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L. Blitz (Berkeley)
The formation and evolution of the Milky Way and the Local Group of galaxies will be reviewed in the context of a new observational and dynamical intepretation of the High Velocity Clouds (HVCs) of atomic hydrogen. The evidence suggests that the HVCs are primarily Local Group objects, and are the remnants of the formation of the Milky Way and M31. Because accretion of the HVCs appears to be taking place at the current epoch, our picture suggests that the Milky Way and M31 are still in the process of formation. At least three new pieces of observational evidence support this basic idea: namely, the observations of H\alpha toward HVCs, the metallicity and abundances of the HVCs, and the pressures measured along two lines of sight. The implications for searches for similar HVC analogues in other galaxy groups will be discussed. An examination of the HI content of the Local Group will be presented which suggests that a large massive halo of hot gas surrounds the Milky Way and M31 with a mean temperature of about 1 - 1.5 \times 106 K and a mean density of about 2.5 \times 10-5 cm-3. The existence of this halo is broadly consistent with being the result of collisions between HVCs prior to accretion. The interaction of the Local Group dwarfs with this hot halo may help to explain their diverse the star formation histories. This work has been partially funded by the National Science Foundation.