What Drives Galaxies? The Milky Way’s Black Hole May Be the Key

On May 12, at nine simultaneous press conferences around the world, astrophysicists revealed the first image of the black hole at the heart of the Milky Way. At first, awesome though it was, the painstakingly produced image of the ring of light around our galaxy’s central pit of darkness seemed to merely prove what experts already expected: The Milky Way’s supermassive black hole exists, it is spinning, and it obeys Albert Einstein’s general theory of relativity.

And yet, on closer inspection, things don’t quite stack up.

From the brightness of the bagel of light, researchers have estimated how quickly matter is falling onto Sagittarius A*—the name given to the Milky Way’s central black hole. The answer is: not quickly at all. “It’s clogged up to a little trickle,” said Priya Natarajan, a cosmologist at Yale University, comparing the galaxy to a broken showerhead. Somehow only a thousandth of the matter that’s flowing into the Milky Way from the surrounding intergalactic medium makes it all the way down and into the hole. “That’s revealing a huge problem,” Natarajan said. “Where is this gas going? What is happening to the flow? It’s very clear that our understanding of black hole growth is suspect.”

Over the past quarter century, astrophysicists have come to recognize what a tight-knit, dynamic relationship exists between many galaxies and the black holes at their centers. “There’s been a really huge transition in the field,” says Ramesh Narayan, a theoretical astrophysicist at Harvard University. “The surprise was that black holes are important as shapers and controllers of how galaxies evolve.”

These giant holes—concentrations of matter so dense that gravity prevents even light from escaping—are like the engines of galaxies, but researchers are only beginning to understand how they operate. Gravity draws dust and gas inward to the galactic center, where it forms a swirling accretion disk around the supermassive black hole, heating up and turning into white-hot plasma. Then, when the black hole engulfs this matter (either in dribs and drabs or in sudden bursts), energy is spat back out into the galaxy in a feedback process. “When you grow a black hole, you are producing energy and dumping it into the surroundings more efficiently than through any other process we know of in nature,” said Eliot Quataert, a theoretical astrophysicist at Princeton University. This feedback affects star formation rates and gas flow patterns throughout the galaxy.

But researchers have only vague ideas about supermassive black holes’ “active” episodes, which turn them into so-called active galactic nuclei (AGNs). “What is the triggering mechanism? What is the off switch? These are the fundamental questions that we’re still trying to get at,” said Kirsten Hall of the Harvard-Smithsonian Center for Astrophysics.

Stellar feedback, which occurs when a star explodes as a supernova, is known to have similar effects as AGN feedback on a smaller scale. These stellar engines are easily big enough to regulate small “dwarf” galaxies, whereas only the giant engines of supermassive black holes can dominate the evolution of the largest “elliptical” galaxies.

Size-wise, the Milky Way, a typical spiral galaxy, sits in the middle. With few obvious signs of activity at its center, our galaxy was long thought to be dominated by stellar feedback. But several recent observations suggest that AGN feedback shapes it as well. By studying the details of the interplay between these feedback mechanisms in our home galaxy—and grappling with puzzles like the current dimness of Sagittarius A*—astrophysicists hope to figure out how galaxies and black holes coevolve in general. The Milky Way “is becoming the most powerful astrophysical laboratory,” said Natarajan. By serving as a microcosm, it “may hold the key.”

Galactic Engines

By the late 1990s, astronomers generally accepted the presence of black holes in galaxies’ centers. By then they could see close enough to these invisible objects to deduce their mass from the movements of stars around them. A strange correlation emerged: The more massive a galaxy is, the heavier its central black hole. “This was particularly tight, and it was totally revolutionary. Somehow the black hole is talking to the galaxy,” said Tiziana Di Matteo, an astrophysicist at Carnegie Mellon University.

The correlation is surprising when you consider that the black hole—big as it is—is a scant fraction of the galaxy’s size. (Sagittarius A* weighs roughly 4 million suns, for instance, while the Milky Way measures some 1.5 trillion solar masses.) Because of this, the black hole’s gravity only pulls with any strength on the innermost region of the galaxy.

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