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, this wind is more of a gentle breeze than a hurricane.
Discovery Using Advanced Telescopes
Using data from the Chile-based ALMA telescope and NASA's orbiting Chandra X-ray Observatory, researchers observed the cosmic neighborhood around the black hole, known as Sagittarius A* (Sgr A*). They identified a vast conical cavity filled with hot, electrically charged gas adjacent to Sgr A*. This cavity, they concluded, was sculpted by wind blowing from the black hole, which swept away or heated the cold gas that once populated the region. The energy required to create such a cavity could only be generated by a supermassive black hole.
What Are Supermassive Black Holes?
Black holes are extraordinarily dense objects with gravity so strong that not even light can escape. Most galaxies have a supermassive black hole at their core that pulls in gas and other material from its surroundings. Scientists decades ago theorized that any active supermassive black hole would expel some gas and other material into space, either as a wind propagating outward or as a focused jet. This behavior had been observed in numerous supermassive black holes in other galaxies, but until now, it had not been confirmed for Sgr A*.
A Half-Century Mystery Solved
"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 in the Astrophysical Journal Letters. Sgr A* has 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, about 9.5 trillion kilometers. It is not as massive as some black holes in other galaxies and is currently in a relatively quiet phase.
The tip of the cone-shaped cavity arises from the immediate vicinity of Sgr A* and expands outward. While researchers are not certain of the cavity's exact scale because it extends beyond their observations, Murchikova estimated it could be about 6.5 light-years in length.
Gentle Breeze vs. Violent Hurricanes
Because of Sgr A*'s current docile state, the wind it generates is not as intense as those from other supermassive black holes. Northwestern University astronomer and study co-leader Mark Gorski compared it to Earth weather. "It is a gentle breeze coming from our supermassive black hole. It doesn't appear to be strong enough to drastically restructure the galactic center," Gorski said.
"Supermassive black holes spend most of their time in this quiet, gentle state. However, sometimes they go through outbursts ranging from thunderstorms to the most violent of hurricanes. Their most intense winds or jets can completely disrupt their host galaxies and regions well beyond," he added.
How the Wind Forms
As gas and other material spiral inward toward a black hole, they reach close to the speed of light, generating enough energy and pressure to hurl some of it outward. "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 explained.
"When we look at distant galaxies 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," Gorski said.
