Nature and nurture: The dynamical ages of in situ and accreted globular clusters in the Milky Way

The bifurcated age-metallicity relation of globular clusters (GCs) in the Milky Way (MW) shows that GCs either originated in situ or accreted into the Galaxy from former satellites of the MW. The effects of the Galactic tidal field can leave signatures on the dynamical evolution and structural properties of GCs. We present a homogeneous census of dynamical ages for a sample of 93 GCs in the MW coupled with the knowledge of their common progenitors from the chemo-dynamical parameters from the Gaia mission, which was unavailable some years ago. We find that the majority of accreted GCs (61%) are dynamically young, but this percentage drops to 38% for in situ GCs. When excluding the enigmatic low-energy (LE) GCs with ambiguous origin, the fraction of dynamically young ex situ GCs rises to 70%. A two tail Kolmogorov-Smirnov test showed that the distribution of dynamical ages of LE GCs cannot be distinguished from the distribution of in situ bulge and disc GCs. Yet the LE GCs are firmly located on the satellite branch of the age-metallicity relation. An explanation for this may be that the progenitor of LE GCs plunged very early into the MW so that the gravitational field of the MW had enough time to act on the associated GCs. The dynamical ages offer statistically robust evidence corroborating the scenario of early accretion proposed by several recent studies regarding the origin of LE GCs.