The Age–Metallicity–Specific Orbital Energy Relation for the Milky Way's Globular Cluster System Confirms the Importance of Accretion for Its Formation

We found that the majority of the Milky Way's metal poor globular clusters lie on a common plane in age-metallicity-orbital energy space, with older or more metal rich clusters being more tightly bound to the Galaxy. This correlation is consistent with the hierarchical assembly of the halo; older, metal poor clusters came from the earliest accreted satellite galaxies and were deposited into the Galaxy's potential back when the Milky Way was smaller and less massive. They remained trapped as the Milky Way continued to grow, while more recently accreted satellites disrupt and deposit their clusters at further radii and higher orbital energies.