  One of the long-standing problems in modern astronomy is the curious division
of Galactic globular clusters, the "Oosterhoff dichotomy," according to the
properties of their RR Lyrae stars. Here, we find that most of the
lowest-metallicity ([Fe/H]<-2.0) clusters, which are essential to an
understanding of this phenomenon, display a planar alignment in the outer halo.
This alignment, combined with evidence from kinematics and stellar population,
indicates a captured origin from a satellite galaxy. We show that, together
with the horizontal-branch evolutionary effect, the factor producing the
dichotomy could be a small time-gap between the cluster-formation epochs in the
Milky Way and the satellite. The results oppose the traditional view that the
metal-poorest clusters represent the indigenous and oldest population of the
Galaxy.
