Supermassive black holes are growing slower because they have less to consume

UNIVERSITY PARK, Pa. — Astronomers have an answer for a long-running mystery in astrophysics: Why are supermassive black holes today growing slower than in the past?

A team of astronomers, led by Penn State researchers, used NASA’s Chandra X-ray Observatory and other X-ray telescopes to find that supermassive black holes are unable to consume material as rapidly as they did in the distant past.

A paper describing these results appeared in the Astrophysical Journal.

Ten billion years ago, there was a period that astronomers call “cosmic noon,” when the growth of supermassive black holes — those with millions to billions of times the mass of the sun — was at its peak across the entire history of the universe. Between cosmic noon and now, however, astronomers studying black holes at different distances from Earth and representing different time periods across the history of the universe 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, a graduate student in astronomy and astrophysics in the Eberly College of Science at Penn State and lead author of the study. “With these X-ray data and supporting observations at other wavelengths, we can test different ideas and narrow down the answer.”

When gas falls into a supermassive black hole, it heats up and produces large amounts of radiation across the electromagnetic spectrum. This includes X-rays — a form of high-energy electromagnetic radiation used in medical imaging that is also generated by extreme astrophysical processes — and radiation at different energy levels, or wavelengths, such as visible or infrared light. Using the fact that black holes that are growing more quickly produce more X-rays, Chandra and other X-ray telescopes have shown for decades a decline in black hole growth by looking at black holes at different distances across the universe.

A key challenge in this study is that both more massive black holes and faster-growing black holes produce brighter X-ray emission. However, the researchers used observations at other wavelengths, including optical and infrared data, to estimate black hole masses and disentangle these two factors.

By analyzing observations of about 1.3 million galaxies and 8,000 growing supermassive black holes from Chandra, the European Space Agency’s XMM-Newton and the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) — a German and Russian-led telescope — the team isolated the “why” behind this black hole slowdown.

“It appears that black holes’ consumption of material has greatly slowed down as the universe has aged,” said co-author Niel Brandt, Eberly Family Chair Professor of Astronomy and Astrophysics and professor of physics at Penn State. “This is probably because the amount of cold gas available for them to ingest has decreased since cosmic noon.”

The 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. They accomplished this by using a combination of surveys ranging from shallow surveys of large regions of the sky to extremely long studies of small fields — an ensemble 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.

“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 co-author Fan Zou, a postdoctoral researcher at the University of Michigan, who earned a doctoral degree in astronomy and astrophysics at Penn State in 2024. “We can find out why over 10 billion years the growth of supermassive black holes has gone from hectic to leisurely to glacial.”

The team ran tests of the three main possible scenarios currently under consideration for the slowdown of black hole growth: Could the decline in black hole growth be caused by less efficient rates of consumption, or by smaller typical black hole masses, or by fewer actively growing black holes?

Their 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 observed after the Big Bang. The researchers said they expect this trend of slower-growing black holes to continue into the future.

NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

The Chandra X-ray Center, the XMM-Newton Program and the U.S. National Science Foundation supported the research.

Editor’s note: A version of this story originally appeared on the Chandra X-ray Center website.