  Ultraviolet (UV) emission from galaxies is associated with hot components,
whether from stellar sources or not. By making use of colour-colour diagrams,
early-type galaxies (ETGs) can be classified in terms of their UV emission in
mainly three categories: residual star-formation, UV weak, and UV upturn. This
thesis aims at investigating galaxies presenting UV upturn by comparing them to
UV weak systems. This investigation has three fronts: (i) the assessment of the
evolution in redshift (z) and stellar mass (logM*) of the fraction of UV upturn
systems in terms of the total of UV bright ETGs; (ii) the stratification of the
first study in terms of emission lines; (iii) the comparison between UV weak
and upturn for passive RSGs in terms of their stellar populations. To tackle
the first front, a Bayesian logistic model was applied. The second front
expands on the first, dividing the sample into emission line classes. The final
front is focused on the study of stellar population properties of UV weak and
upturn systems, by making use of catalogues from GAMA-DR3. The results show
that the fraction of UV upturn systems rises up to z~0.25, followed by a
decline which remains to be confirmed. By stratifying the sample into emission
line classes, the galaxies classified as retired/passive -- the ones associated
with evolved stellar phases -- dominate the behaviour with z and logM*. By
analysing the stellar populations of both types of systems, different
characteristics emerge such as median ages, metallicities, and time since last
burst of star formation. These results seem to indicate that UV upturn systems
have narrower star-formation histories, higher metallicities, and slightly
older populations.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics;High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics