IAU Symposium Abstract S186-008T

Moving Groups in the Galactic Halo

Luis A. Aguilar
Univ. Nac. Aut. de Mexico, Ensenada BC, M'exico < aguilar@bufadora.astrosen.unam.mx >

The idea that the Galactic Halo has been formed to a large extent by the accretion and tidal disruption of satellite systems has been gaining strength. The discovery of a retrograde rotating stellar group (Majewski et al., 1992), general patchiness in the kinematics of halo stars in kinematic surveys (Majewski et al., 1996), the realization that most of the Milky Way satellites lie near two great circles in the sky (Lynden-Bell, 1976) and the discovery of an elongated dwarf galaxy in Sagittarius (Ibata et al., 1994), all add credence to this idea. On the theoretical arena, the apparent fragility of galactic disks (T'oth and Ostriker, 1992) no longer seems to be a problem for the accretion scenario (Vel'azquez and White, 1997). The tidal ``streamers'' that result from tidal disruption seem to be long lived (Barnes 1996) and can be exploited to devise algorithms to search for them in galactic surveys (Johnston et al., 1996). The phase space portrait of the halo, far from being a smooth distribution, should consist of a patchy aggregation of tidally disrupted systems that have been phase mixed over wide swaths in the sky, but which retain kinematic memory of their existence as a coherent entity. The challenges to discover these moving groups in the halo are enormous due to the distances involved and the fact that they can span large angles in the sky. The availability of astrometric databases of unprecedent accuracies (HIPPARCOS) and the plans for follow ups (e.g. GAIA), offer a unique opportunity to search for these moving groups. Together with these databases, new search techniques must be devised (Chen et al. 1997, Hoogerwerf and Aguilar, 1997).