Above and below the plane of the Milky Way, there are two enormous bubbles of hot gas. Their formation is closely linked to the center of our galaxy and the supermassive black hole that lays there. New research now sheds light on the origin of clouds of gas found within the bubbles – with a surprise.
The general idea was that as the bubbles formed, material from the Milky Way's disk was launched into the vast structures, each extending 25,000 light-years from the galaxies. That is roughly how far the solar system is from the center of the Milky Way.
The work, published In Nature Astronomy, showed that material did come from the disk, but also from the halo.
The Milky Way's halo is the spherical region surrounding its galaxies. Unlike the disk, which is packed with a lot of gas and dust organized in spiral arms, the halo is scarcely populated. The stars orbiting there tend to be more primitive – or we could say pristine.
Because stars are responsible for creating all the elements in the Universe that are not the primordial hydrogen and helium (plus a dash of lithium), by measuring the chemical composition of these clouds, the team established how polluted they are. From there, they can possibly work out where they came from.
The value measured is called metallicity (as astronomers call everything that is not hydrogen or helium a “metal”) and the Fermi Bubbles' high-velocity clouds have a wide range of metallicity. Some have one-fifth of the metallicity seen in the Sun. This means these clouds have been barely polluted by heavier elements, so are unlikely to originate in a region as active as the Milky Way’s core.
However, others have over three times the metallicity seen in the Sun. Those definitely need lots of stars becoming red giants, and even a few supernovae, to sprinkle these clouds of heavy elements. Hardly the stuff you would find in the more metal-poor halo.
The origin of the clouds is not exactly clear. While the mechanism behind the Fermi Bubbles could play a role, the halo is also a place for the remaining bits of galaxy mergers to be. Some of the stars there used to be in other galaxies that the Milky Way ate eons ago.