Combined X-ray studies and supercomputer simulations track 12 billion years of cosmic black hole growth

By combining cutting-edge X-ray observations with cutting-edge computer simulations of galaxy formation over cosmic history, the researchers have provided the best modeling to date of the growth of supermassive black holes found at galaxies. Using this hybrid approach, a research team led by Penn State astronomers has compiled a comprehensive picture of the growth of black holes over 12 billion years, since the beginning of the universe, about 1.8 billion years ago, until today, 13.8 billion years old.

Two articles make up the research, one published in The Astrophysics Journal, and a still unpublished one which will be submitted to the same journal. The results will be presented at the 244th meeting of the American Astronomical Society, which will be held June 9-13 at the Monona Terrace Convention Center in Madison, Wisconsin.

“Supermassive black holes at the centers of galaxies have a mass millions to billions of times that of the sun,” said Fan Zou, a Penn State graduate student and first author of the papers. “How do they become such monsters? It’s a question astronomers have studied for decades, but it’s been difficult to keep track of 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 powerful X-rays, and this is the key to tracking their growth through accretion,” said W. Niel Brandt, professor of astronomy and astrophysics at the Eberly family chair and professor of astrophysics. physics at Penn State and leader of the research team. “We measured accretion-induced growth using X-ray sky survey data accumulated over more than 20 years from three of the most powerful X-ray facilities ever launched into space. “

The research team used complementary data from NASA’s Chandra X-ray Observatory, the European Space Agency’s X-ray Multi-Mirror Mission (XMM-Newton), and the NASA’s eROSITA telescope. Max Planck Institute for Extraterrestrial Physics. In total, they measured accretion-induced growth in a sample of 1.3 million galaxies containing more than 8,000 rapidly growing black holes.

“All the galaxies and black holes in our sample are very well characterized at multiple wavelengths, with superb measurements in the infrared, optical, ultraviolet and X-ray bands,” Zou said. “This allows us to draw strong conclusions and the data show that in all cosmic epochs, the most massive galaxies have grown their black holes by accretion more quickly. Thanks to the quality of the data, we were able to quantify this important phenomenon much better than in previous works.

The second way supermassive black holes grow is through mergers, where two supermassive black holes collide and merge to form a single, even more massive black hole. To track merger growth, 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 growth by accretion with simulated growth by merger to reproduce the growth history of supermassive black holes,” Brandt said. “With this new approach, we believe we have produced the most realistic picture of the growth of supermassive black holes to date.”

The researchers found that in most cases, accretion dominated the growth of black holes. Mergers have made notable secondary contributions, particularly during the last 5 billion years of cosmic time, for the most massive black holes. Overall, 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 settled 7 billion years ago, while previously in the universe many new holes continued to emerge.

“With our approach, we can track how the central black holes of the local universe likely expanded over cosmic time,” Zou said. “As an example, we considered the growth of the supermassive black hole at the center of our galaxy, the Milky Way, which has a mass of 4 million solar masses. Our results indicate that the black hole in our galaxy has grown probably developed 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 of the University of Michigan; Bin Luo of Nanjing University in China; Qingling Ni at the Max Planck Institute for Extraterrestrial Physics in Germany; Yongquan Xue of the University of Science and Technology of China; and Guang Yang at the University of Groningen in the Netherlands.