$\tau$ranging from 0.1 to 15 Gyr), metallicities (from $ Z = 0.02 {Z}_\odot$ to $Z = 2.5 {Z}_\odot$) and dust extinctions ( 0 $<$ E(B-V) $<$ 1.1, using a Calzetti extinction curve). Details are given in Table 1 of Fontana et al. (2004), in Fontana et al. (2006) and in Grazian et al. (2006). The age of each object is constrained to be less than the age of the Universe at the relevant redshift. We also adopted a minimum age of 10 Myrs. We are aware that exponential star formation histories may not be the correct choice in some cases, but allowing $\tau$ to vary we can actually reproduce both an instantaneous burst (when $\tau$ is very small) and a constant star formation rate (when $\tau$ is $>>$ than the age of the galaxy.)\
At variance with our previous works (P07, P09) we use here the new version of the stellar population model described in detail in Charlot & Bruzual (2009). The main improvement over the previous Bruzual & Charlot (2003, BC03) model is an updated treatment of the emission from the stars that are in the thermally-pulsing asymptotic giant branch (TP-AGB) phase of stellar evolution from the models of 