Effect of radiation from black holes on life

 Do radiation from black holes could give Life a boost?    

Scientists at Dartmouth and the University of Exeter have discovered that radiation emitted by supermassive black holes can have a surprising, nurturing effect on life. According to NASA, supermassive black holes, often found at the heart of galaxies, are astronomical objects with a gravitational pull so strong that not even light can escape it that are so large they can "shape the evolution of galaxies and influence the formation of stars and planets." At the centre 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. 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 toehold.

But while they are normally thought of as a destructive force, it turns out that under the right conditions, they can actually help life grow and thrive. While pulling matter in, some active galactic nuclei (AGN) release ultraviolet (UV) radiation, which can reach nearby planets similarly to how the sun's radiation reaches Earth. The study published in The Astrophysical Journal explored via computer simulations how this UV radiation might transform a planet's atmosphere to help or hinder life. It's not an environment you'd expect a plant or animal to thrive in. But in a surprising recent study 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. "Once life exists, and has oxygenated the atmosphere, the radiation becomes less devastating and possibly even a good thing," says Kendall Sippy, 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 Earth, and on Earth-like planets of varying atmospheric composition. They found that when oxygen was already present, the radiation would set off chemical reactions causing the planet's protective ozone layer to grow. This is due to the way UV radiation reacts with oxygen, splitting the molecules into single atoms that recombine to form ozone, which deflects dangerous radiation back into space. The more oxygenated the atmosphere, the greater the effect. High-energy light reacts readily with oxygen, splitting the molecule into single atoms which 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 favourable climate to a similar process which happened about two billion years ago with the first oxygen-producing microbes.

Kendall Sippy, explains that once life exists, and has already oxygenated the atmosphere, radiation from AGNs becomes "less devastating and possibly even a good thing." However, for AGN radiation to be beneficial to the ozone, planets must be distant enough from the source of radiation. "Once that bridge is crossed, the planet becomes more resilient to UV radiation and protected from potential extinction events," she says. 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. 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.

"If life can quickly oxygenate a planet's atmosphere, ozone can help regulate the atmosphere to favour 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 mechanism, life may die out fast." The study looked at what might happen to Earth-like plants in older galaxies like "red nugget relics", where stars are all clustered near the black hole, and found out that in this case, the AGN radiation would be too strong and deadly for life. 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.

In bigger galaxies like our own, stars are spread farther apart, distant enough from the black hole to be safe from its dangerous radiation. Using the programming language Julia, researcher's 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." While Earth is too far from Sagittarius A*, the Milky Way's central supermassive black hole, to feel its effects, scientists involved in the study wanted to see what would happen to life on Earth if we were much closer to a hypothetical AGN, and so exposed to radiation billions of times greater. 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 on 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. They surprisingly found out that with modern oxygen levels, the ozone layer would grow and shield us from dangerous radiation in just a matter of days. Ozone is a gas made up of three oxygen atoms, which occurs naturally in small amounts in the stratosphere, protecting Earth from the sun's UV radiation, which would otherwise make the surface of our planet sterile. After graduating from Dartmouth, Sippy left for Middlebury College to work as a post-baccalaureate researcher in the lab of McKinley Brumback, Guarini Ph.D. Brumback had worked in Hickox's lab as a Ph.D. 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. Back in the 70s, scientists discovered that chemicals used in the production of items such as aerosol sprays, foam-blowing agents, solvents and refrigerants, had created a hole in the ozone over the Antarctic spring. In 1987, countries worldwide signed the Montreal Protocol, agreeing to phase out these harmful chemicals for the good of our planet. Since then, our ozone layer has been on a healing process, and scientists estimate it could fully recover by 2066.