In-situ versus accreted Milky Way globular clusters: a new classification method and implications for cluster formation

ABSTRACT We present a new scheme for the classification of the in-situ and accreted globular clusters (GCs). The scheme uses total energy E and z-component of the orbital angular momentum and is calibrated using the Al/Fe abundance ratio. We demonstrate that this classification results in two GC populations with distinct spatial, kinematic, and chemical abundance distributions. The in-situ GCs are distributed within the central 10 kpc of the Galaxy in a flattened configuration aligned with the Milky Way (MW) disc, while the accreted GCs have a wide distribution of distances and a spatial distribution close to spherical. In-situ and accreted GCs have different Fe/H distributions with the well-known bimodality present only in the metallicity distribution of the in-situ GCs. Furthermore, the accreted and in-situ GCs are well separated in the plane of Al/Fe-Mg/Fe abundance ratios and follow distinct sequences in the age– Fe/H plane. The in-situ GCs in our classification show a clear disc spin-up signature – the increase of median Vϕ at metallicities −1. 3 Fe/H −1 similar to the spin-up in the in-situ field stars. This signature signals the MW’s disc formation, which occurred ≈11. 7−12. 7 Gyr ago (or at z ≈ 3. 1−5. 3) according to in-situ GC ages. In-situ GCs with metallicities of Fe/H -1. 3 were thus born in the MW disc, while lower metallicity in-situ GCs were born during early, turbulent, pre-disc stages of the evolution of the Galaxy and are part of its Aurora stellar component.