How Black Holes Could Nurture Life

At the center of most large galaxies, including our own Milky Way, sits a supermassive black hole. Interstellar gas periodically falls into the orbit of these bottomless pits, switching the black hole into active galactic nucleus (AGN)-mode, blasting high-energy radiation across the galaxy.

It's not an environment you'd expect a plant or animal to thrive in. But in a surprising new study in the Astrophysical Journal, researchers at Dartmouth and the University of Exeter show that AGN radiation can have a paradoxically nurturing effect on life. Rather than doom a species to oblivion, it can help assure its success.

The study may be the first to concretely measure, via computer simulations, how an AGN's ultraviolet radiation can transform a planet's atmosphere to help or hinder life. Consistent with studies looking at the effects of solar radiation, the researchers found that the benefits—or harms—depend on how close the planet is to the source of the radiation, and whether life has already gained a toe hold.

"Once life exists, and has oxygenated the atmosphere, the radiation becomes less devastating and possibly even a good thing," says Kendall Sippy '24, the lead author of the study. "Once that bridge is crossed, the planet becomes more resilient to UV radiation and protected from potential extinction events."

The researchers simulated the effects of AGN radiation on not only Earth, but Earth-like planets of varying atmospheric composition. If oxygen was already present, they found, the radiation would set off chemical reactions causing the planet's protective ozone layer to grow. The more oxygenated the atmosphere, the greater the effect.

High-energy light reacts readily with oxygen, splitting the molecule into single atoms that recombine to form ozone. As O3 builds up in the upper atmosphere, it deflects more and more dangerous radiation back into space. Earth owes its favorable climate to a similar process that happened about two billion years ago with the first oxygen-producing microbes.

Radiation from the sun helped Earth's fledgling life oxygenate, and add ozone, to the atmosphere. As our planet's protective ozone blanket thickened, it allowed life to flourish, producing more oxygen, and yet more ozone. Under the Gaia hypothesis, these beneficial feedback loops allowed complex life to emerge.

"If life can quickly oxygenate a planet's atmosphere, ozone can help regulate the atmosphere to favor the conditions life needs to grow," says study co-author Jake Eager-Nash, who is currently a postdoc at the University of Victoria. "Without a climate-regulating feedback mechanisms, life may die out fast."

Earth, in real life, is not close enough to its resident black hole, Sagittarius A, to feel its effects, even in AGN-mode. But the researchers wanted to see what could happen if Earth were much closer to a hypothetical AGN, and thus exposed to radiation billions of times greater.

Recreating Earth's oxygen-free atmosphere in the Archean, they found that the radiation would all but preclude life from developing. But as oxygen levels rose, nearing modern levels, Earth's ozone layer would grow and shield the ground below from dangerous radiation.

"With modern oxygen levels, this would take a few days, which would hopefully mean that life could survive," says Eager-Nash. "We were surprised by how quickly ozone levels would respond."

When they looked at what could happen on an Earth-like planet in an older galaxy, with stars clustered closer to its AGN, they found a much different picture. In a "red nugget relic" galaxy like NGC 1277, the effects would be lethal. Stars in more massive galaxies with an elliptical shape, like Messier-87, or our spiral Milky Way, are spread out more, and thus, farther from an AGN's dangerous radiation.

The stars align aboard the Queen Mary 2

Sippy came to Dartmouth with a keen interest in black holes, and by the end of second term, had joined the lab of Ryan Hickox, professor and chair of the Department of Physics and Astronomy. Later, while debating a potential senior project on AGN radiation, fate intervened.

Heading to England for a sabbatical in 2023, Hickox booked a trip on the Queen Mary 2 so he could bring his dog, Benjamin. Aboard the ship, he got to chatting with an astrophysicist from Exeter, Nathan Mayne, who was a guest speaker on the ship. They quickly realized they had a mutual interest in radiation, and that the PALEO software Mayne had been using to model solar radiation on exoplanet atmospheres could be applied to the more powerful rays of an AGN.

The encounter would clear the way for Sippy to work with Eager-Nash, then a PhD student in Mayne's lab. Using the programing language Julia, they input into their model the initial concentrations of oxygen, and other atmospheric gases, on their Earth-like planet.

"It models every chemical reaction that could take place," says Sippy. "It returns plots of how much radiation is hitting the surface at different wavelengths, and the concentration of each gas in your model atmosphere, at different points in time."

The feedback loop they discovered in an oxygenated atmosphere was unexpected. "Our collaborators don't work on black hole radiation so they were unfamiliar with the spectrum of a black hole and how much brighter an AGN could get than a star depending how close you are to it," says Hickox.

Without the kismet that brought the two labs together, the project might never have happened. "It's the kind of insight you can only really get by combining different sets of expertise," he adds.

After graduating from Dartmouth, Sippy left for Middlebury College, to work as a post-baccalaureate researcher in the lab of McKinley Brumback, Guarini PhD '20. Brumback had worked in Hickox's lab as a PhD student and is now an assistant professor of physics at Middlebury studying accreting neutron star X-ray binaries.

She brought a unique perspective to the project. In the X-ray binaries that she studies, a neutron star pulls matter from a normal star, causing in-falling material to heat up and emit X-rays.

While an AGN can take up to millions of years to flip between active and inactive states, X-ray binaries can change in mere days to months. "A lot of the same physics that applies to AGNs applies to X-ray binaries, but the time scales are much faster than for an AGN," she says.

McKinley contributed to the AGN analysis and served as a "slightly removed reader" to make sure the paper was accessible to non-experts, she says.

"Thanks to Kendall's excellent writing, it definitely was!"