How Globular Clusters Shape the Streams of Stars in the Milky Way
Stellar streams are long, thin trails of stars that form when globular clusters—dense groups of stars—are pulled apart by the Milky Way's gravity. These streams are valuable tools for studying both the visible and dark matter in the galaxy. Previously, gaps in these streams were attributed to dark matter sub-halos, but this paper, led by Salvatore Ferrone, explores whether interactions with other globular clusters can also cause such gaps.
The Case of Palomar 5
The authors focused on the Palomar 5 globular cluster, which has remarkably long tidal tails. By simulating how this cluster interacts with the gravitational pull of the galaxy and other globular clusters, they aimed to identify whether these interactions create gaps in its stellar streams.
Methods
The researchers used a computer simulation to model the movements of Palomar 5 and 165 other globular clusters over 5 billion years. By tracking how stars in Palomar 5’s stream were influenced by close encounters with other clusters, they could determine which interactions caused gaps. The simulations incorporated the galaxy's gravitational forces but simplified certain details, like the internal evolution of the globular clusters, to save computation time.
Results
The simulations revealed that interactions with globular clusters like NGC 2808, NGC 7078, and NGC 104 frequently created gaps in Palomar 5’s stellar streams. On average, 1.5 gaps appeared per simulation, often in the leading part of the stream. These gaps varied in size, depending on the mass, distance, and speed of the interacting globular cluster. Surprisingly, even fast-moving clusters could create gaps if they passed close enough to the stream.
Implications
The findings suggest that globular clusters play a more significant role in shaping stellar streams than previously thought. This complicates efforts to use gaps in streams to study dark matter, as researchers must now account for the influence of globular clusters.
Conclusion
While the study showed that globular cluster interactions create gaps, these gaps were not always in the observed portions of Palomar 5’s stream. This highlights the complexity of disentangling the effects of dark matter and baryonic (normal) matter on stellar streams. Future research could expand these simulations to other clusters and refine the models to better match observations.
Source: Ferrone
