The Bottom-Light Present Day Mass Function of the Peculiar Globular
  Cluster NGC 6535
  Dynamical mass calculations have suggested that the Milky Way globular
cluster NGC 6535 belongs to a population of clusters with high mass-to-light
ratios, possibly due to a bottom-heavy stellar initial mass function. We use
published Hubble Space Telescope data to measure the present day stellar mass
function of this cluster within its half-light radius and instead find that it
is bottom-light, exacerbating the discrepancy between the dynamical measurement
and its known stellar content. The cluster's proximity to the Milky Way bulge
and its relatively strong velocity anisotropy are both reasons to be suspicious
of the dynamical mass measurement, but we find that neither straightforwardly
explains the sense and magnitude of the discrepancy. Although there are
alternative potential explanations for the high mass-to-light ratio, such as
the presence of large numbers of stellar remnants or dark matter, we find this
cluster to be sufficiently perplexing that we now exclude it from a discussion
of possible variations in the initial mass function. Because this was the sole
known old, Milky Way cluster in the population of high dynamical mass-to-light
ratio clusters, some possible explanations for the difference in cluster
properties are again open for consideration.
