Testing Newtonian Gravity with AAOmega: Mass-To-Light Profiles and
  Metallicity Calibrations From 47 Tuc and M55
  Globular clusters are an important test bed for Newtonian gravity in the
weak-acceleration regime, which is vital to our understanding of the nature of
the gravitational interaction. Recent claims have been made that the velocity
dispersion profiles of globular clusters flatten out at large radii, despite an
apparent paucity of dark matter in such objects, indicating the need for a
modification of gravitational theories. We continue our investigation of this
claim, with the largest spectral samples ever obtained of 47 Tucanae and M55.
Furthermore, this large sample allows for an accurate metallicity calibration
based on the equivalent widths of the calcium triplet lines and K band
magnitude of the Tip of the Red Giant Branch. Assuming an isothermal
distribution, the rotations of each cluster are also measured with both
clusters exhibiting clear rotation signatures. The global velocity dispersions
of NGC 121 and Kron 3, two globular clusters in the Small Magellanic Cloud, are
also calculated. By applying a simple dynamical model to the velocity
dispersion profiles of 47 Tuc and M55, we calculate their mass-to-light
profiles, total masses and central velocity dispersions. We find no
statistically significant flattening of the velocity dispersion at large radii
for M55, and a marked increase in the profile of 47 Tuc for radii greater than
approximately half the tidal radius. We interpret this increase as an
evaporation signature, indicating that 47 Tuc is undergoing, or has undergone,
core-collapse, but find no requirement for dark matter or a modification of
gravitational theories in either cluster.
