Unveiling the Faint Edges of Star Clusters in the Milky Way
Understanding the gravitational forces in our Milky Way galaxy helps astronomers piece together its history and structure. This force is shaped by dark matter, an invisible substance that dominates the mass of galaxies. To study these forces, scientists often look at the motions and shapes of globular clusters (GCs), dense, ancient groups of stars that orbit the Milky Way. When these clusters interact with the Galaxy’s gravitational field, they can shed stars, creating faint “extra-tidal” features, like envelopes or streams of stars extending beyond the main body of the cluster. Detecting these faint features is difficult, but important because they act as tracers of both the clusters’ disruption and the Galaxy’s gravitational potential. In this study, Chiti, Tavangar, and collaborators searched for such features in 19 clusters using data from the DELVE survey, and tested whether future surveys like LSST could detect even fainter ones.
Data & Methods
The team began their search in the DELVE DR2 (Data Release 2) dataset, which maps over 21,000 square degrees of the sky using images from the DECam camera. They focused on regions within two degrees of each cluster center. Stars were selected using their brightness and colors, and the authors carefully removed stars that did not belong to the clusters by comparing their motions and distances using the Gaia satellite’s precise measurements. To find extra-tidal features, they created density maps of the stars, highlighting over-densities that extended beyond the “Jacobi radius,” a theoretical boundary where stars are no longer tightly bound to the cluster. Special care was taken to ensure these detections were not just due to noise or uneven data coverage.
Observational Results
Out of the 19 clusters studied, most showed results consistent with previous work. However, the team identified a clear extra-tidal envelope around NGC 5897 and tentative evidence for a similar feature around NGC 7492. For NGC 5897, they found a statistically significant excess of stars extending beyond its Jacobi radius, at about 4.6σ significance, which is strong evidence for a real feature. This envelope does not align with the cluster’s motion, suggesting it may have been caused by a recent close approach to the Milky Way’s center. NGC 7492 showed a weaker, but noticeable, excess of stars beyond its boundary at about 2.8σ significance.
Simulations & Testing Detectability
To better understand these features, the authors simulated the tidal disruption of clusters like NGC 5897 using a software package called Gala. They also simulated deeper photometric data to mimic what the upcoming LSST survey will provide. The simulations showed that while LSST’s deeper observations will improve the ability to detect these faint envelopes, additional techniques, like filtering by star metallicities or more precise motion measurements, may still be necessary to clearly identify streams against the foreground of Milky Way stars.
Discussion & Conclusion
The discovery of an extra-tidal envelope around NGC 5897 adds to the growing list of clusters with known tidal features, and suggests that even more remain to be discovered. The evidence around NGC 7492 is less clear, but intriguing, and further observations may resolve its nature. This study shows that wide-field, deep imaging surveys can reveal subtle signatures of tidal disruption, helping astronomers trace the history of globular clusters and the shape of the Milky Way’s dark matter halo. The authors also highlight the challenges of identifying such faint features and emphasize the importance of combining future deep photometry with additional data to distinguish real streams from background noise.
Source: Chiti
