Mapping the Galactic Halo with blue horizontal branch stars from the 2dF
  quasar redshift survey
  We use 666 blue horizontal branch (BHB) stars from the 2Qz redshift survey to
map the Galactic halo in four dimensions (position, distance and velocity). We
find that the halo extends to at least 100 kpc in Galactocentric distance, and
obeys a single power-law density profile of index ~-2.5 in two different
directions separated by 150 degrees on the sky. This suggests that the halo is
spherical. Our map shows no large kinematically coherent structures (streams,
clouds or plumes) and appears homogeneous. However, we find that at least 20%
of the stars in the halo reside in substructures and that these substructures
are dynamically young. The velocity dispersion profile of the halo appears to
increase towards large radii while the stellar velocity distribution is non
Gaussian beyond 60 kpc. We argue that the outer halo consists of a multitude of
low luminosity overlapping tidal streams from recently accreted objects.
