Dynamical masses and mass-to-light ratios of resolved massive star clusters -- II. Results for 26 star clusters in the Magellanic Clouds

TL;DR

This study measures the masses and mass-to-light ratios of 26 Magellanic Cloud star clusters using spectroscopy, revealing discrepancies with simple stellar population models and suggesting dynamical effects and IMF variations influence cluster properties.

Contribution

It provides new dynamical mass and M/L ratios for 26 star clusters, incorporating a large spectroscopic dataset and a customized membership estimation technique, and compares results with stellar population models.

Findings

Observed M/L ratios are about 40% lower than simple models predict.

Modified models with dynamical effects and bottom-light IMFs improve agreement.

Bottom-heavy IMFs are ruled out due to higher predicted M/L ratios.

Abstract

We present spectroscopy of individual stars in 26 Magellanic Cloud (MC) star clusters with the aim of estimating dynamical masses and $V$-band mass-to-light ($M/L_V$) ratios over a wide range in age and metallicity. We obtained 3137 high-resolution stellar spectra with M2FS on the \textit{Magellan}/Clay Telescope. Combined with 239 published spectroscopic results of comparable quality, we produced a final sample of 2787 stars with good quality spectra for kinematic analysis in the target clusters. Line-of-sight velocities measured from these spectra and stellar positions within each cluster were used in a customized expectation-maximization (EM) technique to estimate cluster membership probabilities. Using appropriate cluster structural parameters and corresponding single-mass dynamical models, this technique ultimately provides self-consistent total mass and $M/L_V$ estimates for each cluster. Mean metallicities for the clusters were also obtained and tied to a scale based on calcium IR triplet metallicites. We present trends of the cluster $M/L_V$ values with cluster age, mass and metallicity, and find that our results run about 40 per cent on average lower than the predictions of a set of simple stellar population (SSP) models. Modified SSP models that account for internal and external dynamical effects greatly improve agreement with our results, as can models that adopt a strongly bottom-light IMF. To the extent that dynamical evolution must occur, a modified IMF is not required to match data and models. In contrast, a bottom-heavy IMF is ruled out for our cluster sample as this would lead to higher predicted $M/L_V$ values, significantly increasing the discrepancy with our observations.