Testing Newtonian gravity with distant globular clusters: NGC1851 and
  NGC1904
  Globular clusters are useful to test the validity of Newtonian dynamics in
the low acceleration regime typical of galaxies, without the complications of
non-baryonic dark matter. Specifically, in absence of disturbing effects, e.g.
tidal heating, their velocity dispersion is expected to vanish at large radii.
If such behaviour is not observed, and in particular if, as observed in
elliptical galaxies, the dispersion is found constant at large radii below a
certain threshold acceleration, this might indicate a break down of Newtonian
dynamics.
  To minimise the effects of tidal heating in this paper we study the velocity
dispersion profile of two distant globular clusters, NGC 1851 and NGC 1904.
  The velocity dispersion profile is derived from accurate radial velocities
measurements, obtained at the ESO 8m VLT telescope. Reliable data for 184 and
146 bona fide cluster star members, respectively for NGC 1851 and NGC 1904,
were obtained.
  These data allow to trace the velocity dispersion profile up to ~2r0, where
r0 is the radius at which the cluster internal acceleration of gravity is a0 =
10e-8 cm/s/s. It is found that in both clusters the velocity dispersion becomes
constant beyond ~r0. These new results are fully in agreement with those found
for other five globular clusters previously investigated as part of this
project. Taken all together, these 7 clusters support the claim that the
velocity dispersion is constant beyond r0, irrespectively of the specific
physical properties of the clusters: mass, size, dynamical history, and
distance from the Milky Way. The strong similarly with the constant velocity
dispersion observed in elliptical galaxies beyond r0 is suggestive of a common
origin for this phenomenon in the two class of objects, and might indicate a
breakdown of Newtonian dynamics below a0.
