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High-velocity clouds (HVCs) are multi-phase gas structures whose velocities(|v_LSR|>100 km/s) are too high to be explained by Galactic disk rotation.While large HVCs are well characterized, compact and small HVCs (with HIangular sizes of a few degrees) are poorly understood. Possible origins forsuch small clouds include Milky Way halo gas or fragments of the MagellanicSystem, but neither their origin nor their connection to the Milky Way halo hasbeen confirmed. We use new Hubble Space Telescope/Cosmic Origins SpectrographUV spectra and Green Bank Telescope HI spectra to measure the metallicities offive small HVCs in the southern Galactic sky projected near the MagellanicSystem. We build a set of distance-dependent Cloudy photoionization models foreach cloud and calculate their ionization-corrected metallicities. All fivesmall HVCs have oxygen metallicities <0.17 Z_sun, indicating they do notoriginate in the disk of the Milky Way. Two of the five have metallicities of0.16-0.17 Z_sun, similar to the Magellanic Stream, suggesting these clouds arefragments of the Magellanic System. The remaining three clouds have much lowermetallicities of 0.02-0.04 Z_sun. While the origin of these low-metallicityclouds is unclear, they could be gaseous mini-halos or gas stripped from dwarfgalaxies by ram pressure or tidal interactions. These results suggest thatsmall HVCs do not all reside in the inner Milky Way halo or the MagellanicSystem, but instead can trace more distant structures.

The Metallicities of Five Small High-Velocity Clouds