Astronomers have an answer for a long-running mystery in astrophysics: Why is there much less growth of supermassive black holes today than in the past?
Study: The Drivers of the Decline in Supermassive Black Hole Growth at z<2 (DOI: 10.3847/1538-4357/ae173d)
Black holes just can't feast like they used to. That's according to a new study from a team of astronomers that included University of Michigan researchers.
Using NASA's Chandra X-ray Observatory and other X-ray telescopes, the team found that supermassive black holes are unable to consume material as rapidly as they did in the distant past. Researchers had known that black holes weren't growing as fast as they once had, but they were unable to pinpoint why before now.
"By combining these data from different X-ray telescopes, we can construct a better picture of how these black holes are growing than any one telescope could do alone," said Fan Zou, a U-M postdoctoral researcher in astronomy who co-authored the study. "We can find out why over ten billion years the growth of supermassive black holes has gone from hectic to leisurely to glacial."
Ten billion years ago, there was a period that astronomers call "cosmic noon." During this epoch, the growth of supermassive black holes-black holes with millions to billions of times the mass of the sun-was the highest it's been in the history of the universe. Between cosmic noon and now, however, astronomers have seen a major slowdown in how much black holes are growing.
"A longstanding mystery has been the cause of this big slowdown," said Zhibo Yu of Penn State University, lead author of the new study. "With these X-ray data and supporting observations at other wavelengths, we can test different ideas and narrow down the answer."
This study builds on previous work Zou did in 2024 during his transition from his doctoral studies at Penn State to his postdoctoral position at U-M. The earlier work asked how supermassive black holes got supermassive in the first place, while the new result provides a reason for why their growth rates have changed.
"This new work advances the previous result and gives us an answer to a very fundamental scientific question," Zou said.
The cold gas buffet closed after cosmic noon
When gas falls into a supermassive black hole, it heats up and produces large amounts of radiation including X-rays. For decades, Chandra and other X-ray telescopes have shown a decline in black hole growth by looking at these black holes at different distances across the universe. Crucially, black holes that are growing more quickly produce more X-rays.
By analyzing observations of about 1.3 million galaxies and 8,000 growing supermassive black holes, the team was able to isolate the 'why' behind this black hole slowdown. The team relied on data from Chandra, the European Space Agency's XMM-Newton and eROSITA, or the extended ROentgen Survey with an Imaging Telescope Array, which is a German and Russian mission.
"It appears that black holes' consumption of material has greatly slowed down as the universe has aged," said co-author Niel Brandt, also of Penn State University. "This is probably because the amount of cold gas available for them to ingest has decreased since cosmic noon."
In this study, researchers determined the brightness and mass of the black holes, and how many galaxies in the survey have X-ray sources, implying that they contain growing supermassive black holes. The team used a combination of surveys ranging from shallow surveys of large regions of the sky to extremely long studies of small fields. This ensemble is often visualized in tiered layers that form a "wedding cake" design.
In the observations, XMM-Newton and eROSITA provided the middle and bottom tiers with wider but shallower observations. Meanwhile, Chandra contributed the top tier with deep observations covering a relatively small area that allowed the detection of fainter and more distant growing black holes.
The team ran tests of the three main possible scenarios currently being considered for the slowdown of black hole growth. Namely, could the decline in black hole growth be caused by less efficient rates of consumption, by smaller typical black hole masses, or by fewer actively growing black holes?
The researchers' analysis of the data, extending over billions of years of cosmic history, led them to the conclusion that black holes are indeed consuming material less rapidly the later they are found after the big bang. The researchers expect this trend of slower-growing black holes to continue into the future.
A key challenge in this study is that both more massive black holes and faster-growing black holes produce brighter X-ray emission. Observations at other wavelengths, including optical and infrared data were used to estimate black hole masses and disentangle these two factors.
