But several recent observations suggest that AGN feedback shapes it as well. With few obvious signs of activity at its center, our galaxy was long thought to be dominated by stellar feedback. Size-wise, the Milky Way, a typical spiral galaxy, sits in the middle. 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. Stellar feedback, which occurs when a star explodes as a supernova, is known to have similar effects as AGN feedback on a smaller scale. “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. This feedback affects star formation rates and gas flow patterns throughout the galaxy.īut researchers have only vague ideas about supermassive black holes’ “active” episodes, which turn them into so-called active galactic nuclei (AGNs). “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. 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. 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. 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. “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. 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.Īnd yet, on closer inspection, things don’t quite stack up.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. 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.
0 Comments
Leave a Reply. |