The large field-of-view Gaia observations of NGC 3201 allows to measure the velocity dispersion in the outskirts of the cluster, which appears significantly higher than what is expected from standard Newtonian dynamics. This peculiarity could suggest the presense of dark matter, left over from the formation in the early universe, or unexpectedly strong tidal heating due to the Milky Way potential. Image credit: P. Bianchini - image on the left: astrosurf.com

With the advances in the field of precision astrometry, the study of both the kinematic and the morphological properties of Milky Way Globular Clusters (MW GCs) is now possible. Globular clusters are dense spheroidal collections of around a million very old stars that orbit around a host galaxy. They are amongst the stellar systems that have been studied for the longest time, dating back to the 17th century.

Their formation, however, presently remains one of the most poorly understood phenomenon of modern astrophysics. Their apparent simplicity has made them the textbook example of simple old stellar population systems, characterised by simple Newtonian dynamics with all their mass provided solely by their stars.

It has nevertheless also long be suspected that such objects could have been born inside very light dark matter halos, but a dark matter effect had never been observed directly in the dynamics of globular clusters until today.

Now, by carefully analysing the Gaia proper motions of stars in the outskirts of the globular cluster NGC 3201, located at 16 thousand light-years from the Sun, a team of astronomers from the Observatoire astronomique de Strasbourg might have detected the gravitational effect of dark matter in a globular cluster for the very first time.

They found that the movements of stars (i.e. the dispersion of the velocities of stars) are much higher than what is a priori expected in the outskirts, even in the presence of the disturbing effect due to the gravitational potential of the Milky Way producing a tidal heating. The measurement could thus indicate that the tidal heating of NGC 3201 has been unexpectedly strong for some unknown reason, or that dark matter is indeed present in the cluster.

Dark matter would indeed start affecting the dynamics of stars in these outer regions, whilst remaining subdominant in the inner regions where the density of stars is extremely high compared to the outskirts. Does this mean that the globular cluster NGC 3201 deviates from the classical Newtonian dynamics, and could this observed discrepancy be accounted for with an alternative gravity model, like modified Newtonian dynamics? This remains to be studied in detail.

In all cases, this measurement is paving the way for a revolution in our understanding of the dynamics, history and formation of globular clusters. Is there truly something peculiar and abnormal with the cluster NGC 3201, or is it a representative case? To find out, the study will be extended to investigate a larger sample of clusters, with the use of Gaia data.

Further reading:

• Exploring the outskirts of globular clusters: the peculiar kinematics of NGC 3201 by P. Bianchini, R. Ibata, B. Famaey (ApJ letter - published today)