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Astron. Astrophys. 351, 920-924 (1999)

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5. Dark matter in HVCs and galaxies

Most of the HVC mass is dark matter, around 85% of the mass of the clouds. The nature of this dark matter is unknown. The dark matter in the HVCs could be some kind of baryonic matter; at least, this is compatible with the nucleosynthesis values: the baryonic matter is limited to 0.018[FORMULA]0.022 (Schramm & Turner 1998) while the observed local stellar density is a 17% of that (Fukugita et al. 1998).

This does not mean that all dark matter has to be baryonic, but the total mass of these clouds could be in baryons. Baryonic dark matter exists (Silk 1996) and a fraction of half of the mass of the Local Group is not excluded by primordial nucleosynthesis of the light elements. Very low mass stars, cold hydrogen and other exotic matter are some candidates. Models derived in part from the observations predict a scenario in which molecular clouds and dark clusters of MACHOs constitute the halo (De Paolis et al. 1995), or very cold molecular hydrogen in the disc (Pfenninger et al. 1994). These may also constitute scenarios for the dark matter in the HVCs.

Since a significant fraction of the present disc material proceeds from HVC accretion, part of the dark matter in the Galactic halo and disc is the same kind of material as the dark matter of the HVCs although the fraction may be different. From Eq. (5), the accretion rate can be derived for any time. For instance, assuming a present rate in the Milky Way of 7.5 [FORMULA]/yr (BL99), twice this for M31 and the total mass of the Local Group [FORMULA] [FORMULA], we find that less than 30% of [FORMULA] has come from the accretion of HVCs during the last 15 Gyr. This is much less than the predictions by BL99, who conclude that practicaly all the mass of the two major disc galaxies should be due to the accretion of HVCs, mainly during early epochs. Unless its non-baryonic nature implies a process which favours only the retention of the baryonic components, this would imply that less than 25% of the matter of the Milky Way or M31 may be constituted by the kind of dark matter which is present in the HVCs. To this fraction, one should, of course, add the dark matter which entered into the constitution of the galaxies when they formed.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999