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