For years, astronomers have been puzzled by why the biggest black holes in the universe have been growing much more slowly over the past 10 billion years. Now, a new study offers a potential solution to this astrophysical enigma: They're starved for gas.

Supermassive black holes (SMBHs) have immense gravitational appetites that allowed them to grow to many millions or billions of times the mass of the sun at surprisingly rapid rates in the first few billion years after the Big Bang. However, SMBHs have been growing ever more slowly since the period known as "cosmic noon," when the universe was less than a quarter of its current age.

The actual reason may be that there's simply less material for them to munch on, scientists suggested in a paper published Dec. 17 in The Astrophysical Journal.

"We knew black holes were growing more slowly, but not why — and it turned out to be that individual black holes are consuming material much less rapidly, rather than there simply being fewer growing black holes or smaller ones," study co-author Fan Zou, an astronomer at the University of Michigan, told Live Science via email.

Beefy black holes on a diet

Studying the growth of black holes is crucial for understanding galactic evolution and star birth, because SMBHs and their host galaxies evolve in a coordinated manner. SMBH sizes also correlate with the total mass of stars and their chaotic movements in a galaxy's bulge — the football-shaped central region where stars are densely packed.

To measure how black hole growth has changed throughout cosmic time, the researchers utilized data from nine extragalactic surveys collected in a "wedding cake" design. These tiered layers include shallow surveys of large, relatively nearby regions of the sky, as well as extremely deep "pencil-beam" looks at smaller fields, gathered from the world's premier X-ray-based space telescopes, including NASA's Chandra X-ray Observatory, the European Space Agency's XMM-Newton and the German-Russian eROSITA.

"X-ray light is arguably the best tracer of black hole growth," lead author Zhibo Yu, an astronomer at Penn State, told Live Science via email. "It is ubiquitously produced by growing supermassive black holes and has high contrast compared to the background star light. It also has high-penetrating power — that's why it's commonly used in medical imaging — so that it is less affected by the obscuring gas and dust in the galaxy."

Altogether, the researchers analyzed multiwavelength observations of 1.3 million galaxies and 8,000 actively feeding, X-ray-spewing SMBHs to determine why the black holes' growth rate has plummeted.

Have black holes had their heyday?

The research tested three main ideas. For example, are the black holes in the modern universe gobbling less matter? Alternatively, are they simply smaller and, therefore, less gravitationally gluttonous than their ancient predecessors? Or are there fewer actively growing black holes overall?

The researchers concluded that the black holes' consumption has slackened as the amount of cold gas for them to gobble has decreased since cosmic noon, approximately 10 billion years ago. "What surprised me most was that we could actually isolate the main reason, and there is indeed a dominating reason instead of a messy mix," Zou explained.

This decrease in growth rates appears to be significant. "Our best estimate is that the decrease is a factor of 22," study co-author Neil Brandt, an astrophysicist at Penn State, told Live Science via email. Although this study did not address the bafflingly fast black hole growth in the very early universe, "it does the best job so far of [addressing] the final 75% of cosmic time — a large majority!"

Future work may focus on additional datasets, such as wide-field X-ray surveys from Chandra and XMM-Newton, as well as multiwavelength data from other observatories. As a result, astronomers will be able to uncover larger SMBH populations, including even older examples and those that are obscured by dense dust and gas.

Finally, the research further confirms that the eras of rampant SMBHs are behind us. "We do not expect many SMBHs to emerge and significantly grow in the future," Zou said. "Actually, we found in 2024 that the number of SMBHs was almost settled by 7 billion years ago — and will likely continue to be so in the future."

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Ivan is a long-time writer who loves learning about technology, history, culture, and just about every major “ology” from “anthro” to “zoo.” Ivan also dabbles in internet comedy, marketing materials, and industry insight articles. An exercise science major, when Ivan isn’t staring at a book or screen he’s probably out in nature or lifting progressively heftier things off the ground. Ivan was born in sunny Romania and now resides in even-sunnier California.