Then, Springel said, “we had the idea to augment our galaxy mergers with supermassive black holes in the center. Whatever was switching off star formation hadn’t been captured in their computer model. But every time the team ran their simulation, it spat out ellipticals that glowed blue. “They are red and dead,” said Springel, of the Max Planck Institute for Astrophysics in Garching, Germany. While spiral galaxies like the Milky Way have many young stars that glow blue, giant elliptical galaxies only contain very old stars that glow red. In the simulations, she and her colleagues could re-create these large featureless blobs, called elliptical galaxies, by merging spiral galaxies many times. Galaxies grow in size and variety until, after enough collisions, they become big and smooth. Wind the clock forward and gravity smashes these dwarfs together in a blaze of spectacular mergers, forming rings, whirlpools, cigars, and every shape in between. They knew the basic picture: Galaxies start out small and dense in the early universe. “We wanted to reproduce the amazing zoo of galaxies that we see in the real universe,” Di Matteo said. “Everyone had to do it very cautiously and gingerly as it was so radical.”Ĭonfirmation of the feedback idea came a few years later, from computer simulations developed by Di Matteo and the astrophysicists Volker Springel and Lars Hernquist. “When I was doing my thesis, we were all obsessed with black holes as a point of no return-just gas going in,” said Natarajan, who helped develop the first AGN feedback models as Rees’ graduate student. Yet few astrophysicists were convinced that the energy of infalling matter could be ejected in such a dramatic way. Or, in Di Matteo’s words, “the black hole eats and then swallows.” A very big galaxy puts more weight on the central black hole, making it harder to blow gas outward, and so the black hole grows bigger before it swallows. In a hand-wavy way, that was the reasoning,” said Rees. Eventually, the outward pressure stops gas from falling into the black hole. The idea was that the more matter a black hole swallows, the brighter it gets, and the increased energy and momentum blows gas outward. The first-ever image of Sagittarius A*, the Milky Way’s supermassive black hole, was taken by a global consortium of radio telescopes known as the Event Horizon Telescope. “The surprise was that black holes are important as shapers and controllers of how galaxies evolve.” “There’s been a really huge transition in the field,” says Ramesh Narayan, a theoretical astrophysicist at Harvard University. 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. “Where is this gas going? What is happening to the flow? It’s very clear that our understanding of black hole growth is suspect.” “That’s revealing a huge problem,” Natarajan said. 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. “It’s clogged up to a little trickle,” said Priya Natarajan, a cosmologist at Yale University, comparing the galaxy to a broken showerhead. Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.įrom 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.
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