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Probing the halo of the Milky Way with horizontal branch stars

Roberto de Propris (CTIO, Chile)

We present a 4-dimensional (position-distance-velocity) map of the Milky Way halo constructed from blue horizontal branch stars in the 2Qz survey.
Our data show that the halo of our galaxy extends to at least 100 kpc, similar to
the very large halo of M31. Comparison with theoretical models shows a comparative lack of structure, but the halo has a web-like or swiss-cheese
appearance. The velocity dispersion profile is qualitatively inconsistent with standard CDM accretion histories, and appears to favor a halo built at very high redshift.