Tomography of stellar halos: what does anisotropy in a stellar halo tell us?

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 $\sim 50$ 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.

19 pages, 4 figures, Significantly revised presentation, Accepted for publication in JCAP

References in corpus (23)

• The Gaia mission
• The accretion origin of the Milky Way's stellar halo
• The Haunted Halos of Andromeda and Triangulum: A panorama of galaxy formation in action
• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• The Dual Origin of Stellar Halos
• An Orphan in the "Field of Streams"
• The stellar halo of the Galaxy
• Detection of a 60 Degree-Long Dwarf Galaxy Debris Stream
• The Galaxy and its stellar halo: insights on their formation from a hybrid cosmological approach
• On the Shoulders of Giants: Properties of the Stellar Halo and the Milky Way Mass Distribution
• The shape of the inner Milky Way halo from observations of the Pal 5 and GD-1 stellar streams
• The Hercules-Aquila Cloud
• The ARTEMIS simulations: stellar haloes of Milky Way-mass galaxies
• Touching The Void: A Striking Drop in Stellar Halo Density Beyond 50 kpc
• The oldest and most metal poor stars in the APOSTLE Local Group simulations
• The Complex Structure of Stars in the Outer Galactic Disk as revealed by Pan-STARRS1
• Deep SDSS optical spectroscopy of distant halo stars II. Iron, calcium, and magnesium abundances
• Chemical Cartography. I. A Carbonicity Map of the Galactic Halo
• How cosmological merger histories shape the diversity of stellar haloes
• The Stellar Density Profile of the Distant Galactic Halo
• The substructures in the local stellar halo from Gaia and LAMOST
• Relating information entropy and mass variance to measure bias and non-Gaussianity
• The global structure of the Milky Way's stellar halo based on the orbits of local metal-poor stars

Cited by in corpus (1)

• Probing the spatial and velocity anisotropies in stellar halos from the Aquarius simulations