Constructing a Pristine View of Extended Globular Cluster Structure

TL;DR

This study uses multi-dimensional data and machine learning to identify and analyze extended structures around Milky Way globular clusters, revealing new features and confirming the importance of metallicity data in such searches.

Contribution

It introduces a large-scale survey combining photometric metallicities and machine learning to detect extended globular cluster structures, including new tentative detections.

Findings

22 GCs have extended structures within 5 degrees

Six of these structures are new tentative detections

Presence of diffuse envelopes and tidal tails around GCs

Abstract

Globular Clusters (GCs) displaying extended structures are becoming increasingly ubiquitous in the Milky Way (MW). Despite their low surface brightness, which makes disentangling the true structure from the MW overwhelmingly difficult, the increasing availability of multi-dimensional data sets has allowed for new detections of extended GC structure. This work utilises the Pristine-Gaia-Synthetic catalogue released as part of the Pristine Surveys first data release to search for tidally stripped stars in the peripheries of MW GCs. Pristine provides photometric [Fe/H] measurements based on CaHK-band photometry. Using unsupervised machine learning techniques, we provided lists of extra-tidal stars for 30 GCs, one of the largest surveys of its kind. We find that (1) 22 GCs that passed our quality cut have extended structure within 5 deg from the cluster centers of which six are new tentative detections, (2) four of those GCs exhibit diffuse envelope-like extra-tidal features, while the remaining GCs exhibit tidal tail-like structures. We measure the position angles of the extended structures, find broad consistency between the position angles and the GC orbits, and discuss our results concerning N-body models. This work demonstrates the effectiveness of adding photometric metallicities to the multi-dimensional search of extended tidal structure and how the upcoming multi-object spectrographs will be crucial for exploring GC peripheries in the coming years.