Multiple Stellar Populations in Globular Clusters

Globular clusters are ancient stellar systems and important tracers of galaxy formation. Nearly all massive, old globular clusters exhibit light-element multiple populations, indicating complex formation histories. Studying both the chemistry and internal motions of globular clusters therefore provides insight into the assembly and evolution of the Milky Way. Bulge globular clusters are particularly valuable targets because they represent the in-situ population of the Galactic bulge, although they are difficult to observe due to severe extinction and stellar crowding. NGC 6440 is a representative bulge cluster: it is massive, metal-rich, located at a distance of approximately 8–9 kiloparsecs, and strongly affected by reddening. Previous studies indicate that it is structurally compact, dynamically evolved, and characterized by a short relaxation time, making it an ideal system for high-precision astrometric studies. This thesis presents a photometric and astrometric analysis of NGC 6440 based on archival Hubble Space Telescope imaging obtained with the WFC3 instrument in the F606W and F814W filters. The dataset consists of 54 exposures taken in 2012. To overcome the effects of crowding and spatially variable extinction, spatially varying point-spread function modelling was applied to produce high-quality stellar catalogues. Photometry derived from these models was used to construct a colour–magnitude diagram, which was subsequently corrected for differential reddening to recover the intrinsic structure of the cluster sequences. Isochrone fitting was carried out using Dartmouth stellar evolution models applied to the de-reddened and member-selected colour–magnitude diagram. The best-fitting solution yields an age of approximately 12 billion years, a metallicity of about minus 0.56 in [Fe/H], and a true distance modulus of roughly 14.7, corresponding to a heliocentric distance of approximately 8.7 kiloparsecs. These results are consistent with previous spectroscopic studies and confirm that NGC 6440 is an old, metal-rich cluster belonging to the Galactic bulge. The central focus of this work is the investigation of the internal kinematics of the cluster using proper-motion measurements. After applying quality and error-based selection criteria, a search for fast-moving stars in the central 4 arcseconds identified three candidates with proper motions significantly larger than the local average. All three are located on the red giant branch and are likely cluster members, although their properties do not provide evidence for the presence of an intermediate-mass black hole. Radial and tangential velocity dispersion profiles were computed in five radial bins extending out to approximately 2.5 arcminutes from the cluster centre. The dispersion remains nearly constant with radius, with average values of about 0.29 milliarcseconds per year in the radial direction and 0.30 milliarcseconds per year in the tangential direction. The corresponding anisotropy parameter is consistent with zero, indicating that the cluster is dynamically isotropic within the region studied. The inferred physical velocity dispersion, assuming a distance of 8.7 kiloparsecs, is approximately 12.1 kilometres per second, in excellent agreement with spectroscopic measurements. Finally, I examined whether the two stellar populations recently identified in NGC 6440 show any difference in their kinematic properties. First- and second-population stars were classified using infrared photometry from the James Webb Space Telescope. Their average velocity dispersions were measured in the outer region of the cluster and found to be statistically indistinguishable. Both populations are consistent with isotropic motion, suggesting that long-term dynamical evolution has erased any initial differences. This supports the conclusion that NGC 6440 is a dynamically old, well-mixed system whose multiple populations share identical present-day kinematics.