Proper Motions and Internal Dynamics in the Core of the Globular Cluster Messier 71

Background:  The Milky Way Galaxy is home to about 160 globular clusters. These clusters are of great importance in studying the kinematics, structure and evolution of the Galactic Halo, the Galaxy itself, and stellar evolution in general. By probing the cores of globular clusters, central intermediate mass black holes (IMBHs) may be detected, providing the missing link between supermassive black holes in the cores of galaxies to stellar mass black holes. Determining if globular clusters harbor IMBHs will help answer questions as to how these clusters are born and what type of conditions allow IMBHs to form.  Historically, to probe the centres of these clusters ground-based spectroscopic studies or proper motion studies with baselines of decades were required. With the advancement of ground based adaptive optics technology, image distortions induced by turbulence in the atmosphere can be eliminated.  Therefore, we are able to investigate the cores of globular clusters, a feat that was previously limited to only space based telescopes. Methods: Using the 8m Gemini North Telescope, we obtained near infrared adaptive optics imaging of the core of Messier 71.  In 2007 we imaged the core of Messier 71 and two years later we re-imaged the same field.  We were very strict in our observing conditions, imaging was only conducted when the seeing conditions were the best available. We then reduced the data using standard astronomical software, however, we processed the data in a non-traditional manner. We did not stack and make a master image of the cluster, as is the norm. Instead we processed each image individually and built model point-spread-functions for each image.  This results in improved accuracy and the ability of resolving the motions of the stars.  We then analyzed the data using several statistical approaches and constructed black hole models to put limits on the existence of any central black hole. Results: We found the orbits of the stars are fairly uniform in direction with respect to the cluster centre. We found several stars in our field that were travelling well beyond the escape velocity of the cluster. However, comparing our field with a galactic simulation we find these stars are very likely galactic field stars and not cluster members. We find the motions of the stars to be constant with radius from the cluster centre and are thus able to put an upper limit to any central black hole to be ~2700 solar masses at 95% confidence. Conclusions: Previous proper motion studies of the cores of globular clusters were either limited to ground based observations with lengthy baselines or expensive space-based observations. We have shown that it is possible to resolve the internal motions in the core of a globular cluster using large ground based telescopes with adaptive optics.  Previously it was speculated that clusters like Messier 71 could contain a central intermediate mass black hole, however we find no evidence for one. This work can be expanded by employing integrated field spectroscopy on the same collection of stars.