The globular cluster system of the Milky Way: accretion in a cosmological context

TL;DR

This study reveals a significant planar arrangement of outer young halo globular clusters in the Milky Way, supporting their accreted origin and linking their distribution to large-scale cosmic filaments.

Contribution

It provides observational evidence of a planar distribution of outer young halo globular clusters, indicating their accreted origin in a cosmological context.

Findings

Outer young halo GCs form a significant plane with 24 +/- 4 kpc thickness.

The plane's orientation matches that of the Milky Way's satellite galaxies.

Distribution inside 10 kpc is isotropic, outside shows a significant planar structure.

Abstract

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