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. It is commonly interpreted as 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 last ~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 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 expected from internal dynamical evolution processes over timescales mainly set by the structure of each system at formation.The Large Magellanic Cloud (LMC) hosts star clusters covering a wide range of ages (from a few million, to several billion years), at odds with the Milky Way where mostly old (t>10 Gyr) globular cluster (GCs) are found. The LMC thus offers a unique opportunity to explore the evolutionary processes of stellar clusters over cosmic time. One of the most intriguing features emerging from these studies 1-4 is the behaviour of the core radius (r c , which characterizes the size of the innermost cluster region) as a function of age (as measured from the cluster stellar population): the youngest clusters are all compact (with r c < 2.5 pc), while the oldest ones span the full range of observed r c

The “dynamical clock”: dating the internal dynamical evolution of star clusters with Blue Straggler Stars

“…These results confirm that the internal dynamical evolution tends to produce clusters with more and more compact cores, and demonstrate that the spread in core size observed for the old LMC GCs is the natural consequence of these processes. On the other hand, the analysis discussed in Ferraro et al (2019) also demonstrates that only low-mass systems have been recently (in the last ∼ 3 Gyr) formed in the LMC, and essentially all of them were generated in the innermost regions of the galaxy. Hence, they surely cannot resemble the progenitors of the currently old clusters, which are much more massive (by up to factors of 100) and orbit at any distance from the centre of the LMC.…”

“…Hence, deeper investigations appeared to be necessary to solve Fig. 9 The application of the "dynamical clock" to extra-Galactic clusters: N relax − A + relation for the five LMC clusters discussed in Ferraro et al (2019) (large red squares). For the sake of comparison, the sample of 48 Galactic GCs previously investigated (Ferraro et al, 2018) is shown with grey circles.…”

“…On the other hand, the dynamical ageing effects are expected to be most evident in old star clusters, with chronological ages comparable to those of the Milky Way systems (12-13 Gyr). Hence, Ferraro et al (2019) selected five 13 Gyr-old GCs in the LMC, showing different core sizes (ranging from 1 to 7 parsec), and measured their stage of internal dynamical evolution via the BSS sedimentation level.…”

An empirical clock to measure the dynamical age of star clusters

“…The tight relation between these two parameters (solid line) demonstrates that the segregation level of BSSs can be efficiently used to evaluate the level of dynamical evolution experienced by the parent cluster. Right Panel: The same as in the left panel (with the sample of Galactic GCs shown in grey) with the five LMC clusters discussed in Ferraro et al (2019) highlighted as red squares.The LMC clusters ranked at increasing value of the dynamical age are NGC1841, Hodge 11, NGC2257, NGC1466 and NGC2210.…”

“…The "dynamical clock" has been recently used by Li et al (2019) to measure the dynamical age of 7 intermediate-age (between 700 Myr and 7 Gyr) clusters in the Large Magellanic Cloud (LMC) finding a low-level of dynamical evolution. Interesting enough, instead Ferraro et al (2019) measured the BSS segregation level in five old LMC GCs (with ages comparable to Galactic GCs) detecting clear-cut differences in their level of dynamical evolution (see the right panel of Figure 2). The figure also shows the impressive matching with the trend defined by the Galactic GCs (grey circles), demonstrating that the "dynamical clock" can be efficiently used in any stellar environment.…”