What drives the galaxies? The Milky Way’s black hole could be the key

May 12 in 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, as impressive as it was, the painstakingly produced image of the ring of light around our galaxy’s central well of darkness seemed to simply prove what experts already expected: the Milky Way’s supermassive black hole exists, is spinning. and obeys the theory of general relativity.

Yet, upon closer inspection, things don’t stack up.

From the brightness of the bagel of light, the researchers estimated the rate at which matter falls on Sagittarius A *, the name given to the central black hole of the Milky Way. The answer is: not fast for nothing. “It’s clogged up to a small trickle,” said Priya Natarajan, a cosmologist at Yale University, comparing the galaxy to a broken shower head. Somehow only one thousandth of the matter flowing into the Milky Way from the surrounding intergalactic medium reaches all the way down and into the hole. “This is turning out to be a huge problem,” Natarajan said. “Where is this gas going? What’s happening to the flow? It is very clear that our understanding of black hole growth is suspect. “

Over the past quarter century, astrophysicists have come to recognize what a close dynamic relationship exists between many galaxies and the black holes at their centers. “There has 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 modelers and controllers of the evolution of galaxies.”

These giant holes – concentrations of matter so dense that gravity prevents even light from escaping – are like the motors of galaxies, but researchers are only beginning to understand how they work. Gravity draws dust and gas towards the galactic center, where it forms a swirling accretion disk around the supermassive black hole, heating up and turning into incandescent plasma. Then, when the black hole swallows this matter (in drops and discharges or sudden bursts), the energy is spit back into the galaxy in a feedback process. “When you grow a black hole, you produce energy and discharge it into the surrounding environment more efficiently than 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 across the galaxy.

But researchers have only vague ideas about the “active” episodes of supermassive black holes, which transform them into so-called active galactic nuclei (AGN). “What is the activation mechanism? What is the off switch? These are the fundamental questions we are still trying to come up with, ”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 to small-scale AGN feedback. These star motors are easily large enough to regulate small “dwarf” galaxies, while only the giant motors of supermassive black holes can dominate the evolution of larger “elliptical” galaxies.

In terms of size, the Milky Way, a typical spiral galaxy, lies in the middle. With few obvious signs of activity at the center, our galaxy was long thought to be dominated by stellar feedback. But several recent observations suggest that AGN’s feedback also shapes it. By studying the details of the interplay between these feedback mechanisms in our natal galaxy and addressing puzzles such as the current darkness of Sagittarius A *, astrophysicists hope to understand how galaxies and black holes co-evolve in general. The Milky Way “is becoming the most powerful astrophysics laboratory,” Natarajan said. Acting as a microcosm, “it can hold the key”.

Galactic Engines

In the late 1990s, astronomers generally accepted the presence of black holes in the centers of galaxies. At that point they could see close enough to these invisible objects to deduce their mass from the movements of the stars around them. A strange correlation has emerged: the more massive a galaxy, 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, however large it is, is a small fraction of the size of the galaxy. (Sagittarius A * weighs about 4 million suns, for example, while the Milky Way measures about 1.5 trillion solar masses.) For this reason, the black hole’s gravity only strongly attracts the innermost region of the galaxy.

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