UNIVERSITY PARK, Pa. — The growth of the biggest black holes in the universe is outrunning the growth rate of the galaxies that they inhabit, according to a new study led by researchers at Penn State.
For years, astronomers have been studying the formation of galaxies with supermassive black holes — those with millions to billions of times the mass of the Sun — at their centers. The prevailing theory suggests that black holes and their host galaxies grow roughly in tandem with each other.
Using data from NASA’s Chandra X-ray Observatory and other telescopes, the new study provides evidence that black holes in massive galaxies have grown much faster than those in less massive ones. The study will appear in the April 2018 issue of the journal Monthly Notices of the Royal Astronomical Society.
“We are trying to reconstruct a race that started billions of years ago,” said Guang Yang, graduate student in astronomy and astrophysics at Penn State and lead author of the study. “We are using extraordinary data taken from different telescopes to figure out how this cosmic competition unfolded.”
Using data from NASA's Chandra X-ray Observatory, the Hubble Space Telescope, and other observatories, Yang and his colleagues studied the growth rate of black holes in galaxies in the distant universe.
“This incredible amount of information, which included data from the Chandra Deep Field-South & North and the COSMOS-Legacy surveys,” said Yang, “allowed us to study the evolution of black holes and their host galaxies starting 12 billion years ago, when the universe was quite young.”
The researchers calculated the ratio between a supermassive black hole's growth rate and the growth rate of its host galaxy. This ratio had been generally thought to be approximately constant for all galaxies. Instead, Yang and colleagues found that this ratio is much higher for more massive galaxies.
"The story appears to be quite different for big and small galaxies," said Fabio Vito, postdoctoral researcher at Penn State and an author of the study. "In big galaxies, the black holes win the race against their hosts. But they lose in small galaxies."
“The black holes in big galaxies grow much faster than would have been expected compared to small galaxies,” said Yang. “For example, if a galaxy is ten times bigger than a second galaxy, then its black hole grows 100 times faster than the second galaxy’s black hole.”
“An obvious question is why,” said Niel Brandt, Verne M. Willaman Professor of Astronomy and Astrophysics and Professor of Physics at Penn State and an author of the paper. “Maybe massive galaxies are more effective at feeding cold gas to their central supermassive black holes than less-massive ones.”
The research team also predicted the accumulated black-hole mass at the current cosmic epoch — the time we are living now — by tracking the growth history of black holes.
Their prediction matched direct measurements for big galaxies, suggesting that the team’s predictions successfully recovered the entire cosmic history of black-hole growth.
"Even though the race has been going on for billions of years," said Chien-Ting Chen, postdoctoral researcher at Penn State and another author of the paper, "we can still watch the entire replay with nothing missed."
In addition to Yang, Vito, Brandt and Chen, the research team at Penn State includes Donald Schneider, department head and distinguished professor of astronomy and astrophysics. The research team also includes Jonathan Trump of the University of Connecticut; Bin Luo of Nanjing University in China; Mouyuan Sun, Yongquan Xue and Junxian Wang of the University of Science and Technology of China; Anton Koekemoer of the Space Telescope Science Institute; and Christian Vignali of the Università di Bologna in Italy. Many of the additional team members are former graduate students or postdoctoral researchers at Penn State.
This work is supported by the Chandra X-Ray Center, the Penn State ACIS Instrument Team, the National Natural Science Foundation of China, the Ministry of Science and Technology of China, the 973 Program, the CAS Frontier Science Key Research Program, and the Fundamental Research Funds for the Central Universities.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.