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

TL;DR

This work tackles the incomplete census of Galactic globular clusters behind the bulge and plane by exploiting RR Lyrae stars as bright tracers. A six-dimensional clustering framework (positions, proper motions, and metallicities) is applied to Gaia DR3 and VVV/VVVx RRab samples, carefully calibrating HDBSCAN to recover known GCs and identify GC-like substructures. The analysis yields high-purity, cohesive recoveries of GC members and uncovers dozens of GC-like RRab groups, including some not clearly associated with known GCs, bulge, or Sgr dSph, up to distances of ~25 kpc. The results provide a catalog of promising candidates for follow-up spectroscopy and radial-velocity measurements, highlighting the potential of 6D clustering to reveal hidden substructures in the most crowded regions of the Milky Way.

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.

Figures (10)

• Figure 1: Sky distribution in Galactic coordinates of our RRab samples after the quality cuts described in Sect. \ref{['sec:quality_cuts']}. RRab stars from Gaia DR3 are shown as black circles, while those from Z24 and VIVACE are shown as red and blue circles, respectively. The lower panel is a zoom-in of the upper one, as highlighted with blue dashed lines. For a better visualization, stars in the VVV samples are plotted on top in the upper panel, while stars in the Gaia sample are plotted on top in the lower panel.
• Figure 2: Comparison of the clusters' [Fe/H] (upper panels) and distance (lower panels) values derived from their RRab stars and their values from the literature. Clusters in the VIVACE, Z24, and Gaia samples are shown in the left, center, and right panels, respectively. The black dotted lines show the zero functions, while the median and MAD of the y-axis values in each panel are highlighted in blue text and as a blue line and shaded region, respectively. The y-axis error bars shown in the upper panels correspond to the errors in our [Fe/H] estimates, while the y-axis error bars in the lower panels correspond to the errors in the RRab-based distances divided by the nominal values from the literature. Clusters with relative distance differences greater than $10 \%$ have their names highlighted in the lower panels.
• Figure 3: Metric distributions of all the RRab groups (blue histograms) obtained, compared with the metrics of the cleaned GCs in our calibrating sample (black histograms) in the VIVACE, Z24, and Gaia samples (upper, middle, and lower panels, respectively). Metric regions that satisfy our cuts are displayed with light-green shaded areas, with the cut threshold of each panel highlighted in green text.
• Figure 4: Distribution of the compact groups with at least three RRab stars in the sky (upper panels) and PM-space (lower panels). Groups found in the Gaia, Z24, and VIVACE samples are shown in the left, middle, and right panels, respectively. Stars in the compact groups are colored according to their heliocentric distances. For easier identification of the groups, their IDs are highlighted in the upper panels, and stars are connected by black lines to a box with their group's ID in the lower panels (the latter is not done for the Gaia groups since all of their stars have very similar PMs). Blue-dashed lines represent the density contours of Sgr dSph stars (in the left panels) and bulge stars (in the middle and right lower panels), while gray crosses represent known GCs. The mean PM error in Gaia are shown in the lower-right corner.
• Figure 5: Same as Fig. \ref{['fig:compact_groups']}, but for the compact pairs of RRab stars in the Z24 and VIVACE samples that were not identified as potentially associated to the Galactic bulge or the Sgr dSph galaxy.