Radial velocities in NGC 7099

Thus, for {tau}>~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_=1.2x10^-8^cm/s^2^ observed in galaxies. 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.