Comparison of Star Formation Rate Estimates from Hα, FIR and Radio Data

We use three indicators of massive star formation, H_α, 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, α, 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≈ 60-100 M⊙ and α ≈–3.1 at the high-mass end of the IMF (M>10 M⊙.Parallelwith 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. This revised version was published online in July 2006 with corrections to the Cover Date.