$S^5$: New insights from deep spectroscopic observations of the tidal tails of the globular clusters NGC 1261 and NGC 1904 (2411.08991v1)

Abstract: As globular clusters (GCs) orbit the Milky Way, their stars are tidally stripped forming tidal tails that follow the orbit of the clusters around the Galaxy. The morphology of these tails is complex and shows correlations with the phase of the orbit and the orbital angular velocity, especially for GCs on eccentric orbits. Here, we focus on two GCs, NGC 1261 and NGC 1904, that have potentially been accreted alongside Gaia-Enceladus and that have shown signatures of having, in addition of tidal tails, structures formed by distributions of extra-tidal stars that are misaligned with the general direction of the clusters' respective orbits. To provide an explanation for the formation of these structures, we make use of spectroscopic measurements from the Southern Stellar Stream Spectroscopic Survey ($S^5$) as well as proper motion measurements from Gaia's third data release (DR3), and apply a Bayesian mixture modeling approach to isolate high-probability member stars. We recover extra-tidal features similar to those found in Shipp et al. (2018) surrounding each cluster. We conduct N-body simulations and compare the expected distribution and variation in the dynamical parameters along the orbit with those of our potential member sample. Furthermore, we use Dark Energy Camera (DECam) photometry to inspect the distribution of the member stars in the color-magnitude diagram (CMD). We find that the potential members agree reasonably with the N-body simulations and that the majority of them follow a simple stellar population-like distribution in the CMD which is characteristic of GCs. In the case of NGC 1904, we clearly detect the tidal debris escaping the inner and outer Lagrange points which are expected to be prominent when at or close to the apocenter of its orbit. Our analysis allows for further exploration of other GCs in the Milky Way that exhibit similar extra-tidal features.

• The paper analyzes the tidal tails of globular clusters NGC 1261 and NGC 1904 using S^5 and Gaia DR3 data, employing Bayesian mixture modeling to distinguish high-probability member stars.
• N-body simulations confirm that the observed spatial distribution and dynamical variations of the tidal tails are consistent with models of stream formation through tidal disruption, aligning well with NGC 1904.
• Analysis reveals complex tidal features like cross-shaped patterns and extended tails, showing that the orientation of these features correlates with the clusters' orbital phases around the Milky Way.

Insights from the Southern Stellar Stream Spectroscopic Survey into the Tidal Tails of NGC 1261 and NGC 1904

This paper presents an in-depth analysis of the tidal tails of the globular clusters NGC 1261 and NGC 1904 using data from the Southern Stellar Stream Spectroscopic Survey (S^5) and Gaia DR3. As globular clusters orbit the Milky Way, they lose stars due to tidal forces, forming complex structures known as tidal tails. This research focuses on characterizing these stellar streams and their implications for understanding the dynamical history and evolution of their host clusters.

Key Findings

• Bayesian Mixture Modelling: The paper implements a Bayesian mixture modelling approach to distinguish high-probability member stars of the clusters from field stars. This statistical method effectively isolates stars that are likely part of the globular clusters' tidal tails, confirming the previous detection of these structures.
• Simulations Consistency: The use of N-body simulations allows for a comparison between observed and predicted stellar distributions. These simulations reveal that the stars' spatial distribution and dynamical variations along the orbit are consistent with expectations of stream formation through tidal disruption, particularly aligning well with the results for NGC 1904.
• Tidal Features: Analysis indicates that NGC 1261 and NGC 1904 display multiple tidal features, including cross-shaped patterns and extended tails, consistent with theoretical models of the gravitational interactions these clusters experience during orbit around the Milky Way. NGC 1904, in particular, reveals signs of significant tidal stripping, with debris patterns indicative of recent apocentric passages.
• Orbital Dynamics: The paper elucidates the correlation between the orientation of the observed tidal tails and the clusters’ orbital phases. NGC 1904 is close to apocenter, resulting in distinctive orientations of its tidal tails relative to the Milky Way's center, supporting prior theoretical predictions of stream morphologies dependent on orbital position.

Implications and Future Directions

The findings provide robust empirical support for the models of tidal tail formation, enhancing our understanding of the dynamical processes governing globular cluster evolution. These results have broader implications for using stellar streams as probes of the Galaxy's potential, offering a lens through which to paper not only the clusters themselves but also the dark matter distribution in the Milky Way halo.

Further exploration can extend this approach to other globular clusters displaying similar tidal features, broadening our knowledge of cluster disruption history and contributing to the precision mapping of the Galaxy's mass distribution. As spectroscopic surveys continue to expand, leveraging data from projects like S^5 and Gaia will be instrumental in unraveling the complex tapestry of stellar interactions in the galactic halo. Additionally, integrating high-resolution spectroscopic data could refine chemical abundance measurements, helping to disentangle individual stream origins and interactions over cosmic time.

This research underscores the necessity for synergy between observational data, advanced statistical methods, and computational simulations in astronomy, driving a comprehensive understanding of the dynamics shaping our Galactic environment.