Using globular clusters to test gravity in the weak acceleration regime

Free access article

Issue		A&A Volume 405, Number 1, July I 2003


Page(s)		L15 - L18
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:20030762

A&A 405, L15-L18 (2003)
DOI: 10.1051/0004-6361:20030762

Letter

Using globular clusters to test gravity in the weak acceleration regime

R. Scarpa, G. Marconi and R. Gilmozzi

European Southern Observatory, 3107 Alonso de Cordova, Santiago, Chile
(Received 21 March 2003 / Accepted 19 May 2003 )

Abstract
We have carried out a study of the velocity dispersion of the stars in the outskirts of the globular cluster $\omega$ Cen, finding that the velocity dispersion remains constant at large radii rather than decrease monotonically. The dispersion starts to be constant for an acceleration of gravity of $a=2.1^{+0.5}_{-0.4}\times 10^{-8}$ cm s ^-2. A similar result is obtained reanalyzing existing data for the globular cluster M15 where the profile flattens out for $a=1.7^{+0.7}_{-0.5}\times 10^{-8}$ cm s ^-2. In both cases the acceleration is comparable to that at which the effect of dark matter becomes relevant in galaxies. Explanations for this result within Newtonian dynamics exist (e.g. cluster evaporation, tidal effects, presence of dark matter) but require ad hoc tuning of the relevant parameters in order to make in both clusters the dispersion profile flat starting exactly at the same acceleration. We suggest that this result, together with a similar one for Palomar 13 and the anomalous behavior of spacecrafts outside the solar system, may indicate a breakdown of Newton's law in the weak acceleration regime. Although not conclusive, these data prompt for the accurate determination of the internal dynamics of as many GCs as possible.

Key words: gravity -- globular cluster -- star dynamics

Offprint request: R. Scarpa, rscarpa@eso.org

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