A torrent of energetic particles appears to be spewing from the center of our Milky Way Galaxy, coming from the gigantic black hole that lies at its heart, according to a new study.
Such jets are common throughout the universe, and most supermassive black holes are thought to produce them. When matter falls into these behemoths, some material also is accelerated away, usually in two straight beams that fly out along the black hole’s spin axis.
The Milky Way’s giant black hole, called Sagittarius A* (pronounced “Sagittarius A-star”) has long been theorized to have jets, but evidence was inconclusive. Now researchers have combined x-ray photographs of the galaxy’s center from NASA’s Chandra space telescope with radio data from the Very Large Array (VLA) observatory in New Mexico to offer the best support yet for the idea of jets from Sagittarius A*. The x-ray photos show a wispy bright line of gas that is emitting x-ray light to one side of the black hole—perhaps indicating the jet itself—and the radio observations highlight a wall of gas that scientists think is a shock front created where the jet is slamming into a cloud, snow-plowing the gas into a clump.
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In a paper accepted for publication in The Astrophysical Journal, Zhiyuan Li of Nanjing University in China, Mark Morris of the University of California, Los Angeles, and Frederick Baganoff of the Massachusetts Institute of Technology show these features share a “striking spatial relationship,” the researchers wrote. “There are many pieces of this story that fit nicely,” Morris says. “It’s not a slam dunk, but to my mind it’s stronger evidence than anyone has seen so far.”
The jet is so hard to find in part because it is relatively faint—the Milky Way’s black hole just is not eating enough to produce really robust jets, researchers say. Furthermore, the center of the galaxy is shrouded in so much gas and dust that nothing is easy to see there from our perspective on Earth. “There’s basically all the gunk between us and the galactic center, plus a big screen of plasma that is sort of like bathroom shower glass, serving to smear out images because of electron scattering,” says astronomer Sera Markoff of the University of Amsterdam, who was not involved in the new study. For example, scientists have detected a weak jet emanating from the black hole at the center of the nearby galaxy M81, but models suggest that if the same jet projected from the Milky Way’s core, we wouldn’t be able to see it. The new study supports the idea that jets may become visible only when they hit something. “Maybe the shock excites and accelerates particles in the jet,” causing the beam to light up, Markoff says.
Jets arise because the black hole is spinning. As matter falls into the black hole, the matter’s magnetic field gets twisted and amplified by the black hole’s spin, and this pumped-up magnetic field launches material outward in the form of jets. If the signals from Chandra and the VLA really are a jet, its direction would reveal the spin axis of the Sagittarius A* black hole. “Lo and behold, the spin axis appears to be the same as the galaxy,” Morris says. “That’s so satisfying, because that’s what you would expect if the black hole has never undergone a major disturbance.”
Some previous studies, however, suggested the jets pointed in a different direction. The new study is well thought out and “will be a benchmark against future claims of jet orientations,” says astronomer Heino Falcke of Radboud University Nijmegen in the Netherlands, who has also argued for a jet at the galactic center, and recently found that theoretical models favor one. “Is it a smoking gun now? Well, there is smoke, but until we find the bullet and the pistol, we can't really convict anyone for sure.”
An opportunity to test some of these ideas should arrive soon from a cloud of gas that is circling the drain of our central black hole. This cloud, called G2, has been on astronomers’ radar for awhile, and predictions vary about what its future holds. In the next couple of years most scientists think the strong gravitational tides near the black hole should rip the cloud to shreds, and some of these scraps should fall in, potentially causing the jet to flare brighter. “Everybody’s watching,” Morris says.