Probing the farthest star clusters to the Small Magellanic Cloud

TL;DR

The study addresses whether three unusually distant SMC clusters are bona fide SMC members or remnants stripped by LMC tides. It uses deep GEMINI GMOS-S imaging to produce high-quality CMDs, applies field-star decontamination and isochrone fitting (via ASteCA) to derive ages, metallicities, and distances, and analyzes structural and kinematic properties with radial-density profiles and Gaia DR3 proper motions to compute 3D velocities. The clusters show a spectrum of kinematic states: OGLE-CL-SMC0237 in the SMC disk rotation, OGLE-CL-SMC0133 marginally rotating, and Lindsay 116 strongly perturbed as a likely LMC-tide–driven survivor, with Lindsay 116 being among the outermost SMC clusters. Ages are around $\,\sim 2.2\$ Gyr and [Fe/H] between $-1.0$ and $-0.7$ dex, and their distances place two in the SMC body while Lindsay 116 lies at a large deprojected radius ($r_{\rm deproj} \approx 15.7$ kpc), supporting a tidal interaction scenario. Overall, the work highlights the importance of deep CMDs and kinematic diagnostics for mapping the 3D structure and interaction history of the SMC–LMC system.

Abstract

The Small Magellanic Cloud (SMC) has been tidally shaped by the interaction with the Large Magellanic Cloud (LMC). The scope of such an interaction has recently been studied from different astrophysical properties of its star cluster population, which point to star clusters placed remarkably outside the known extension of the galaxy. In this work we report results for three of the recently identified most external SMC star clusters, OGLE-CL-SMC0133, OGLE-CL-SMC0237, and Lindsay~116, using deep GEMINI GMOS imaging. Once we confidently cleaned their color-magnitude diagrams from field star contamination, we estimated their fundamental parameters applying likelihood techniques. We also derived their structural parameters from normalized star number density radial profiles. Based on {\it Gaia} astrometric data, complemented with kinematics information available in the literature, we computed the 3D components of their space velocities. With similar ages (~ 2.2 Gyr) and moderately metal-poor overall abundances ([Fe/H] = -1.0 - -0.7 dex), OGLE-CL-SMC0237 is placed at 2.6 kpc from the SMC center and shares its disk rotation; OGLE-CL-SMC0133 is located at 7.6 kpc from the galaxy center and exhibits a kinematics marginally similar to the SMC rotation disk, while Lindsay~116 placed at 15.7 kpc from the center of the SMC is facing strong perturbations of its orbital motion with respect to an ordered rotational trajectory. Furthermore, its internal dynamical evolution would seem to be accelerated -- it seems kinematically older -- in comparison with star clusters in the outskirts of relatively isolated galaxies. These outcomes lead to conclude that Lindsay~116 is subject to LMC tides.

Figures (7)

• Figure 1: Color-magnitude diagrams of the GMOS-S FOV centered on OGLE-CL-SMC0133 (left), OGLE-CL-SMC0237 (middle), and Lindsay 116 (right). The solid line represent the limiting magnitude reached by the SMASH database in those star cluster fields.
• Figure 2: Cleaned color-magnitude diagrams of OGLE-CL-SMC0133 (left), OGLE-CL-SMC0237 (middle), and Lindsay 116 (right). The color bar represents (in %) how dissimilar the magnitude and color of a star are from the magnitude and color distributions of the projected surrounding SMC field-star population.
• Figure 3: Color-magnitude diagrams of OGLE-CL-SMC0133 (left), OGLE-CL-SMC0237 (middle), and Lindsay 116 (right) for stars with $P > 70\%$, with the isochrone of betal12 corresponding to the best fitted parameters values superimposed (see text for details).
• Figure 4: Normalized observed and background-subtracted star number density radial profiles of OGLE-CL-SMC0133 (left), OGLE-CL-SMC0237 (middle) and Lindsay 116 (right) drawn with open and black filled circles, respectively, with uncertainties represented by error bars. The horizontal line represents the adopted mean background level. Blue, orange, and magenta solid lines are the best-fitted king62's, plummer11's, and eff87's models, respectively.
• Figure 5: Left: Equal-area Hammer projection of the SMC in equatorial coordinates. Black dots represent the star clusters cataloged in bicaetal2020. Right: distribution of heliocentric distances of SMC star clusters. The solid vertical line represent the SMC center derived by graczyketal2020.