Local Goup HVCs: Status of the Evidence

Abstract

The evidence for locating the High Velocity Clouds in the Local Group is summarized and evaluated. Recent measurements of the H$α$ surface brightness and metallicity of a number of HVCs appear to be fatal to the Galactic fountain as a significant contributor to the HVC phenomenon, but not to the existence of the fountain itself. Observations of extragalactic analogues to HVCs remain the {\it sine qua non} for deciding whether the Local Group hypothesis is viable, but the constraints based on existing surveys appear to be rather weak. MgII quasar absorption lines restrict how many HVC analogues exist at intermediate redshift, depending on where these lines originate. It is concluded that the evidence remains ambiguous, none of the main hypotheses is fully consistent with all of the data, and the Local Group hypothesis remains a viable explanation for the HVC phenomenon.