The Sausage Globular Clusters (1805.00453v2)

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.

• The paper identifies and characterizes a distinct group of 8 "Sausage Globular Clusters" within the Milky Way's halo using Gaia data and action space analysis.
• These Sausage GCs exhibit extreme radial anisotropy and eccentric orbits, with $\beta \sim 0.95$ and $e \gtrsim 0.80$, mirroring the kinematics of the "Gaia Sausage" structure.
• The existence and properties of the Sausage GCs provide strong evidence for a significant merger event where the Milky Way accreted a massive dwarf galaxy at redshift $z \lesssim 3$.

An Examination of the `Sausage' Globular Clusters and Their Implications for Galactic Dynamics

The paper, "The Sausage' Globular Clusters" by Myeong et al., presents a compelling investigation into a subset of globular clusters (GCs) within the Milky Way galaxy, identified in action space, that are associated with theGaia Sausage.' This cosmic structure has gained attention due to its peculiar velocity space characteristics and its implications for understanding the dynamics of our galaxy.

The analysis carried out by Myeong et al. leverages data from the Gaia Data Release 2, supplemented by proper motion measurements from the Hubble Space Telescope, to explore the kinematic and dynamic properties of 91 Milky Way globular clusters. By plotting these clusters in action space, the paper not only reinforces the hypothesis of a recent massive accretion event but also identifies a distinct group of clusters, termed the `Sausage Globular Clusters', which exhibit unique dynamical properties indicative of such an event.

Key Findings and Analysis

Several noteworthy insights emerge from the research:

• Action Space Analysis: The paper illuminates the presence of 15 old halo GCs with energies exceeding a characteristic value, distinguishing them as accreted objects rather than structures formed in situ. Within this subset, eight GCs, termed the `Sausage Globular Clusters', are markedly clustered in azimuthal and vertical action yet positioned at extreme radial actions. This spatial distribution substantiates the strong radial anisotropy and eccentric orbits ($\beta \sim 0.95$, $e \gtrsim 0.80$) that these GCs exhibit.
• Velocity Space Distributions: The velocity distributions of these GCs strain the typical isotropy expected and mirror the characteristics of the `Gaia Sausage,' reinforcing the narrative of a violent accretion event. The extreme radial anisotropy of the Sausage GCs is evident in their velocity dispersion.
• Age-Metallicity Relationship: Using the age-metallicity plane, the Sausage GCs delineate a track that diverges from the majority of the Milky Way's GCs, which suggests an extragalactic origin congruent with a dwarf spheroidal progenitor. This pattern aligns with expectations for globular clusters born out of major accretion events.

Theoretical and Practical Implications

The identification and characterization of the `Sausage Globular Clusters' provide robust evidence for a significant merger event in the Milky Way's history, estimated to have transpired at a redshift $z \lesssim 3$. The implications are substantial, suggesting that the Milky Way's assembly involved the absorption of a massive dwarf galaxy, considerably altering its halo structure and kinematic signature. This realization deepens our understanding of the hierarchical assembly of galaxies and highlights the importance of such accretive processes in the formation of galactic structures.

Prospects for Future Research

This research opens several avenues for further investigation:

• Numerical Simulations: To simulate the dynamics of the accretion event more accurately and predict the long-term stability and evolution of the `Sausage Globular Clusters.'
• High-Resolution Observations: Future Gaia data releases and observations with next-generation telescopes can provide improved astrometric data which may reveal additional members of the `Sausage Globular Clusters' and enhance our understanding of their spatial distribution.
• Chemical Abundances Studies: Investigating the detailed chemical composition of these clusters could yield insights into their formation environments and validate the model of a massive progenitor galaxy.

In conclusion, Myeong et al.'s analysis of the `Sausage Globular Clusters' accentuates the necessity of employing advanced kinematic analyses in interpreting the galaxy's evolutionary history. The forthcoming enhancements in observational capabilities and computational modeling will doubtlessly refine our comprehension of such accretive phenomena and their profound effects on galactic morphology and dynamics.