The black hole at the centre of the Milky Way swallowed a clump of interstellar hydrogen at least 6 million years ago, before ‘burping’ up hot plasma that can still be seen today as two bubbles stretching north and south above the galactic centre.
This is the latest discovery made using the Hubble Space Telescope.
The two bubbles, called the Fermi Bubbles, were discovered in 2010 by NASA’s Fermi Gamma-ray Space Telescope, but Hubble has now been able to reveal how old they are, and more information about their composition.
The bubbles weigh the equivalent of millions of our Suns, and are situated just above and below our galactic centre.
The edge of the the northern bubble extends 23,000 lightyears above the Galaxy.
These outflows from black holes are common, and are the result of material being sucked in. The energy created by this process heats some of the infalling material, which then escapes in the form of a massive jet firing out into space.
In the case of the supermassive black hole at the centre of the Milky Way, it has left two enormous gassy structures.
The team were able to date and analyse the northern bubble by observing the ultraviolet light from quasars - which are bright cores of distant active galaxies - as it passed through it.
This analysis provided information about the speed, composition and temperature of the gas inside.
It revealed the temperature of gas in the bubble to be just under 10,000°C, racing through the bubble at 3,200,000 km per hour.
"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," says Rongmon Bordoloi of the Massachusetts Institute of Technology, who led the study.
"The Hubble data open a whole new window on the Fermi Bubbles," says co-author Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland.
"Before, we knew how big they were and how much radiation they emitted; now we know how fast they are moving and which chemical elements they contain.
That's an important step forward."