Galactic Regulators

You might already know that the gravitational pull of black holes is so extreme that not even light can escape it. But new research suggests that black holes do more than just gobble up any star that passes by: they emit energy, and this energy is regulating the creation of stars and planets.

Caleb Scharf, an expert in astrophysics and astrobiology at Columbia University and author of Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos (published in August), outlined the new view of black holes in a talk last month at the New York Academy of Sciences. The audience, which ranged from students in T-shirts and jeans to balding men in business suits, listened intently as Scharf described how matter is ripped apart as it is pulled into black holes, and how the energy from this tumult shoots into the cosmos as sub-atomic particles.

“A pet black hole would solve our energy needs,” Scharf said, since the conversion of matter to energy in a black hole is fifty times more efficient than nuclear fusion.

In most galaxies, the mass of stars outweighs the mass of the black hole by a 1,000 to one ratio. “The very nature of galaxies and the number of stars they contain is intimately linked to black holes,” Scharf told the audience. By spewing out energy, black holes are acting as galactic regulators, “keeping things in balance.”

Using a striking image of a galaxy known as Perseus A, Scharf illustrated what energy does once it is sent back into space. Perseus A, like many other galaxies, has a black hole at its center. The galaxy appeared on the projector screen behind Scharf as a grainy shot of what looked like a swirl of green dust splotched with dark patches. These dark patches represent “cavities produced by stuff squirted from the black hole,” said Scharf, comparing the process to blowing bubbles through a straw.

The bubbles represented by Perseus A’s cavities indicate trillions of trillions of watts of energy pushed out by its black hole, he explained.

By sending ripples through the extremely hot gas that surrounds Perseus A, gas that would otherwise cool down and build stars, the black hole is regulating how many stars the galaxy can make and how large they can become.

“The ripples transfer energy into the surrounding gas and help slow its ability to cool down,” Scharf said in an email afterwards. The estimated amount of energy in these ripples “is just right to explain the actual number of stars we see forming at any given time, which is much smaller than it would be otherwise.”

Tod Lauer, an astronomer at the National Optical Astronomy Observatory in Tucson, Arizona, agreed that without accounting for feedback from black holes, “we don’t get galaxies like what we see today.”

The regulatory role that black holes play may have broad implications for life in the universe, according to Scharf. If black holes influence the formation of stars in galaxies, they indirectly influence us. Stars, Scharf explained, produce elements that in turn produce planets, chemistry, and “at least in one place, life.”

In this one place, our solar system, we have a sun that is larger than 75 percent of other stars in the Milky Way galaxy. Its size means that it produces more ultraviolet light, which can split molecules and encourage what Scharf called “complex chemistry,” influencing the composition of planets that can form. Life and the geochemical nature of our planet have always been linked, he added.

Black holes, Scharf said at the close of his lecture, influence the proportions of large and small stars, and the life they can produce.

Scientists are just beginning to explore the mechanics of these galactic regulators, said National Optical Astronomy Observatory’s Lauer. “It’s frontier research right now to understand how this works.”