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

TL;DR

This study uses the age-metallicity-orbital energy relation of the Milky Way's globular clusters to confirm that accretion of dwarf galaxies significantly contributed to the galaxy's formation, revealing distinct formation histories.

Contribution

It introduces a novel method to distinguish in situ and accreted globular clusters based on their orbital energy, age, and metallicity relations, supported by new precise measurements.

Findings

Metal-poor clusters are weakly bound and likely accreted from dwarf galaxies.

Metal-rich clusters are tightly bound, indicating in situ formation or early accretion.

The observed relations support galaxy formation models in a ΛCDM universe.

Abstract

Globular clusters can form inside their host galaxies at high redshift when gas densities were higher and gas-rich mergers were common. They can also form inside lower-mass galaxies that have since been accreted and tidally disrupted, leaving their globular cluster complement bound to higher-mass halos. We argue that the age-metallicity-specific orbital energy relation in a galaxy's globular cluster system can be used to identify its origin. Gas-rich mergers should produce tightly bound systems in which metal-rich clusters are younger than metal-poor clusters. Globular clusters formed in massive disks and then scattered into a halo should have no relationship between age and specific orbital energy. Accreted globular clusters should produce weakly bound systems in which age and metallicity are correlated with each other but inversely correlated with specific orbital energy. We use precise relative ages, self-consistent metallicities, and space-based proper motion-informed orbits to show that the Milky Way's metal-poor globular cluster system lies in a plane in age-metallicity-specific orbital energy space. We find that relatively young or metal-poor globular clusters are weakly bound to the Milky Way, while relatively old or metal-rich globular clusters are tightly bound to the Galaxy. While metal-rich globular clusters may be formed either in situ or ex situ, our results suggest that metal-poor clusters formed outside of the Milky Way in now-disrupted dwarf galaxies. We predict that this relationship between age, metallicity, and specific orbital energy in a $L^{*}$ galaxy's globular cluster system is a natural outcome of galaxy formation in a $\Lambda$CDM universe.