Testing Newtonian gravity with distant globular clusters: NGC 1851 and NGC 1904

Context. Globular clusters are useful for testing the validity of Newtonian dynamics in the low acceleration regime typical of galaxies, without the complications of non-baryonic dark matter. In the absence of disturbing effects, such as tidal heating, the velocity dispersion of globular clusters is expected to vanish at large radii. If this is not observed, and in particular if, as observed in elliptical galaxies, the dispersion is found to be constant at large radii below a certain threshold acceleration, this might indicate a breakdown of Newtonian dynamics.