An Aligned Stream of Low-Metallicity Clusters in the Halo of the Milky Way

Abstract

One of the long-standing problems in modern astronomy is the curious division of Galactic globular clusters, the &#34;Oosterhoff dichotomy,&#34; 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.