Using essentially the BL99 model for HVCs and a non-decreasing accretion rate of these clouds with low metallicity onto the Milky Way, we have deduced that their present abundance in the Local Group must be greater than some 40% to 50% of its total mass. This is in agreement with the dynamical measurements of the total mass in the Local Group and may account for a significant fraction of the baryonic dark matter, although a major fraction of non-baryonic matter within these clouds is not excluded. The calculations shown here are a first approximation, in which collapse or expansion of the Local Group was not taken into account but, the volume variation, if any, should be slow enough for us to neglect its effects.
At present, the nature of this dark matter (around 85% of the total mass of the clouds) cannot be determined, but the hypothesis of an important contribution to the total mass of the Local Group may be a clue towards finding it. Rotation curve anomalies and warps (Jiang & Binney 1999) in spiral galaxies could also be due to the existence of these HVCs, as a response to the torque the HVC system can exert on the disc of a large spiral.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999