Multi-band photometric studies of extragalactic globular cluster systems in galaxy groups and clusters: getting ready for the LSST and Euclid data releases

The formation and evolution of cosmic structures is an important question in astrophysics. The Lambda Cold Dark Matter (ΛCDM) model, which suggest a Universe dominated by dark energy (Λ) and cold dark matter, has successfully explained key observations, including galaxy distribution and cosmic web properties. However, recent observations have challenged ΛCDM, notably the Hubble tension – the discrepancy in the expansion rate of the Universe between the local and early-Universe – and other differences in galaxy clustering and the cosmic microwave background. These challenges suggest potential new physics beyond ΛCDM, requiring further exploration into structure formation processes. Globular clusters (GCs) are valuable tracers for studying the Universe at various scales. As ancient stellar clusters which formed early in galaxy evolution, GCs trace galaxy halos across large distances and reveal key properties of their host galaxies, such as age, metallicity and kinematics, where other indicators may fail. GCs also help in building the cosmic distance ladder through their luminosity functions. By examining their spatial distributions, kinematics, and metallicities, GCs offer insights into galaxy formation, merger histories and hierarchical assembly within the ΛCDM framework, helping to address some of the open questions in cosmology. However, photometric studies of extragalactic GCs face challenges from contamination, limited field-of-view (FoV) and insufficient multi-passband data. This complicates the process of obtaining pure and complete GC samples which are critical for characterizing GC systems in the Local Universe. In this thesis project, I first developed tools and techniques for GC identification in deep, large-field optical ground-based surveys, using data from the VLT Survey Telescope (VST) Elliptical GAlaxy Survey (VEGAS) team. These GC catalogues are then used to characterise the properties of the GC systems and their host environments. In particular, I analysed the VEGAS data from two interacting galaxy groups, dominated by the bright ellipticals NGC 5018 and NGC 3640. The study of the global properties of GC systems using large-format VST images is fundamentally relevant as preparatory work for ongoing and future large-area, deep imaging surveys. In the final part of this thesis, I present the first results on GC studies based on Euclid data. The methods and analysis techniques developed here have proven valuable for application to GCs observed by Euclid. Looking ahead, similar extensions to the Legacy Survey of Space and Time (LSST) from the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope will enable deeper investigations of extragalactic GC systems across extensive sky regions.