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

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2. Accretion theory

The view of HVCs as gas being accreted by the Galaxy was first suggested by Oort (1966, 1970), who postulated that gas left over from the formation of the Galaxy is now reaching the Galactic disc. During their approach, the clouds are heated and recooled forming the HVCs at heights of 1 to 3 kpc. A simple model with the Galaxy accreting these clouds encounters directly many problems; for instance, it cannot justify observations with positive velocities. The possibility of an extragalactic origin of these clouds associated with the Local Group was discussed by Verschuur (1969, 1975), Einasto et al. (1976), Eichler (1976), Giovanelli (1980, 1981), Arp (1985), Bajaja et al. (1987) and others, but their models apparently failed to fully explain the observations.

The difficulties of this hypothesis were solved in BL99 aided by new observational evidence. The kinematic anomalies are explained in terms of the infall of remnants of the formation of the Local Group towards its barycentre. A typical cloud at a distance of 1 Mpc has a diameter of 30 kpc and contains [FORMULA] [FORMULA] of HI gas, and a total mass of [FORMULA] [FORMULA] (given 85% of dark mass). Its density would be [FORMULA] cm-3. Braun & Burton (1999) analyzed a sample of HVCs similar to that of BL99, but chose isolated clouds, avoiding the contamination of some complexes related to the Magellanic Stream or the Outer Arm Extension, finding a size of 15 kpc and a similar HI mass. The dynamics of the Local Group is dominated by the attraction of M31 (with twice the mass of the Milky Way) and the Milky Way. When a cloud is within 100 kpc comoving distance of one of these galaxies it can be taken as accreted. The simulated spatial and kinematic distributions resemble the observed distributions rather well. Most of the HVCs are located either near the direction of M31, towards the barycenter of the Local Group, or in the antibarycenter direction. The distribution of velocities with angular distance from the solar apex is that expected for Local Group barycentre infall (Braun & Burton 1999).

Low metallicity is observed in some HVCs. Sembach et al. (1999) observed some highly ionized HVCs whose ionic ratios are well explained via photoionization by extragalactic background radiation combined with some ultraviolet starlight. The observations by Tufte et al. (1998) of a set of HVCs also suggest photoionization. Hence, these clouds must have low density ([FORMULA] cm-3), and be bigger than a few kpc and mainly ionized ([FORMULA]), which indicate that they must be far away from the neutral Galactic gas. These clouds would have total masses of [FORMULA] [FORMULA]. If the clouds are intergalactic in nature, their metallicities could well be [FORMULA] or lower. Low metallicities were also detected in an HVC through sulphur abundance measurements, yielding [FORMULA] times solar, much lower than the solar value (Wakker et al. 1999a). This metallicity supports the extragalactic origin at least for this object because gas ejected from the current Galaxy would share its near-solar metallicity.

As an additional datum, it is observed that the HVCs properties are similar to those found for high redshift "Ly[FORMULA] forest" clouds (BL99). This implies that this type of clouds is also abundant at very large distances from the Local Group and that they are present in the primordial scenarios of galaxy formation.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999