A Limit on the Metallicity of Compact High‐Velocity Clouds

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Abstract
There is a fortuitous coincidence in the positions of the quasar TonS210 and the compact H I high velocity cloud CHVC224.0-83.4-197 on the sky. Using Far Ultraviolet Spectroscopic Explorer observations of the metal-line absorption in this cloud and sensitive H I 21cm 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.Comment: 15 pages, 3 postscript figures + 1 JPEG figure (reduced from postscript for size considerations), accepted for publication in ApJ (June 2002
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