Size diversity of old Large Magellanic Cloud clusters as determined by internal dynamical evolution

Abstract

The distribution of size as a function of age observed for star clusters in the Large Magellanic Cloud (LMC) is very puzzling: young clusters are all compact, while the oldest systems show both small and large sizes. This phenomenon is commonly interpreted as being due to a population of binary black holes driving a progressive expansion of cluster cores. Here we propose, instead, that it is the natural consequence of the fact that only relatively low-mass clusters have formed in the past ~3 Gyr in the LMC and only the most compact systems survived and are observable. The spread in size displayed by the oldest (and most massive) clusters, instead, can be explained in terms of initial conditions and internal dynamical evolution. To quantitatively explore the role of the latter, we selected a sample of five coeval and old LMC clusters with different sizes, and we estimated their dynamical age from the level of central segregation of blue straggler stars (the so-called dynamical clock). Similarly to what is found in the Milky Way, we indeed measure different levels of dynamical evolution among the selected coeval clusters, with large-core systems being dynamically younger than those with small size. This behaviour is fully consistent with what is expected from internal dynamical evolution processes over timescales mainly set by the structure of each system at formation.

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Fig. 1: The drastic change in LMC cluster properties as a function of cosmic time.
Fig. 2: The age of the five LMC clusters.
Fig. 3: BSS selection.
Fig. 4: Measure of the A+ parameter
Fig. 5: Quantifying the internal dynamical evolution and its effect on the cluster physical size.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This research is part of the project COSMIC-LAB at the University of Bologna. The research is based on data acquired with the NASA/ESA HST at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.

Author information

F.R.F. designed the study and coordinated the activity. E.D., M.C. and S.R. analysed the photometric datasets. F.R.F, B.L. and E.D. wrote the first draft of the paper. G.B., A.M. and C.P. critically contributed to the paper presentation. All the authors contributed to the discussion of the results and commented on the manuscript.

Correspondence to F. R. Ferraro.

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Supplementary Figs. 1 and 2 and Table 1.

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