Star formation rates of distant luminous infrared galaxies derived from Hα and IR luminosities

We present a study of the star formation rate ( SFR ) for a sample of 16 distant galaxies detected by ISOCAM at 15  μ m in the CFRS0300+00 and CFRS1400+52 fields. Their high quality and intermediate resolution VLT/FORS spectra have allowed a proper correction of the Balmer emission lines from the underlying absorption. Extinction estimates using the H β /H γ and the H α /H β Balmer decrement are in excellent agreement, providing a robust measurement of the instantaneous SFR based on the extinction-corrected H α luminosity. Star formation has also been estimated exploiting the correlations between IR luminosity and those at MIR and radio wavelengths. Our study shows that the relationship between the two SFR estimates follow two distinct regimes: (1) for galaxies with SFR IR below ~100 $M_{\odot}$/yr, the SFR deduced from H α measurements is a good approximation of the global SFR and (2) for galaxies near ULIRG regime. The corrected H α SFR understimated the SFR by a factor of 1.5 to 2. Our analyses suggest that heavily extincted regions completely hidden in optical bands (such as those found in Arp 220) contribute less than 20% of the global budget of star formation up to $z=1$.