THE OUTSKIRTS OF GLOBULAR CLUSTERS AS MODIFIED GRAVITY PROBES

In the context of theories of gravity modified to account for the observed dynamics of galactic systems without the need to invoke the existence of dark matter, a prediction often appears regarding low-acceleration systems: wherever a falls below a(0), one should expect a transition from the classical to the modified gravity regime. This modified gravity regime will be characterized by equilibrium velocities that become independent of distance and that scale with the fourth root of the total baryonic mass, V-4 alpha M. The two conditions above are the well-known flat rotation curves and Tully-Fisher relations of the galactic regime. Recently, however, a similar phenomenology has been hinted at, at the outskirts of Galactic globular clusters, precisely in the region where a < a(0). Radial profiles of the projected velocity dispersion have been observed to stop decreasing along Keplerian expectations and to level off at constant values beyond the radii where a < a(0). We have constructed gravitational equilibrium dynamical models for a number of globular clusters for which the above gravitational anomaly has been reported, using a modified Newtonian force law that yields equilibrium velocities equivalent to modified Newtonian dynamics. We find models having an inner Newtonian region and an outer modified gravity regime, which reproduce all observational constraints, surface brightness profiles, total masses, and line-of-sight velocity dispersion profiles, can be easily constructed. Through the use of detailed single stellar population models tuned individually to each of the globular clusters in question, we derive estimates of the total masses for these systems. Interestingly, we find that the asymptotic values of the velocity dispersion profiles are consistent with scaling with the fourth root of the total masses, as expected under modified gravity scenarios.