The UV upturn phenomenon in the hierarchical universe

Recent studies show that an old stellar population with high metallicity in the monolithic paradigm can explain the UV upturn. Numerical simulations and empirical studies however point out that massive early-type galaxies have evolved hierarchically with an extended star formation history. This obviously has an impact on our traditional understanding on the UV upturn and requires a new investigation on its origin. We report on our investigation on the evolutionary history of model galaxy SEDs in the hierarchical scenario. The use of conventional population models (calibrated to the monolithic picture) in combination with merger trees and extended star formation fails to reproduce the observed UV upturn. If a hierarchical picture is thought to be more realistic than a monolithic one, new calibration on the population models is required.