universe reaches about 40% of its age, the overall demography of supermassive black holes is very similar to the demography of supermassive black holes that we see in the local universe,” Zou stated.
Astronomers modeled our galaxy’s black hole, Sagittarius A*, and found that most of its mass formed later in cosmic time. The Milky Way’s development was primarily due to accretion, with most mergers with other galaxies occurring over 8–10 billion years ago.
The European Space Agency’s Gaia mission discovered evidence of a dwarf galaxy colliding with the Milky Way 2-3 billion years ago.
Dwarf galaxies are thought to contain intermediate-mass black holes that are tens to hundreds of thousands of times the mass of our sun, and one may combine with Sagittarius A* to increase our black hole’s mass.
Because the data only go back 1.8 billion years after the Big Bang, they don’t explain how the seeds of supermassive black holes arose. Cosmologists face a problem, as the Hubble Space Telescope and James Webb Space Telescope have discovered enormous black holes earlier in the universe.
It’s unclear how they became millions of times the mass of our sun in less than a billion years.
A study on the findings was published in March in The Astrophysical Journal, and a second paper is currently being prepared.
