Tomography of stellar halos: what does anisotropy in a stellar halo tell
  us?
  The stellar halo of the Milky Way is known to have a highly lumpy structure
due to the presence of tidal debris and streams accreted from the satellite
galaxies. The abundance and distribution of these substructures can provide a
wealth of information on the assembly history of the Milky Way. We use some
information-theoretic measures to study the anisotropy in a set of Milky
Way-sized stellar halos from the Bullock & Johnston suite of simulations. Our
analysis shows that the radial anisotropy in each stellar halo increases with
the distance from its centre and eventually plateaus out beyond a certain
radius. All the stellar halo have a very smooth structure within a radius of
$\sim 50$ kpc and a highly anisotropic structure in the outskirts. At a given
radius, the polar and azimuthal anisotropies in a stellar halo have two
distinct components: (i) a nearly unvarying component from the smooth
distribution of stars and (ii) a highly fluctuating component possibly from the
substructures. We destroy the substructures and any non-spherical shape of the
halo by randomizing the polar and azimuthal coordinates of the stellar
particles while keeping their radial distances fixed. We observe that the
fluctuating part of the anisotropy is completely eliminated, and the unvarying
part of the anisotropy is significantly reduced after the sphericalization. A
comparison between the original halo and its sphericalized versions reveals
that the unvarying part of the anisotropy originates from the discreteness
noise and the non-spherical shape of the halo whereas the substructures
contribute to the fluctuating part. We show that such distinction between the
anisotropies can constrain the shape of the stellar halo and its substructures.
Finally, we map the distribution of the individual substructures by combining
the polar and azimuthal anisotropy profiles of the halo at different radial
distances.
