The Anisotropic Distribution of Galactic Satellites

We present a study of the spatial distribution of subhalos in galactic darkmatter halos using dissipationless cosmological simulations of the concordanceLCDM model. We find that subhalos are distributed anisotropically and arepreferentially located along the major axes of the triaxial mass distributionsof their hosts. The Kolmogorov-Smirnov probability for drawing our simulatedsubhalo sample from an isotropic distribution is P_KS \simeq 1.5 \times10^{-4}. An isotropic distribution of subhalos is thus not the correct nullhypothesis for testing the CDM paradigm. The nearly planar distribution ofobserved Milky Way (MW) satellites is marginally consistent (probability \simeq0.02) with being drawn randomly from the subhalo distribution in oursimulations. Furthermore, if we select the subhalos likely to be luminous, wefind a distribution that is consistent with the observed MW satellites. Infact, we show that subsamples of the subhalo population with acentrally-concentrated radial distribution, similar to that of the MW dwarfs,typically exhibit a comparable degree of planarity. We explore the origin ofthe observed subhalo anisotropy and conclude that it is likely due to (1)preferential accretion of subhalos along filaments, often closely aligned withthe major axis of the host halo, and (2) evolution of satellite orbits withinthe prolate, triaxial potentials typical of CDM halos. Agreement betweenpredictions and observations requires the major axis of the outer dark matterhalo of the Milky Way to be nearly perpendicular to the disk. We discusspossible observational tests of such disk-halo alignment with current largegalaxy surveys.Comment: 14 pages (including appendix), 9 figures. Accepted for Publication in ApJ. Minor changes to reflect referee's comment