High-velocity clouds as dark matter in the Local Group

Abstract

The High-Velocity Clouds (HVCs) observed in the Galactic neighbourhood, have been proposed to be remnants of the formation of the galaxies in the Local Group, having distances, and thus masses, predominantly of dark matter, considerably larger than hitherto hypothesized. This hypothesis is plausibly supported by observational evidence that their kinematical centre is the Local Group barycentre. Evolutionary models to account for the evolution of the light elements in the Galaxy demand infall of metal poor gas to the plane, which could well be supplied by these HVCs. Modelling the time dependence of this infall, taking into account that an accreting galaxy shows an increasing cross-section to the infalling clouds, and produces increasing mean infall velocities, we deduce that the HVCs must currently represent at least around one half of the total mass of the Local Group, given that the accretion rate, as inferred from chemical evolution, has not decreased significantly during the disc lifetime. This fraction is consistent with dynamical estimates of the relative masses of the Local Group as a whole and its constituent galaxies. The HVCs may thus form a significant constituent of baryonic, and of non-baryonic, dark matter.