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

TL;DR

This study measures the present-day stellar mass function of globular cluster NGC 6535 using Hubble data, revealing it to be bottom-light and challenging previous assumptions of a bottom-heavy initial mass function based on dynamical mass estimates.

Contribution

It provides the first direct measurement of NGC 6535's stellar mass function, showing it is bottom-light and questioning prior dynamical mass-based inferences about its initial mass function.

Findings

NGC 6535's present-day mass function is bottom-light.

Dynamical mass estimates may be unreliable for this cluster.

The cluster's properties challenge the assumption of a bottom-heavy initial mass function.

Abstract

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.