HI and OVI high-velocity clouds and the Local Group

Abstract

I examine some of the evidence relevant to the idea that high-velocity clouds (HVCs) are gas clouds distributed throughout the Local Group, as proposed by Blitz et al. (1999) and Braun & Burton (1999). This model makes several predictions: a) the clouds have low metallicities; b) there should be no detectable Halpha emission; c) analogues near other galaxies should exist; and d) many faint HVCs in the region around M31 can be found. Low metallicities are indeed found in several HVCs, although they are also expected in several other models. Halpha emission detected in most HVCs and, when examined more closely, distant (D>200 kpc) HVCs should be almost fully ionized, implying that most HVCs with HI must lie near the Milky Way. No clear extragalactic analogues have been found, even though the current data appear sensitive enough. The final prediction (d) has not yet been tested. On balance there appears to be no strong evidence for neutral gas clouds distributed throughout the Local Group, but there may be many such clouds within 100 or so kpc from the Milky Way (and M31). On the other hand, some (but not all) of the high-velocity OVI recently discovered may originate in hot gas distributed throughout the Local Group.