THE GLOBULAR CLUSTER SYSTEM OF THE MILKY WAY: ACCRETION IN A COSMOLOGICAL CONTEXT

Abstract

We examine the significance of a planar arrangement in the spatial\ndistribution of the Milky Way's globular clusters (GCs). We find that, when\nseparated on the basis of horizontal branch morphology and metallicity, the\nouter-most canonical young halo GC sample (at galactocentric radii in excess of\n10 kpc) exhibit an anisotropic distribution that may be equated to a plane (24\n+/- 4) kpc thick (rms) and inclined at 8 degrees +/- 5 degrees to the polar\naxis of the Milky Way disk. To quantify the significance of this plane we\ndetermine the fraction of times that an isotropic distribution replicates the\nobserved distribution in Monte-Carlo trials. The plane is found to remain\nsignificant at the >95% level outside a galactocentric radius of 10 kpc, inside\nthis radius the spatial distribution is apparently isotropic. In contrast, the\nspatial distribution of the old halo sample outside 10 kpc is well matched by\nan isotropic distribution. The plane described by the outer young halo globular\nclusters is indistinguishable in orientation from that presented by the\nsatellite galaxies of the Milky Way. Simulations have shown that the planar\narrangement of satellites can arise as filaments of the surrounding large scale\nstructure feed into the Milky Way's potential. We therefore propose that our\nresults are direct observational evidence for the accreted origin of the outer\nyoung halo globular cluster population. This conclusion confirms numerous lines\nof evidence that have similarly indicated an accreted origin for this set of\nclusters from the inferred cluster properties.\n