Surface Brightness Fluctuations to Constrain Galaxy Stellar Populations

The purpose of this work is to demonstrate that Surface Brightness Fluctuations (SBF) preserve an unexploited potential for studying stellar populations in galaxies. Throughout this thesis, the methodology associated with the SBF has been thoroughly reviewed. We introduced a previously unaccounted type of uncertainty, arising from the stochastic nature of SBF. This uncertainty was quantified through Monte Carlo simulations, assessing its reliability across diverse conditions, including radial gradients. This modelling is publicly available as a code for the community to use. In this context, the methodology for SBF extraction has been reassessed to provide recommendations and highlight the most sensitive aspects for inference. Notably, there is a strong correlation between the number of available pixels and the ability to recover a reasonable uncertainty. Additionally, there is a risk of obtaining biased values if pixels with fluxes less than approximately ten times the SBF ! estimate are included. Subsequently, a novel method for fitting composite stellar populations has been developed by combining standard and SBF colour-colour diagrams. This approach allows the characterisation of primary and secondary stellar populations in several Early-Type galaxies, where the presence of 1% to 10% of old, very low-metallicity components has been estimated in otherwise metal-rich galaxies. These components may be linked to the earliest stages of chemical evolution in this type of galaxy. It also enables breaking the age-metallicity degeneracy, which is characteristic of aged stellar populations, and sequencing star formation histories when using multi-band colour combinations. Achieving these results relies significantly on obtaining consistent and cohesive SBF measurements. At last, a multi-band SBF measurement was conducted for the elliptical galaxy M60, applying the curated methodology described above. Among the most relevant findings are the importance of equivalent masking across all bands, which proved crucial for deriving SBF, and the quantification of the inherent uncertainty in SBF through Monte Carlo simulations. A preliminary measurement of the stellar components of M60 using standard and SBF colour-colour diagrams revealed a predominantly old and metal-rich population, alongside a small secondary fraction of equally old but low-metallicity stars, in line with a monolithic initial formation scenario. These results demonstrate that the research expands the capabilities of SBF to uncover small stellar components that might otherwise remain undetected in standard studies based on integrated light.