In a study published yesterday in The Astrophysical Journal Letters, astrophysicists at Northwestern University presented possible evidence of a giant, cone-shaped hole in the cold gas around Sagittarius A*, the supermassive black hole at the center of the galaxy. When the team calculated how much energy was needed to create this cavity, they found that there must be some input from the black hole – something like a powerful wind or jet. To reach these conclusions, the team compiled five years of observations by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
“Unless a black hole exists in a perfect vacuum, it must have wind somehow,” Mark Gorski, co-lead author of the study, said in a statement. “With the new observations, this is the first time we have a clean enough view to see the imprint of the wind. We looked at the data and said, ‘There it is. This is the thing everyone has been looking for for 50 years.'”
height of anarchy
All large galaxies have a supermassive black hole at their center. According to the study, these black holes are important for the evolution of the galaxy. Since black holes “nourish” material, the material is pulled inwards in a spiral manner and eventually reaches the speed of light. The paper explains that the energy and pressure generated in this process produces hot, powerful winds that blow outward from the black hole.
Current theories are quite good at capturing these processes—in part because astrophysicists predicted in 1971 that Sagittarius A* would also generate winds and jets. However, given the logistical challenges of studying black holes, the study said there has since been “no universally accepted signature of active wind from Sagittarius A*”.
“To observe our own black hole, we have to look at the bottom of our galaxy,” explained co-lead author Elena Merchikova. “That means we have to wade through gas, dust, and ionized structures, and you can’t really see them all that easily.”
Given the chaos
With the ALMA data, Gorsky and Marchikova managed to see all the gas, dust, and ionized structures. After carefully removing the black hole’s radio glow, they landed on a sharp image of a giant, cone-shaped cavity devoid of cold molecular gas. The discovery “shocked” the couple, according to the statement.
“It’s a severe shortage of material,” Gorski said. “We calculated how much energy was needed to create this cavity. It’s more energy than can be provided by the stars in that region. Basically, it must have input from the supermassive black hole. And, if you follow the shape of the cone, it points directly at the black hole.”
In fact, the cone was so strange that Marchikova’s first instinct was to doubt her own analysis. But the researchers’ fears were dispelled as additional X-ray data from the Chandra observatory “fit perfectly” with their initial results, Gorsky said.
no exception
Once the team was certain of their discovery, the researchers compiled an inventory of everything they currently understood about the wind, such as its direction, activity, and strength. Study estimates indicate that the wind has been active for at least 20,000 years. Compared to some other known galactic jets, the wind is on the weak side.
Perhaps most importantly, Sagittarius A* is a “typical example” of an underpowered, dormant supermassive black hole in the universe, the paper says. Of course, “it is very tempting to study black holes when they are in the fireworks phase, but in fact this is not their prime state,” Marchikova said. Given Sagittarius A*’s relative proximity to us, the findings could inform future investigations into our local black holes and, by extension, provide new insights into the activity of similar entities.
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