Abstract
Abstract
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 that uses a hybrid
approach coupling semi-analytic and N-body techniques. Our analysis shows that the whole-sky
anisotropy in each stellar halo increases with the distance from its centre and eventually
plateaus out beyond a certain radius. All the stellar halos have a very smooth structure within a
radius of ∼ 50 kpc and a highly anisotropic structure in the outskirts. At a given radius,
the anisotropies at a fixed polar or azimuthal angle have two distinct components: (i) an
approximately isotropic component and (ii) a component with large density fluctuations on small
spatial scales. We remove the contributions of 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 approximately uniform component of the anisotropy is significantly reduced
after the sphericalization. A comparison between the original halos and their sphericalized
versions reveals that the approximately uniform 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 has the potential to
constrain the shape of the stellar halo and its substructures.
Subject
Astronomy and Astrophysics