Dynamical Mass-To-light Ratios of Populous Star Clusters in the Magellanic Clouds

Abstract

The mass-to-light (M/L) ratio is a fundamental astrophysical parameter that converts the luminosity of a stellar system---which is comparatively easy to measure---to a corresponding baryonic mass. Typically, M/L ratios are estimated using stellar population synthesis models. Testing such models requires independent mass estimates (specifically, only of the baryons) which usually require kinematic data and is considerably more involved than photometric measures. Galaxies are particularly difficult to employ for this task given their typically complex stellar population mixtures and their dark-matter contents. Star clusters, in contrast, are comprised of comparatively simple stellar populations and contain no dark matter, making them ideal laboratories to test M/L predictions.

This thesis aims to estimate in a consistent manner dynamical masses and V-band M/L ratios (M/L_V) of a large sample of star clusters spanning a wide range in age and metallicity. To accomplish this, I have obtained 3137 high-resolution stellar spectra of individual stars in 26 populous star clusters of the Magellanic Clouds using the M2FS multi-object spectrograph on the Magellan/Clay Telescope. Combined with 239 published spectroscopic results of comparable quality, I have produced a final sample of 2787 individual stars suitable for kinematic analysis in the target clusters. Line-of-sight (LOS) velocities measured from these spectra and stellar positions within each cluster were used within a customized expectation-maximization (EM) technique to estimate cluster membership probabilities. Using the 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 estimated from the spectra and tied to a scale based on calcium IR triplet measurements and high-precision, high-resolution metallicity estimates.

I describe trends of the cluster M/L_V values with cluster age, mass and metallicity, and compare the relations with predictions from simple stellar population (SSP) models. The new observational M/L_V results parallel the systematic behavior of the SSP models as a function of age, but are on average about 40% lower than model predictions. 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 stellar initial mass function (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, suggested by other studies, is strongly ruled out for our cluster sample as this would lead to higher predicted M/L_V values, exacerbating the discrepancy with our observations.