High-velocity clouds as dark matter in the Local Group

The High-Velocity Clouds (HVCs) observed in the Galactic neighbourhood, hav e been proposed to be remnants of the formation of the galaxies in the Loca l Group, having distances, and thus masses, predominantly of dark matter, c onsiderably larger than hitherto hypothesized. This hypothesis is plausibly supported by observational evidence that their kinematical centre is the L ocal Group barycentre. Evolutionary models to account for the evolution of the light elements in the Galaxy demand infall of metal poor gas to the pla ne, which could well be supplied by these HVCs. Modelling the time dependen ce of this infall, taking into account that an accreting galaxy shows an in creasing cross-section to the infalling clouds, and produces increasing mea n infall velocities, we deduce that the HVCs must currently represent at le ast around one half of the total mass of the Local Group, given that the ac cretion rate, as inferred from chemical evolution, has not decreased signif icantly during the disc lifetime. This fraction is consistent with dynamica l estimates of the relative masses of the Local Group as a whole and its co nstituent galaxies. The HVCs may thus form a significant constituent of bar yonic, and of non-baryonic, dark matter.