A Two-Point Correlation Function For Galactic Halo Stars
  We describe a correlation function statistic that quantifies the amount of
spatial and kinematic substructure in the stellar halo. We test this statistic
using model stellar halo realizations constructed from the Aquarius suite of
six high-resolution N-body simulations in combination with the Galform
semi-analytic galaxy formation model. These simulations show considerable
scatter in the properties of stellar haloes. We find that our statistic can
distinguish between these plausible alternatives for the global structure of
the Milky Way stellar halo. We compare with observational data and show that
pencil beam surveys of ~100 tracer stars (such as the Spaghetti Survey) are not
sufficient to constrain the degree of structure in the Milky Way halo with this
statistic. Larger area surveys with >1000 tracer stars (such as BHB stars in
the Sloan Digital Sky Survey) provide much tighter constraints on comparisons
between models and data. In our simulations, we find examples of haloes with
spatial and kinematic substructure consistent with the available Milky Way
data.
