Evidence for Two Distinct Stellar Initial Mass Functions : Probing for Clues to the Dichotomy

TL;DR

This study investigates the stellar initial mass function in globular clusters, revealing two distinct classes with different mass-to-light ratios, but the cause of this bimodality remains unclear and uncorrelated with common formation or structural parameters.

Contribution

It provides new velocity dispersion measurements for 11 globular clusters, expanding the sample to analyze the bimodal nature of the stellar initial mass function.

Findings

Clusters fall into two classes with distinct mass-to-light ratios.

The bimodality does not correlate with age, metallicity, or structural parameters.

Mass-to-light ratios match those of early and late type galaxies.

Abstract

We present new measurements of the velocity dispersions of eleven Local Group globular clusters using spatially integrated spectra, to expand our sample of clusters with precise integrated-light velocity dispersions to 29, over 4 different host galaxies. This sample allows us to further our investigation of the stellar mass function among clusters, with a particular emphasis on a search for the driver of the apparent bimodal nature of the inferred stellar initial mass function. We confirm our previous result that clusters fall into two classes. If, as we argue, this behavior reflects a variation in the stellar initial mass function, the cause of that variation is not clear. The variations do not correlate with formation epoch as quantified by age, metallicity quantified by $[ {\rm Fe/H}] $, host galaxy, or internal structure as quantified by velocity dispersion, physical size, relaxation time, or luminosity. The stellar mass-to-light ratios, $\Upsilon_*$, of the high and low $\Upsilon_*$ cluster populations are well-matched to those found in recent studies of early and late type galaxies, respectively.