There is a fortuitous coincidence in the positions of the quasar Ton S210 and the compact H i high velocity cloud CHVC 224.0-83.4-197 on the sky .
Using Far Ultraviolet Spectroscopic Explorer observations of the metal-line absorption in this cloud and sensitive H i 21 cm emission observations obtained with the multibeam system at Parkes Observatory , we determine a metallicity of ( O/H ) < 0.46 solar at a confidence of 3 \sigma .
The metallicity of the high velocity gas is consistent with either an extragalactic or Magellanic Cloud origin , but is not consistent with a location inside the Milky Way unless the chemical history of the gas is considerably different from that of the interstellar medium in the Galactic disk and halo .
Combined with measurements of highly ionized species ( C iii and O vi ) at high velocities , this metallicity limit indicates that the cloud has a substantial halo of ionized gas ; there is as much ionized gas as neutral gas directly along the Ton S210 sight line .
We suggest several observational tests that would improve the metallicity determination substantially and help to distinguish between possible origins for the high velocity gas .
Additional observations of this sight line would be valuable since the number of compact HVCs positioned in front of background sources bright enough for high resolution absorption-line studies is extremely limited .