is therefore a complicated function of redshift and it has a local minimum around z$\sim 3$. For the purpose of the present work, we just note that the two sub-samples have a consistent redshift distribution.

SED fitting
===========

The main physical properties of the galaxies such as total stellar mass, continuum-based star formation rate, stellar age, dust extinction E(B-V) and so on, were obtained through a spectral fitting technique which has been developed in previous papers (Fontana et al. 2003, F06), and is similar to those adopted by other groups in the literature (e.g. Dickinson et al. 2003; Drory et al. 2004). Briefly, a grid of spectral templates is computed from standard spectral synthesis models, and the expected magnitudes in our filter set are calculated. The derived template library is compared with the available photometry and the best-fit model template is adopted according to a $\chi^2$ minimization. During the fitting process, the redshift is fixed to its spectroscopic value. The physical parameters associated with each galaxy are obtained from the best-fit template up to 5.5 $\mu $m rest-frame. This analysis assumes that the overall galaxy SED can be represented as a purely stellar SED, extincted by a single attenuation law, 