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

Abstract: The Galactic Center 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). While these clouds have kinematics that can be modeled by a biconical nuclear wind launched from the Galactic center, their exact origin is unknown because, until now, there has been little information on their heavy-metal abundance (metallicity). Here we show that FB HVCs have a wide range of metallicities from <20% solar to ~320% solar. This result is based on the first metallicity survey of FB HVCs. These metallicities challenge the previously accepted tenet that all FB HVCs are launched from the Galactic center into the Fermi Bubbles with solar or super-solar 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 circumgalactic medium that the Fermi Bubbles expand into, rather than material carried outward 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 halos while simultaneously removing gas from their disks.