High-velocity clouds (HVCs) consist of neutral hydrogen at radial velocities approximately km s-1, incompatible with a simple model of differential galactic rotation (Wakker & van Woerden 1997). Their range of radial LSR velocities extends indeed up to -500 km s-1+300 km s-1. Several hypotheses try to explain the origins of the HVCs (see review in Wakker & van Woerden 1997; Wakker et al. 1999b). At least three different sources appear to be needed: one for the Magellanic Stream and related clouds, one for the Outer Arm Extension, and at least one for the "other HVCs" (Wakker & van Woerden 1997).
One hypothesis, recently put forward by Blitz et al. (1999, hereafter BL99) as the most plausible one to explain "other" HVCs, claims that they are remnants of the formation of the Local Group at a scale distance of 1 Mpc, i.e. far from the Galaxy. These clouds are falling towards the Local Group barycenter and some of them will be accreted by the Milky Way if they move close enough to it.
Here, we hypothesize that these clouds may constitute a major fraction of the mass of the Local Group. The fraction of sky covered by HVCs with km s-1 and cm-2 is 8%, excluding the Outer Arm Extension and the Magellanic Stream (Wakker 1991). If they are relatively distant, as the estimated mass of a cloud is proportional to (d is the distance), they could represent a larger contribution to the mass of the Local Group than hitherto assumed.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999