A clustering-based search for substructures in the Galactic plane and bulge using RR Lyrae stars as tracers

TL;DR

This study uses clustering of RR Lyrae stars with six-dimensional data to identify potential new globular clusters in the Galactic plane and bulge, improving detection in crowded and obscured regions.

Contribution

It introduces a hierarchical clustering method applied to RR Lyrae stars with six-dimensional data to find new Galactic substructures, including potential undiscovered globular clusters.

Findings

Recovered known globular clusters using RR Lyrae groups.

Derived first RR Lyrae-based distances for BH 140 and NGC 5986.

Identified new small RR Lyrae groups possibly representing undiscovered clusters.

Abstract

Although many globular clusters (GCs) have been identified in the Galaxy, their population is estimated to be incomplete, especially in regions with strong crowding and interstellar extinction such as the Galactic bulge and plane.RR Lyrae stars, as bright standard candles and tracers of old stellar populations, are powerful tools for finding GCs in these regions, and large catalogs of such stars have recently become available. We aim to construct a sample of RR Lyrae stars with six-dimensional information (three-dimensional positions, proper motions, and metallicities) in the Galactic plane and bulge, and to exploit it using a hierarchical clustering algorithm to search for Galactic substructures. We build a sample of fundamental-mode RR Lyrae (RRab) stars with positions, distances, proper motions, and photometric metallicity estimates from Gaia and VVV data. Using a clustering algorithm calibrated to optimize GC recovery, we identify groups of RRab stars with similar locations in the six-dimensional parameter space. The most promising groups are selected by comparison with the properties of known GCs. We recover many RRab groups associated with known Galactic GCs and derive the first RR Lyrae-based distances for BH 140 and NGC 5986. We also detect small groups of two to three RRab stars at distances up to ~25 kpc that are not associated with any known GC, but display GC-like distributions in all six parameters. Several of these groups, mostly pairs, lie toward the Galactic bulge but show distinct proper motions or distances, suggesting they may not belong to the bulge population. Overall, our approach identifies dozens of GC-like RRab groups in the Galactic plane and bulge, which are excellent targets for follow-up observations. Future radial velocity measurements can test whether the RRab members of these groups are truly co-moving.