Comparison of star formation rate estimates from H-alpha, FIR and radio data

We use three indicators of massive star formation, H-alpha, FIR and non-thermal radio luminosities, to compare estimates of the star formation rate (SFR) for a sample of 34 spiral galaxies. To adjust the SFR values obtained from these indicators, we considered the slope, alpha, and/or the upper mass limit M-up of the initial mass function (IMF) as free parameters. The best agreement between the indicators is found for M-up approximate to 60-100 M. and alpha approximate to -3.1 at the high-mass end of the IMF (M >10 M.). Parallel with the SFR we also estimated the FIR excess X-FIR, defined as the fraction of the observed FIR not directly related to young massive stars. X-FIR is found to be well correlated with types of spiral galaxies and their colours (B-V): the redder a galaxy, the higher its FIR excess. We conclude that for any parameters of the IMF the observed FIR flux of early-type spiral galaxies needs an additional source of energy apart from massive star radiation.