Astronomy and Astrophysics – Astrophysics – Galaxy Astrophysics
Scientific paper
2011-08-26
Astronomy and Astrophysics
Astrophysics
Galaxy Astrophysics
10 pages, 6 figures, submitted to MNRAS
Scientific paper
The velocity dispersion profile in globular clusters (GCs) is explained here without having to rely on dark matter or a modification of Newtonian dynamics (MOND). The flattening of the velocity dispersion at large radii in certain Milky Way GCs, or lack thereof, is explained by recourse to the stability of the three-body problem in Newtonian dynamics. The previous paper in this series determined an analytical formula for the transition radius between stable and unstable orbits for a star in a globular cluster. This stability boundary is used here to predict where the velocity dispersion profile is expected to flatten in GCs, given known orbital parameters of the GC-galaxy orbit. Published observational data for the velocity dispersion as a function of radius of 8 Milky Way globular clusters with approximately known orbital parameters has been used here. We find that the stability radius predicts where the velocity flattens as well as, and in some cases better than, the MOND models do. In particular it is noted that the cluster masses in the MOND models are often incompatible with the velocity dispersion requirements in the inner region of the cluster. However when the stability boundary is determined from the stability in the three-body problem using Newtonian dynamics, it is found to fit the observations well, and there is no need for any MOND type theories to explain the observations. A warning is also made against fitting the velocity dispersion of globular clusters without knowing their orbital parameters, particularly for clusters with strong evidence of ongoing tidal disruption, such as NGC 288.
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