and their contribution to the 24 micron flux. Nevertheless we point out that there are still many sources of uncertainties in the conversion from 24 micron flux to total IR luminosity, due to the modeling of the PAH features, which depend on the galaxy metallicity, galaxy environment, galaxy size, star formation history, and on the ionizing radiation field (Calzetti et al. 2007).

The derived $SFR_{MIPS}$ are on average higher than both $SFR_{SED}$ and $SFR_{UV}$. We also find a clear trend that the $SFR_{MIPS}/ SFR_{SED}$ tend to be $\sim 1$ for objects with moderate star formation rates, while becoming $>>1$ for objects with large SFR, as shown in Figure \[fig:mips1\]. Both  and   seem to follow this trend. One possibility is that, in objects showing $SFR_{MIPS} >>SFR_{SED}$, the MIPS emission is not (entirely) due to star formation but might be still partially due to AGN contribution, even after the exclusion of the clear obscured AGNs. However since the trend extends also to values lower than unity and is present at all redshifts (see Santini et al. 2009), it is possible that it reflects a change in some intrinsic physical property of star-forming galaxies, such as metallicity. Galaxies with sub-solar metallicities have lower 