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Analyses of strong gravitational lenses, galaxy-scale kinematics, andabsorption line stellar population synthesis (SPS) have all concluded that thestellar initial mass function (IMF) varies within the massive early-type galaxy(ETG) population. However, the physical mechanism that drives variation in theIMF is an outstanding question. Here we use new SPS models to consider adiverse set of compact, low-velocity dispersion stellar systems: globularclusters (GCs), an ultra-compact dwarf (UCD), and the compact elliptical (cE)galaxy M32. We compare our results to massive ETGs and available dynamicalmeasurements. We find that the GCs have stellar mass-to-light ratios (M/L) thatare either consistent with a Kroupa IMF or are slightly bottom-light while theUCD and cE have mildly elevated M/L. The separation in derived IMFs for systemswith similar metallicities and abundance patterns indicates that our SPS modelscan distinguish abundance and IMF effects. Variation among the sample in thispaper is only $\sim 50\%$ in normalized M/L compared to the $\sim 4\times$among the ETG sample. This suggests that metallicity is not the sole driver ofIMF variability and additional parameters need to be considered.

Initial Mass Function Variability (or not) Among \\ Low-Velocity Dispersion, Compact Stellar Systems