Milky Way's Supermassive Black Hole Ate Its Last Big Meal 6 Million Years Ago

Most galaxies in the observable universe contain a supermassive black hole at their center, one that is either active and surrounded by an accretion disk of dust, gas and other debris, or is dormant — lurking at the center, patiently awaiting its next meal.

Sagittarius A* — the black hole at the center of our galaxy Milky Way — has been more or less sedate for quite a while, and now researchers have finally figured out when it gobbled down its last big meal. Analysis of gigantic structures known as Fermi Bubbles has revealed that the supermassive black hole consumed a large clump of infalling gas roughly 6 million years ago.

The researchers’ conclusions were based on observations of ultraviolet light from 47 distant quasars — which are bright cores of distant active galaxies — made using the Hubble Space Telescope’s cosmic origins spectrograph. Specifically, the spectrograph was used to study the light from these quasars as it passed through the Fermi Bubbles, the giant bubbles of gas burped out by Sagittarius A* after its meal.

Read: Scientists Create A Better Model To Simulate Accretion Disk Flow Around Milky Way’s Supermassive Black Hole

The light from these quasars carries with it information about the speed, composition and temperature of gas in these bubbles — information that can be used to determine their age.

“For the first time, we have traced the motion of cool gas throughout one of the bubbles, which allowed us to map the velocity of the gas and calculate when the bubbles formed,” lead researcher Rongmon Bordoloi from the Massachusetts Institute of Technology said in a statement released Thursday. “What we find is that a very strong, energetic event happened 6 million to 9 million years ago. It may have been a cloud of gas flowing into the black hole, which fired off jets of matter, forming the twin lobes of hot gas seen in X-ray and gamma-ray observations. Ever since then, the black hole has just been eating snacks.”

The researchers, whose findings were published in the Astrophysical Journal, also found that the bubbles contain gas at a temperature of approximately 17,700 degrees Fahrenheit — much cooler than super-hot gas in the black hole’s outflow. In addition, by mapping the motion of particles in the structure, the astronomers estimated that the minimum mass of the cool gas is equivalent to 2 million suns.

“We have traced the outflows of other galaxies, but we have never been able to actually map the motion of the gas,” Bordoloi said. “The only reason we could do it here is because we are inside the Milky Way. This vantage point gives us a front-row seat to map out the kinematic structure of the Milky Way outflow.”