Data from The Metallicity of Compact High-Velocity Clouds

Compact high-velocity clouds (CHVCs) are atomic hydrogen (H I) clumps with angular sizes of < 2 degrees whose velocities (|vLSR| ≥ 100 km/s) are too high to be explained by Galactic disk rotation. Possible origins for CHVCs include Milky Way halo clouds or fragments of the Magellanic System, but neither their origin nor their connection to the Milky Way halo has been confirmed. We use new Hubble Space Telescope/Cosmic Origins Spectrograph UV spectra and Green Bank Telescope H I spectra to measure the metallicities of five CHVCs in the southern Galactic sky projected near the Magellanic System. We build a set of distance-dependent Cloudy photoionization models for each CHVC and calculate their ionization-corrected metallicities. Additionally, we quantify beam-smearing effects on the metallicity measurements using H I data of CHVCs at both high- (2′) and low- (9′) resolution. All five CHVCs have oxygen metallicities ≤0.17 Z_solar, indicating they do not originate in the disk of the Milky Way. Two of the five have metallicities of 0.16–0.17 Z_solar, similar to the Magellanic Stream, suggesting these clouds are fragments of the Magellanic System. The remaining three CHVCs have much lower metallicities of 0.02–0.04 Z_solar. These three low-metallicity CHVCs could be either low-mass dwarf galaxies with no detectable star formation, H I stripped from dwarf galaxies by ram pressure or tidal interactions, or gaseous dark matter minihalos. These results imply that not all CHVCs reside in the Milky Way halo or the Magellanic System, but instead can trace more distant structures in the Local Group.