Emission-lines calibrations of the star formation rate from the Sloan Digital Sky Survey

Aims. Up to now, the study of the star formation rate in galaxies has been based mainly on the Ha emission-line luminosity. However, this standard calibration cannot be applied at all redshifts if there is only one instrumental setup. Surveys based on optical spectroscopy do not observe the Ha emission line at redshifts higher than z similar to 0.5. Our goal is to study existing star formation rate calibrations and to provide new ones, still based on emission-line luminosities, which can be applied for various instrumental setups. Methods. We used the SDSS public data release DR4, which gives star formation rates and emission-line luminosities for more than 100 000 starforming galaxies observed at low redshifts. We take advantage of this statistically significant sample in order to study the relations, based on these data, between the star formation rate and the luminosities of some well-chosen emission lines. We correct the emission-line measurements for dust attenuation using the same attenuation curve as the one used to derive the star formation rates. Results. We confirm that the best results are obtained when relating star formation rates to the Ha emission-line luminosity, itself corrected for dust attenuation. This calibration has an uncertainty of 0.17 dex. We show that one has to carefully check the method used to derive the dust attenuation and to use the adequate calibration: in some cases, the standard scaling law has to be replaced by a more general power law. When data are corrected for dust attenuation but the H alpha emission line not observed, the use of the H beta emission line, if possible, has to be preferred to the [OII]lambda 3727 emission line. In the case of uncorrected data, the correction for dust attenuation can be assumed as a constant mean value, but we show that such a method leads to poor results in terms of dispersion and residual slope. Self-consistent corrections, such as previous studies based on the absolute magnitude, give better results in terms of dispersion but still suffer from systematic shifts and/or residual slopes. The best results with data not corrected for dust attenuation are obtained when using the observed [OII]lambda 3727 and H beta emission lines together. This calibration has an uncertainty of 0.23 dex.