Euclid: Early Release Observations -- Unveiling the morphology of two Milky Way globular clusters out to their periphery

Abstract: As part of the Euclid Early Release Observations (ERO) programme, we analyse deep, wide-field imaging from the VIS and NISP instruments of two Milky Way globular clusters (GCs), namely NGC 6254 (M10) and NGC 6397, to look for observational evidence of their dynamical interaction with the Milky Way. We search for such an interaction in the form of structural and morphological features in the clusters' outermost regions, which are suggestive of the development of tidal tails on scales larger than those sampled by the ERO programme. Our multi-band photometric analysis results in deep and well-behaved colour-magnitude diagrams that, in turn, enable an accurate membership selection. The surface brightness profiles built from these samples of member stars are the deepest ever obtained for these two Milky Way GCs, reaching down to $\sim30.0$ mag~arcsec$^{-2}$, which is about $1.5$ mag arcsec$^{-2}$ below the current limit. The investigation of the two-dimensional density map of NGC 6254 reveals an elongated morphology of the cluster peripheries in the direction and with the amplitude predicted by $N$-body simulations of the cluster's dynamical evolution, at high statistical significance. We interpret this as strong evidence for the first detection of tidally induced morphological distortion around this cluster. The density map of NGC 6397 reveals a slightly elliptical morphology, in agreement with previous studies, which requires further investigation on larger scales to be properly interpreted. This ERO project thus demonstrates the power of Euclid in studying the outer regions of GCs at an unprecedented level of detail, thanks to the combination of large field of view, high spatial resolution, and depth enabled by the telescope. Our results highlight the future Euclid survey as the ideal data set to investigate GC tidal tails and stellar streams.

• The paper reveals unprecedented surface brightness profiles that indicate increased tidal radii and morphological distortions in cluster NGC 6254.
• It employs deep, wide-field imaging from Euclid’s VIS and NISP instruments to construct precise color-magnitude diagrams and detailed surface brightness profiles.
• The study quantitatively compares observations with N-body simulations, reinforcing evidence of tidal distortions from the Milky Way's gravitational influence.

Overview of the Euclid Early Release Observations of Milky Way Globular Clusters

The paper, "Euclid: Early Release Observations -- Unveiling the morphology of two Milky Way globular clusters out to their periphery," presents a detailed analysis of observations of two Milky Way globular clusters, NGC 6254 (M10) and NGC 6397, using data from the Euclid mission's Early Release Observations (ERO). The objective was to uncover morphological features in the clusters' peripheral regions, indicative of their dynamical interactions with the Milky Way, such as tidally induced distortions.

Methodology and Observations

The study employs the VIS and NISP instruments aboard the Euclid satellite to capture deep, wide-field imaging of these clusters. The data covers a significant portion of the clusters' peripheries, enabling the creation of high-precision color-magnitude diagrams (CMDs) and surface brightness profiles. The CMDs exhibit a clear main sequence down to faint magnitudes, providing insights into the stellar composition and characteristics of these clusters.

Key Results

1. Surface Brightness Profiles: The analysis yielded the deepest surface brightness profiles ever obtained for these clusters, reaching limits of approximately 30.0 mag arcsec⁻² for NGC 6254. The profiles were fitted using King models to derive structural parameters. The results suggest an increased tidal radius for NGC 6254 compared to previous estimates, a finding attributed to the survey's ability to detect low-mass stars.
2. Morphological Features: A significant finding is the identification of an elongated morphology for NGC 6254, aligned northwest, consistent with tidal distortion. This aligns with predictions from N-body simulations modeling the cluster's interaction with the Milky Way's gravitational field. For NGC 6397, the observed elongation was less pronounced and its interpretation as a tidal feature remains inconclusive without observations extending beyond its tidal radius.
3. Comparison with Simulations: The observed morphological elongations in NGC 6254's outer regions were quantitatively supported by N-body simulations, which predicted similar elongation directions and ellipticity variations with distance from the cluster center.

Implications

These findings highlight the efficacy of the Euclid mission in enhancing our understanding of the outer regions of globular clusters, with significant implications for studying stellar dynamics and the interplay between clusters and galactic potentials. The detection of tidal distortions offers a window into the clusters' histories and interactions with the Milky Way, contributing to the broader narrative of galactic structure and evolution.

Future Prospects

The paper suggests that the forthcoming Euclid survey, with its expansive coverage and depth, is ideally suited for pursuing similar studies of many more globular clusters within its footprint. The ability to analyze tidal tails and stellar streams associated with these clusters will provide unprecedented insights into their formation, evolution, and interactions with the Milky Way.

The datasets and insights from Euclid are poised to significantly advance the field of galactic archaeology by enabling a comprehensive analysis of the faintest stellar populations across globular clusters, augmenting our understanding of the Milky Way's assembly history.