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

Kavli Affiliate: Andrey Kravtsov

| First 5 Authors: Vasily Belokurov, Andrey Kravtsov, , ,

| Summary:

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 [Al/Fe] abundance ratio.
We demonstrate that such classification results in the 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 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 $rm [Fe/H]$ 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 $rm [Al/Fe]-[Mg/Fe]$ abundance ratios and follow distinct
sequences in the age–$rm [Fe/H]$ plane. The in-situ GCs in our classification
show a clear disc spin-up signature — the increase of median $V_phi$ at
metallicities $rm [Fe/H]approx -1.3div -1$ similar to the spin-up in the
in-situ field stars. This signature signals the MW’s disc formation, which
occurred $approx 11.7-12.7$ Gyrs ago (or at $zapprox 3.1-5.3$) according to
GC ages. In-situ GCs with metallicities of $rm [Fe/H]gtrsim -1.3$ were thus
born in the Milky Way 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.

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