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Dynamical mass calculations have suggested that the Milky Way globularcluster NGC 6535 belongs to a population of clusters with high mass-to-lightratios, possibly due to a bottom-heavy stellar initial mass function. We usepublished Hubble Space Telescope data to measure the present day stellar massfunction of this cluster within its half-light radius and instead find that itis bottom-light, exacerbating the discrepancy between the dynamical measurementand its known stellar content. The cluster's proximity to the Milky Way bulgeand its relatively strong velocity anisotropy are both reasons to be suspiciousof the dynamical mass measurement, but we find that neither straightforwardlyexplains the sense and magnitude of the discrepancy. Although there arealternative potential explanations for the high mass-to-light ratio, such asthe presence of large numbers of stellar remnants or dark matter, we find thiscluster to be sufficiently perplexing that we now exclude it from a discussionof possible variations in the initial mass function. Because this was the soleknown old, Milky Way cluster in the population of high dynamical mass-to-lightratio clusters, some possible explanations for the difference in clusterproperties are again open for consideration.

The Bottom-Light Present Day Mass Function of the Peculiar Globular Cluster NGC 6535