What moves galaxies? The Milky Way’s black hole may be the key

on May 12 At nine simultaneous press conferences around the world, astrophysicists have revealed the first image of a black hole at the heart of the Milky Way. At first, as impressive as the painstakingly produced image of the ring of light around our galaxy’s central dark hole seemed to prove just what experts had already predicted: There is a supermassive black hole of the Milky Way, rotating, and subdued by Albert Einstein. General theory of relativity.

However, upon closer inspection, things don’t quite stack up.

From the brightness of the cake of light, the researchers estimated how fast the material would fall on Sagittarius A* – the name given to the Milky Way’s central black hole. Answer: Not at all fast. “I got a little clogged up,” Yale cosmologist Priya Natarajan said, comparing the galaxy to a broken showerhead. Somehow, only a thousandth of the matter flowing into the Milky Way from the surrounding galactic medium makes it all the way down and into the crater. “This reveals a big problem,” Natarajan said. “Where does this gas go? What happens to the stream? It is very clear that our understanding of black hole growth is questionable.”

Over the past quarter century, astrophysicists have realized what a dynamic, coherent relationship that exists between many galaxies and the black holes at their centers. “There’s been a really big shift in the field,” says Ramesh Narayan, a theoretical astrophysicist at Harvard University. “The surprise was that black holes are important as both modulators and controllers of how galaxies evolve.”

These giant holes – concentrations of matter so dense that gravity prevents even light from escaping – are like galactic motors, but researchers are only just beginning to understand how they work. Gravity pulls dust and gas inward to the galactic center, where a swirling accretion disk forms around the supermassive black hole, heating up and turning into hot white plasma. Then, when this material is swallowed up by the black hole (either in droplets or in sudden bursts), the energy is spit back into the galaxy in a feedback process. “When you grow a black hole, you produce energy and release it into the surrounding environment more efficiently than any other process we know of in nature,” said Elliott Quatert, a theoretical astrophysicist at Princeton University. This feedback affects the rates of star formation and gas flow patterns throughout the galaxy.

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

Stellar reactions, which occur when a star explodes as a supernova, are known to have effects similar to those of active galactic nuclei on a smaller scale. These stellar engines are large enough to organize small “dwarf” galaxies, while only giant drives for supermassive black holes are able to control the evolution of the largest “elliptical” galaxies.

In terms of size, the Milky Way, a typical spiral galaxy, lies in the middle. With few clear signs of activity at its center, our galaxy has long been thought to be dominated by stellar feedback. But several recent observations indicate that reactions of active galactic nuclei also shape it. By studying the details of the interaction between these feedback mechanisms in our home galaxy—and grappling with puzzles like Sagittarius A*’s current grimace—Castroian physicists hope to learn how galaxies and black holes in general evolve. The Milky Way “has become the most powerful astrophysical laboratory,” Natarajan said. By presenting it as a microcosm, he “might hold the key”.

Galaxy drives

By the late 1990s, astronomers generally accepted the existence of black holes at the centers of galaxies. By then, they could see close to these invisible objects to elicit their mass from the motions of the stars around them. A strange correlation has arisen: the more massive the galaxy, the larger its central black hole. “This was particularly narrow, and it was quite revolutionary. In a way, the black hole is talking to the galaxy,” said Tiziana Di Matteo, an astrophysicist at Carnegie Mellon University.

The correlation is amazing when you consider that a black hole – as big as it is – is a tiny fraction of the size of a galaxy. (Sagittarius A* weighs about 4 million suns, for example, while the Milky Way measures about 1.5 trillion solar masses.) Because of this, the gravity of a black hole pulls only with any force on the innermost region of the galaxy.

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