Star formation, metallicity and dust properties derived from the Stromlo-APM galaxy survey spectra

We have derived star formation rates (SFRs), gas-phase oxygen abundances and effective dust absorption optical depths for a sample of galaxies drawn from the Stromlo-APM redshift survey using the new model of Charlot & Longhetti (CLO1), which provides a physically consistent description of the effects of stars, gas and dust on the integrated spectra of galaxies. Our sample consists of 705 galaxies with measurements of the fluxes and equivalent widths of Halpha, [O II] and one or both of [N II] and [S II]. For a subset of the galaxies, 60- and 100-mum IRAS fluxes are available. We compare the star formation rates derived using the model with those derived using standard estimators based on the Halpha, the [O II] and the far-infrared luminosities of the galaxies. The CL01 SFR estimates agree well with those derived from the IRAS fluxes, but are typically a factor of similar to3 higher than those derived from the Halpha or the [O II] fluxes, even after the usual mean attenuation correction of A(Halpha) = 1 mag is applied to the data. We show that the reason for this discrepancy is that the standard Halpha estimator neglects the absorption of ionizing photons by dust in H II regions and the contamination of Halpha emission by stellar absorption. We also use our sample to study variations in star formation and metallicity as a function of galaxy absolute b(J) magnitude. For this sample, the star formation rate per unit b(J) luminosity is independent of magnitude. The gas-phase oxygen abundance does increase with b(J) luminosity, although the scatter in metallicity at fixed magnitude is large.