Roaming black holes may tell the hidden history of galaxies

If you want to understand the origins of black holes, Yale astronomers say, you need to look beyond the centers of galaxies and start searching in the nooks and crannies. That’s where you’ll find the “wanderers”—black holes whose journeys may tell the story of how the first black holes formed.

In a new study published in The Astrophysical Journal Letters, astronomers Emma Jane Weller and Priyamvada Natarajan from Yale and Colin Burke from the University of North Texas found that black holes—which are normally located at the centers of galaxies—can be displaced by galactic mergers and wander far from their original locations for billions of years. This process is even more pronounced in smaller galaxies, where there is less gravity pulling a wandering black hole toward the center.

“Our results show that considering wandering black holes, in addition to centered black holes, is essential for understanding the origins and dynamics of massive black holes and the histories of their host galaxies,” said Weller, who led the research and is an astronomy student in Yale’s Graduate School of Arts and Sciences.

Seeds and galactic histories

The earliest black holes are thought to have originated from “seeds” in the young universe. Some theories propose that the first stars in the universe left behind relatively “light” seeds (smaller black holes); other theories propose that “heavy” seeds (larger black holes) formed even earlier, via processes such as the direct collapse of pristine gas.

Natarajan, the Joseph S. and Sophia S. Fruton Professor of Astronomy and professor of physics in Yale’s Faculty of Arts and Sciences, and Weller’s thesis adviser, is a leading proponent of the “heavy seeds” theory.

“What is exciting about our findings is that black holes seem to remember more than the circumstances of their birth,” Natarajan said. “Their present-day locations carry the imprint of everything that has happened to their host galaxies. By separating black holes at galactic centers from those that are wandering, we can begin to disentangle these two histories.”

Tracking black holes beyond galactic centers

For the study, the researchers used ASTRID, a cosmological simulation that follows the evolution of dark matter, gas, stars and black holes. The team examined galaxies with stellar masses ranging from 10 million to 1 trillion times the mass of the sun, tracing their evolution across approximately 12.6 billion years.

One important feature of the ASTRID simulation is that it does not artificially pin black holes to the centers of their galaxies. Instead, it models the gravitational forces that cause black holes to move through their surroundings and gradually sink toward their galactic centers. This enabled the researchers to distinguish between centered black holes and wanderers.

“Some of the universe’s most revealing black holes may be the ones that have wandered away,” Natarajan said.

Small galaxies retain telling clues

Indeed, for low-mass galaxies in their simulation, the researchers found that the number of galaxies with a wandering black hole increased, while the number of galaxies with a centered black hole decreased.

The ASTRID simulation pointed to other intriguing possibilities. It suggests that low-mass galaxies retain information about their initial “seed” population, even after billions of years of growth and mergers. It also suggests that low-mass galaxies that have stopped forming stars are more likely to contain centered black holes, while star-forming galaxies are more likely to host wanderers.

“Black holes are remarkable cosmic archivists,” Natarajan said. “Their abundance tells us something about how they were born, while their locations preserve a record of how their galaxies were assembled.”

Multiple searches for wandering black holes

The findings offer a framework for a more comprehensive approach to finding and understanding the origin of black holes, the researchers say. It’s an approach that would include deep X-ray observations, optical and infrared spectroscopy, radio measurements and searches for brief “flares” caused by stars being torn apart by black holes.

“Each of these methods can probe different parts of the population,” Weller said. “By combining observations with the results from simulations, we may be able to identify the imprints of black hole formation and galaxy evolution.”

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Lisa Lock

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Robert Egan

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