Combined X-ray surveys and supercomputer simulations trace 12 billion years of cosmic black hole growth

By combining leading X-ray observations with state-of-the-art supercomputer simulations of galaxy accretion throughout cosmic history, scientists have provided the best modeling yet of the growth of supermassive black holes found at the centers of galaxies. Using this hybrid approach, a research team led by Penn State astronomers derived a complete picture of the growth of black holes over 12 billion years, from the beginning of the universe at about 1.8 billion years old to the present at 13.8 billion years old.

The research consists of two articles, one published in The Astrophysical Journaland one not yet published to be submitted to the same journal. The results will be presented at the 244th meeting of the American Astronomical Society, June 9-13 at the Monona Terrace Convention Center in Madison, Wisconsin.

“Supermassive black holes at the centers of galaxies have millions to billions of times the mass of the Sun,” said Fan Zou, a graduate student at Penn State and first author of the paper. “How do they become such monsters? That’s a question astronomers have been studying for decades, but it’s been difficult to track all the ways black holes can reliably grow.”

Supermassive black holes grow through a combination of two main channels. They consume cold gas from their host galaxy – a process called accretion – and can merge with other supermassive black holes when the galaxies collide.

“During the process of consuming gas from their host galaxies, black holes emit strong X-rays, and this is key to tracking their accretion growth,” said W. Niel Brandt, Eberly Family Professor of Astronomy and Astrophysics and Professor of Physics at Penn State and leader of the research team. “We measured accretion-driven growth using X-ray sky survey data collected over more than 20 years from three of the most powerful X-ray instruments ever launched into space.”

The research team used additional data from NASA’s Chandra X-ray Observatory, the European Space Agency’s X-ray Multi-Mirror Newton (XMM-Newton) mission, and the Max Planck Institute for Extraterrestrial Physics’ eROSITA telescope. In total, they measured accretion-driven growth in a sample of 1.3 million galaxies that contained over 8,000 rapidly accreting black holes.

“All the galaxies and black holes in our sample are very well characterized at multiple wavelengths, with excellent measurements in the infrared, optical, ultraviolet and X-ray bands,” said Zou. “This allows for robust conclusions, and the data show that at all cosmic epochs, more massive galaxies grew their black holes faster by accretion. Thanks to the quality of the data, we were able to quantify this important phenomenon much better than in past work.” .”

The second way supermassive black holes grow is through mergers, where two supermassive black holes collide and merge together to form a single, even more massive black hole. To track the growth of mergers, the team used IllustrisTNG, a set of supercomputer simulations that model the formation, evolution and merger of galaxies from shortly after the Big Bang to the present.

“In our hybrid approach, we combine observed accretion growth with simulated growth through mergers to reproduce the growth history of supermassive black holes,” Brandt said. “With this new approach, we believe we have created the most realistic picture of the growth of supermassive black holes to date.”

Scientists have found that in most cases of black hole growth, accretion dominates. Mergers have produced remarkable secondary contributions, especially during the last 5 billion years of cosmic time for the most massive black holes. All in all, supermassive black holes of all masses grew much faster when the universe was younger. For this reason, the total number of supermassive black holes was almost established 7 billion years ago, while many new ones appeared in the universe earlier.

“With our approach, we can track how central black holes in the local universe most likely grew over cosmic time,” Zou said. “As an example, we considered the growth of the supermassive black hole at the center of our Milky Way galaxy, which has a mass of 4 million solar masses. Our results suggest that our galaxy’s black hole most likely grew relatively late in cosmic time.” “

In addition to Zou and Brandt, the research team includes Zhibo Yu, a graduate student at Penn State; Hyungsuk Tak, assistant professor of statistics and astronomy and astrophysics at Penn State; Elena Gallo at the University of Michigan; Bin Luo at Nanjing University in China; Qingling Ni at the Max Planck Institute for Extraterrestrial Physics in Germany; Yongquan Xue at the University of Science and Technology of China; and Guang Yang at the University of Groningen in the Netherlands.