As part of an ongoing program to test Newton ’ s law of gravity in the low acceleration regime using globular clusters , we present here new results obtained for NGC 6171 .
Combining VLT spectra for 107 stars with data from the literature , we were able to trace the velocity dispersion profile up to \sim 16 pc from the cluster center , probing accelerations of gravity down to 3.5 \times 10 ^ { -9 } cm s ^ { -2 } .
The velocity dispersion is found to remain constant at large radii ( with an asymptotic values of 2.7 km s ^ { -1 } ) rather than follow the Keplerian falloff .
Similar results were previously found for the globular clusters \omega Centauri and M15 .
We have now studied three clusters and all three have been found to have a flat dispersion profile beyond the radius where their internal acceleration of gravity is \sim a _ { 0 } = 1.2 \times 10 ^ { -8 } cm s ^ { -2 } .
Whether this indicates a failure of Newtonian dynamics or some more conventional dynamical effect ( e.g. , tidal heating ) is still unclear .
However , the similarities emerging between globular clusters and elliptical galaxies seem to favor the first of the two possibilities .