Tracing the Origins of Globular Clusters Through Their Tidal Tails

Globular clusters are massive groups of stars bound together by gravity, and over time, they lose stars due to the gravitational pull of the Milky Way. These escaping stars form long, thin streams called tidal tails. By studying these tails, astronomers can learn about the history of the clusters—whether they formed inside the Milky Way (in-situ) or were once part of smaller galaxies that merged with it. In this paper, Andrés E. Piatti examines a recently created catalog of simulated (or mock) extra-tidal stars to test whether they accurately reflect the real tidal tails of globular clusters.

Building Mock Tidal Tails

The study relies on a catalog of simulated extra-tidal stars, produced by Grondin et al. (2024). This catalog contains stars that were artificially generated to model how real globular clusters eject stars over time. These stars were created under the assumption that the clusters formed in the Milky Way, rather than being accreted from smaller galaxies. If the properties of these simulated stars—such as their width, velocity dispersion, and angular momentum—match those of real tidal tails, then the simulations could be useful for future studies.

Measuring Key Properties

Piatti selects a sample of globular clusters that are widely believed to have formed in the Milky Way’s bulge (central region) or disk (outer region). He analyzes the width of the tidal tails, as well as how the stars' velocities are spread out. Specifically, he examines the z-component of angular momentum (a measure of how the stars move relative to the galaxy's rotation), the line-of-sight velocity dispersion (how fast the stars move toward or away from Earth), and the tangential velocity dispersion (how fast they move sideways).

By comparing these properties to predictions from earlier studies—such as those by Malhan et al. (2021, 2022)—he aims to determine if the simulated stars match what is expected for in-situ globular clusters.

Do the Simulations Match Reality?

Piatti’s findings suggest a mismatch between the mock tidal tails and real ones. While the tidal tail widths mostly agree with an in-situ formation, the velocity dispersions do not. The spread of velocities in the simulated tidal tails is much larger than what is typically expected for globular clusters formed in the Milky Way. Instead, the simulations more closely resemble clusters that originated in dwarf galaxies before merging with the Milky Way.

Additionally, he finds that as the tidal tails get longer, their width and angular momentum dispersion increase, while the bulk velocity dispersion remains around 12 km/s. This trend is seen for both bulge and disk clusters, suggesting a common pattern in their evolution.

What This Means for Future Research

These findings raise important questions. If the mock stars do not accurately represent real tidal tails, it suggests that the assumptions made in the simulations—particularly about how stars are ejected—may need to be refined. Alternatively, it could mean that some globular clusters thought to have formed in the Milky Way actually have an external origin.

By highlighting these discrepancies, Piatti’s work helps refine future studies of globular clusters and their tidal tails. Understanding these structures is crucial for piecing together the history of the Milky Way and the smaller galaxies that may have contributed to its formation.

Source: Piatti