At the center of the Milky Way lurks a shadowy giant, a supermassive black hole that draws in stars and gas on which it feasts. As with all black holes, this one is an overeager eater, surrounded by piled-up material it cannot immediately consume—an accretion disk, a glowing maelstrom of gas heated by friction as it swirls around the giant’s maw. Now, new observations are hinting at the presence of never-before-seen cooler regions of the disk surrounding the Milky Way’s central black hole, opening a new window on our galaxy’s dark heart.
Despite the presence of these so-called galactic nuclei in the middle of most galaxies—supermassive black holes remain a cosmic mystery. Because they possess gravitational forces strong enough to trap light, black holes are by definition things that cannot really be directly seen. To spot and study them, scientists must rely on the black holes’ gravitational interactions with other objects. Thanks to the great size and relative closeness of our own galaxy’s behemoth, astronomers can even glimpse just how its gravity guides flows of gas down its gullet.
“This is the first indication of accretion with a direction of rotation,” says Elena Murchikova, a theoretical astrophysicist at the Institute for Advanced Study in Princeton, N.J., and lead author of a study reporting the new results, published June 5 in the journal Nature. Murchikova and her colleagues used data gathered in 2015 by the Atacama Large Millimeter/submillimeter Array (ALMA) to discover the thin accretion disk of material extremely close to the black hole that is sending streams of material spiraling inward. Earlier studies had revealed a more distant disk of warm gas farther away from the black hole which may feed the newfound cooler disk.
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Although researchers were previously able to glimpse the disk of warm gas around the Milky Way’s black hole, they had never before been able to directly measure the disk’s rotation. “This is the first evidence for seeing the flow around a black hole directly,” says Roger Blandford, a co-author on the study who is noted for his contributions to black hole studies. Current models of how the Milky Way’s supermassive black hole, known as Sagittarius A*, consumes its gas rely on assumptions about the exact distribution of accretion flows, and none of them deal with cool gas, hindering our understanding of just how our galaxy’s shadowy giant gobbles its meals. By tracking the movement of cooler gas, the new research provides firmer, data-based footing for researchers hoping to make more daring theoretical leaps.
“This is amazing,” says Smadar Naoz, a theoretical astrophysicist at the University of California, Los Angeles, who was not part of the study. Naoz, who is part of UCLA’s Galactic Center Group, is intrigued by the origin of the cold gas and its potential interactions with stars and other objects in the galactic center. Naoz suspects that even deeper probing of the environment around Sagittarius A* could result in breakthroughs in our understanding of harder-to-observe supermassive black holes all across the universe. “If we can understand how things work together, maybe we can have a good picture of other galactic nuclei,” she says. Black holes in such nuclei are thought to help shape the overall architecture, composition and evolution of galaxies and galaxy clusters. Learning the intricacies of their behavior could unveil fundamental new details about how some of the universe’s largest and most important structures came to be.
Studying the flows of cool gas around Sagittarius A* might even resolve the enigma of G2, a strange object that scientists once predicted would be torn apart during a close encounter with the black hole in 2014. To their surprise, instead of being shredded by the black hole’s gravitational pull, G2 survived its perilous pass around the giant relatively unscathed, leading some researchers to posit it was a shrouded star rather than a cloud of dust. Naoz and other theorists can now try modeling G2 as either a star or a cloud as it interacts with the cool debris disk, perhaps finally arriving at an explanation for its bizarre behavior and determining its true nature. It is even possible, Naoz says, that G2 and other similar objects could be feeding the disk. “This is a theorist’s dream, a new puzzle that we can figure out,” she says.
A Controversial Result
Probing the region around the supermassive black hole is a challenge, and right now only ALMA is up to the task (other projects, such as the Event Horizon Telescope radio array, can also zoom in on Sagittarius A* but can not presently glimpse cool gas there). The radiation from the vicinity of Sagittarius A* is about 2,000 times brighter than the signal from the cool gas, Murchikova says. She compares it with photographing a candle held directly in front of the sun, then subtracting out the sunlight. “You need a really perfect camera for that,” she says. ALMA fits the bill.
In part because only one instrument can so thoroughly probe this elusive region, not everyone is convinced that Murchikova and her colleagues have actually spotted a cool disk. Reinhard Genzel, a veteran of black hole studies at the Max Planck Institute for Extraterrestrial Physics (MPE) who served as a referee on the paper, says he was “very intrigued” when he first saw the result because no one had previously managed to observe the rotation of gas so close to a black hole. And although ALMA provides incredible details as yet unmatched by other instruments, he questions whether the facility’s more recent observations would confirm the presence of the cool cloud. He says that no such confirmatory data has appeared. Similarly, Stefan Gillessen, an astronomer at MPE who led the team that discovered G2 in 2012, says the new observations complement those he and his colleagues have made of the galactic center since 2004. Yet Gillessen’s observations have revealed no sign of cold gas.
An absence of cold gas in Gillessen’s data should come as no surprise, Murchikova says. Although combining years of observations can sharpen the view of Sagittarius A*, such approaches are not guaranteed to tease out the presence of an exceedingly faint signal. “We had a project which nothing except ALMA can possibly do,” she says. The telescope has been upgraded and expanded since it saw first light in 2011, which could explain why earlier ALMA observations such as Gillessen’s could have missed the gas.
A cool gas disk circling the center of the galaxy should also affect its surroundings in distinct, discernible ways. For instance, Gillessen says that such a disk would most likely have impacted G2’s traverse through the galactic center. Although the object has slowed somewhat, it has not shown signs of colliding with or crashing through an unseen disk of gas, he says. Geoff Bower, an astronomer at Academia Sinica Institute of Astronomy and Astrophysics in Hawaii who used ALMA and other instruments to study G2 in 2013, also says his team saw no evidence of the object interacting with cold gas.
Naoz, however, is not convinced that the cloud would have had a strong effect on G2. The disk is very diluted, with a mass between 10 and a 100,000 times less than that of our sun spread across an enormous volume of space around the black hole. Consequently, she says, it might not be able to affect G2 much at all. “It’s like shooting a bullet through a cloud,” she says. “The bullet doesn’t care.” Even if the cold gas did nudge G2, Murchikova says that changes in the orbit would be undetectably minuscule.
Murchikova and her colleagues plan to study the putative cold-gas reservoir again with ALMA, this time at an even higher resolution thanks to the instrument’s recent updates. The detailed observations may help to explain another mystery—why the gas is so much brighter than expected—while providing a second, confirmatory glimpse of the elusive mass of cool material.
In the meantime, the new research is providing an intriguing if controversial probe of what is happening at the center of the Milky Way and galaxies elsewhere in the cosmos. “Sagittarius A* is the ultimate laboratory for black hole accretion physics,” Bower says. “Every new piece of information gives a better picture of the very complex process by which gas falls onto our black hole. This informs our whole picture of how black holes and galaxies grow through the history of the universe.”