A Violent Past
Our galaxy has a colossal secret. Towering above and below the galactic plane are two enormous structures known as the Fermi Bubbles, invisible to the naked eye but glowing fiercely in gamma rays. Discovered in 2010 by the Fermi Gamma-ray Space Telescope, these bubbles form a massive hourglass shape centered on the Milky Way's core. Each lobe extends 25,000 light-years into space, making the total structure 50,000 light-years tall—roughly half the diameter of the entire Milky Way. They are filled with a mix of very hot gas, cosmic rays and magnetic fields.
These bubbles suggest a cataclysmic event that occurred a few million years ago. The two leading theories both involve the galactic center. One model suggests the bubbles are the remnants of a powerful jet of matter blasted out from Sagittarius A*, the supermassive black hole at our galaxy's heart. This could have happened when the black hole devoured a massive cloud of gas and dust, releasing energy. An alternative theory proposes that a burst of rapid star formation and subsequent supernova explosions near the galactic center provided the energy required to inflate these structures. The total energy needed is estimated to be equivalent to about 100,000 supernovae.
Bubbles within Bubbles
The mystery deepened in 2020 with the discovery of even larger, though less energetic, structures encompassing the Fermi Bubbles. Detected by the eROSITA X-ray telescope, these "eROSITA bubbles" stretch nearly 45,000 light-years from the galactic center. They are thought to be a shockwave pushed out by the same event that created the Fermi Bubbles. The eROSITA bubbles shine in X-rays, showing disturbances in the hot gas surrounding the Milky Way.
Inside the Fermi Bubbles, which are mostly composed of gas heated to about 1 million degrees Kelvin, astronomers found something unexpected: clouds of cold, dense neutral hydrogen gas. These clouds, with temperatures around 10,000 degrees Kelvin, should have been destroyed by the hot, fast-moving environment within a few million years. Their survival suggests the bubbles are younger than previously thought, likely forming just a few million years ago. The gas within the bubbles is expanding at speeds approaching 2.2 million miles per hour (3.5 million km/h). The gamma rays they emit are produced by cosmic ray particles, accelerated to near the speed of light, interacting with lower-energy photons.