The Sausage Globular Clusters

TL;DR

This paper identifies globular clusters associated with the Gaia Sausage merger event in the Milky Way by analyzing their action space properties, revealing a distinct, radially anisotropic group consistent with a dwarf galaxy origin.

Contribution

It introduces a method to identify Sausage Globular Clusters using action space analysis and characterizes their properties, linking them to the Gaia Sausage merger event.

Findings

15 old halo GCs have high energy and are clustered in action space.

These GCs are highly eccentric and radially anisotropic.

They form a distinct age-metallicity track consistent with dwarf galaxy origin.

Abstract

The Gaia Sausage is an elongated structure in velocity space discovered by Belokurov et al. (2018) using the kinematics of metal-rich halo stars. It was created by a massive dwarf galaxy ($\sim 5 \times 10^{10} M_\odot$) on a strongly radial orbit that merged with the Milky Way at a redshift $z\lesssim 3$. We search forthe associated Sausage Globular Clusters by analysing the structure of 91 Milky Way globular clusters (GCs) in action space using the Gaia Data Release 2 catalogue, complemented with Hubble Space Telescope proper motions. There is a characteristic energy $E_{\rm crit}$ which separates the in situ objects, such as the bulge/disc clusters, from the accreted objects, such as the young halo clusters. There are 15 old halo GCs that have $E > E_{\rm crit}$. Eight of the high energy, old halo GCs are strongly clumped in azimuthal and vertical action, yet strung out like beads on a chain at extreme radial action. They are very radially anisotropic ($\beta \sim 0.95$) and move on orbits that are all highly eccentric ($e \gtrsim 0.80$). They also form a track in the age-metallicity plane distinct from the bulk of the Milky Way GCs and compatible with a dwarf spheroidal origin. These properties are consistent with GCs associated with the merger event that gave rise to the Gaia Sausage.