mid-IR emission (at least at $8~\mu$m, Calzetti et al. 2007) and higher UV luminosity than solar ones, for a given level of SFR. The observed trend could therefore be related to a metallicity trend. Unfortunately, a direct check of this is not feasible. Reliable metallicities cannot be inferred from broad-band SED fitting, and high resolution spectroscopy is necessary to properly distinguish between SEDs characterized by different lines and hence metallicities.

Finally the observed trend could also be due to a failure of the assumed modelling of star formation history for example in galaxies with very high star formation.

Spectral properties
===================

The majority of the spectra were retrieved from the GOODS VIMOS database. Since relatively few spectra and redshifts were obtained from the other programs, we decided to make a composite spectrum using only the VIMOS data. The homogeneity of the data set in terms of resolution, depth and calibration accuracy, makes it easier to add up the spectra, even if the number of sources is reduced.\
The 1-D spectra are distributed in fits format from the GOODS database[^1]: they are calibrated both in flux and wavelength. For details on reduction and calibration see Popesso et al. (2009). The final 