The relation between the mass-to-light ratio and the relaxation state of globular clusters

Abstract

The internal dynamics of globular clusters (GCs) is strongly affected by two-body interactions that bring the systems to a state of partial energy equipartition. Using a set of Monte Carlo clusters simulations, we investigate the role of the onset of energy equipartition in shaping the mass-to-light ratio (M/L) in GCs. Our simulations show that the M/L profiles cannot be considered constant and their specific shape strongly depends on the dynamical age of the clusters. Dynamically younger clusters display a central peak up to M/L 25 M /L caused by the retention of dark remnants; this peak flattens out for dynamically older clusters. Moreover, we find that also the global values of M/L correlate with the dynamical state of a cluster quantified as either the number of relaxation times a system has experienced n rel or the equipartition parameter m eq : clusters closer to full equipartition (higher n rel or lower m eq ) display a lower M/L. We show that the decrease of M/L is primarily driven by the dynamical ejection of dark remnants, rather than by the escape of low-mass stars. The predictions of our models are in good agreement with observations of GCs in the Milky Way and M31, indicating that differences in relaxation state alone can explain variations of M/L up to a factor of 3. Our characterization of the M/L as a function of relaxation state is of primary relevance for the application and interpretation of dynamical models.

A simple two-component description of energy equipartition and mass segregation for anisotropic globular clusters

“…By means of a relatively large number of particles (N= 5 × 10 5 − 2 × 10 6 ), these simulations provide a realistic description of the long-term evolution of globular clusters, starting with a single stellar population. Selected snapshots taken from these simulations have already been studied as simulated states by Bianchini et al (2016) and Bianchini et al (2017b) to characterize energy equipartition and mass segregation in realistic systems. These authors focused on projected quantities to make useful comparisons with observations.…”

“…The values of n rel for the selected snapshots are taken from Tab. 3 of Bianchini et al (2017b). The snapshots correspond to pre-core collapse conditions (with respect to the gravothermal catastrophe).…”

“…3.2), might be a bold step, and subject to unacceptable limitations. Therefore, before declaring that the proposed conceptually simple models may offer a reliable tool to study some dynamical mechanisms or to interpret the observations (with the required discussion of the size and role of massto-light gradients; see Bianchini et al 2017b, Sect. 4.1 of de Vita et al 2016, and Hénault-Brunet et al 2019 for a three-component description), we need to test whether the simple two-component models do indeed have the power to describe the structural properties of the extremely complex globular clusters, as is nowadays well-described by state-of-the-art simulations.…”

Correlation between mass segregation and structural concentration in relaxed stellar clusters

“…Noyola et al 2008;Lanzoni et al 2013;Lützgendorf et al 2015). In this context, a variety of different tracers for the BHs presence have been proposed, with one of them relying on the characterization of the long-term dynamical evolution of GCs (Gill et al 2008;Pasquato et al 2009;Trenti & van der Marel 2013;Peuten et al 2016;Bianchini et al 2017;Weatherford et al 2018;Arca Sedda et al 2018;Askar et al 2018), which is driven by the tendency of the system to evolve towards a state of energy equipartition through twobody relaxation (see Binney & Tremaine 2008).…”

The WAGGS project-III. Discrepant mass-to-light ratios of Galactic globular clusters at high metallicity

“…More compact clusters evolve faster dynamically, decreasing their M/L accordingly. Bianchini et al (2017) suggest that possible differences in internal configurations between metal-rich and metal-poor clusters (e.g. metal-rich clusters being more dense) could produce a different dynamical evolution of the M/L, therefore decreasing the M/L of metal-rich clusters.…”