After five decades of searching, astronomers have finally detected the wind emanating from the supermassive black hole at the center of our Milky Way galaxy. However, it turns out to be more of a gentle breeze than a hurricane.
Using data from the Chile-based ALMA telescope and NASA's orbiting Chandra X-ray Observatory, researchers studied the cosmic neighborhood around the black hole, known as Sagittarius A* (Sgr A*). They spotted a vast conical cavity filled with hot, electrically charged gas adjacent to Sgr A*, which they concluded was sculpted by wind blowing from the black hole. This wind swept away or heated up the cold gas that had populated the region.
The energy needed to create such a cavity could only be generated by a supermassive black hole. Black holes are extraordinarily dense objects with gravity so strong that not even light can escape. Galaxies typically have a supermassive black hole at their core that pulls in gas and other material from its surroundings.
Scientists decades ago posited that any active supermassive black hole, due to its physics, would expel some gas and other material into space, either as a wind propagating outward or as a focused jet. They subsequently identified such behavior in numerous supermassive black holes in other galaxies, but until now had been unable to demonstrate that Sgr A* also did this.
"This discovery resolves a half-century-old mystery," said Lena Murchikova, a professor of physics and astronomy at Northwestern University in Illinois and co-leader of the study published this week in the Astrophysical Journal Letters.
Sgr A* possesses about 4 million times the mass of our Sun and is located about 26,000 light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). It is not as massive as some of its counterparts in other galaxies and has been determined to be in a relatively quiescent phase.
The tip of the cone-shaped cavity arises from the immediate vicinity of Sgr A* and expands outward. While the researchers are not certain of the scale of the cavity because it reaches beyond the field of view of their observations, Murchikova said it could extend to about 6.5 light-years in length.
"While some gas keeps falling in, other gas is ejected. In fact, more of the gas is ejected than falls into the black hole. This ejected gas is the wind we are talking about," Murchikova said. "When we look at distant galaxies far-far away, it is much easier to see violent phenomena. We see huge, powerful jets ripping through the galaxy and everything else in their path. We see violent winds ejecting nearly all gas from their galaxies."
The difference between a jet and a wind is purely geometric. "Jets are narrow and don't expand very much as they leave their source, often producing a beam of matter. Winds, however, are wider and expand as they leave their source. It's almost like the difference between a laser pointer and a flashlight," said Gorski, a co-author of the study.
