Context :

Aims : A test of Newton ’ s law of gravity in the low acceleration regime using globular clusters is presented and new results for the core collapsed globular cluster NGC 7099 given .

Methods : The run of the gravitational potential as a function of distance is probed by studying the velocity dispersion profile of the cluster , as derived from a set of 125 radial velocities with accuracy better than 1 km s ^ { -1 } .
The velocity dispersion profile is traced up to \sim 18 pc from the cluster center .

Results : The dispersion is found to be maximal at the center , then decrease until 10 \pm 2 pc from the center , well inside the cluster tidal radius of 42 pc .
After that the dispersion remains basically constant with an average value of 2.2 \pm 0.3 km s ^ { -1 } .
Assuming a total V mag of M ( V ) = -7.43 mags for NGC 7099 , the acceleration at 10 \pm 2 pc from the center is 1.1 ^ { +0.4 } _ { -0.3 } \tau \times 10 ^ { -8 } cm s ^ { -2 } , where \tau is the mass-to-light ratio .
Thus , for \tau \la 2 typical of globular clusters , the flattening of the velocity dispersion profile occurs for a value of the internal acceleration of gravity that is fully consistent with a _ { 0 } = 1.2 \times 10 ^ { -8 } cm s ^ { -2 } observed in galaxies .

Conclusions : This new result for NGC 7099 brings to 4 the clusters with velocity dispersion profile probing acceleration below a _ { 0 } .
All four have been found to have a flat dispersion profile at large radii where the acceleration is below a _ { 0 } , thereby mimicking elliptical galaxies qualitatively and quantitatively .
Whether this indicates a failure of Newtonian dynamics in the low acceleration limit or some more conventional dynamical effect ( e.g. , tidal heating ) is still unclear .
However , the similarities emerging between very different globular clusters , as well as between globular clusters and elliptical galaxies , seem to favor the first of these two possibilities .