Diverse metallicities of Fermi bubble clouds indicate dual origins in the disk and halo

The Galactic Centre is surrounded by two giant plasma lobes known as the Fermi bubbles, extending ~10 kpc both above and below the Galactic plane. Spectroscopic observations of Fermi bubble directions at radio, ultraviolet and optical wavelengths have detected multi-phase gas clouds thought to be embedded within the bubbles, referred to as Fermi bubble high-velocity clouds (FB HVCs). Although these clouds have kinematics that can be modelled by a biconical nuclear wind launched from the Galactic Centre, their exact origin is unknown because there has so far been little information on their heavy metal abundances (metallicities). Here we show that FB HVCs have a wide range of metallicities from <20% of solar to ~320% of solar, based on a metallicity survey of twelve FB HVCs. These metallicities challenge the previously accepted tenet that all FB HVCs are launched from the Galactic Centre into the Fermi bubbles with solar or supersolar metallicities. Instead, we suggest that FB HVCs originate in both the Milky Way’s disk and halo. As such, some of these clouds may characterize the circumgalactic medium that the Fermi bubbles expand into, rather than material carried outwards by the nuclear wind, changing the canonical picture of FB HVCs. More broadly, these results reveal that nuclear outflows from spiral galaxies can operate by sweeping up gas in their haloes while simultaneously removing gas from their disks.