The bifurcated age–metallicity relation of Milky Way globular clusters and its implications for the accretion history of the galaxy

摘要: We use recently derived ages for 61 Milky Way (MW) globular clusters (GCs) to show that their age-metallicity relation (AMR) can be divided into two distinct, parallel sequences at [Fe/H] $\ga -1.8$. Approximately one-third of the form an offset sequence spans full range in age ($\sim 10.5$--13 Gyr), but is more metal rich a given by $\sim 0.6$ dex [Fe/H]. All one orbital properties are consistent with membership MW disk. They not simply most metal-rich GCs, which have long been known disk-like kinematics, they all ages. The slope mass-metallicity (MMR) galaxies implies metallicity branches AMR corresponds mass decrement 2 dex, suggesting host galaxy masses $M_{*} \sim 10^{7-8} \msol$ GCs belong metal-poor AMR. suggest branch consists formed in-situ disk, while were relatively low-mass (dwarf) and later accreted MW. observed disk stars, LMC, SMC WLM dwarf shown this interpretation, relative distribution implied progenitor halo GC excellent agreement subhalo function predicted simulations. A notable implication bifurcated AMR, identical mean spread ages, poor difficult reconcile formation latter population.