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A good correlation has been found in star-forming galaxies between the soft X-ray and the far infrared or radio luminosities. In this work we analyze the relation between the soft X-ray and far infrared luminosities as predicted by evolutionary population synthesis models, aiming first to test the validity of the soft X-ray luminosity as a star formation rate estimator, using the already known calibration of the FIR luminosity as a proxy, and second proposing a calibration based on the predictions of evolutionary synthesis models. We have computed the soft X-ray and far infrared luminosities expected for a massive starburst as a function of evolutionary state, efficiency of the conversion of mechanical energy into soft X-ray luminosity, star formation regime (instantaneous or extended) and dust abundance, and have compared the predictions with observational values for a sample of 62 star-forming galaxies taken from the literature. The observational Lsoftx/Lfir ratios are consistent with the predictions by the models under realistic assumptions (young starbursts, efficiency in the re-processing of mechanical energy of few percent), confirming the direct relation of the diffuse soft X-ray emission with the star formation episode. The soft X-ray emission of the diffuse, extended gas surrounding massive star forming regions can be used as a Star Formation Rate estimator. The empirical calibrations presented in the literature are supported by the predictions of evolutionary synthesis models and by the analysis of a larger sample of star-forming galaxies, but it is biased towards galaxies dominated by relatively unevolved starbursts., Comment: 8 pages, 6 figures. Accepted by Astronomy and Astrophysics. New version including language style revision and 3 new references

The study of massive star formation needs some basic tools, among which reliable SFR tracers are crucial ones. We are presently revising the calibration of SFR tracers at different wavelength ranges using last generation evolutionary population synthesis codes. The FIR luminosity produced by the heated dust in star forming regions is commonly used to characterize a starburst. The X-ray luminosity has been found to show a narrow correlation with it, and therefore it has been proposed as an additional tracer of potential interest for high $z$ galaxies, whose FIR emission is redshifted to the sub-mm range. In this communication we analyze the evolution of the X-ray to FIR luminosities ratio as predicted by the CMHK population synthesis models, both for constant stellar formation and instantaneous bursts. The results are compared to some sample data taken from the literature. The conclusion drawn from the comparison is that the empirical calibration of the soft X-ray luminosity seems a valid SFR tracer only for starbursts around a rather short period of time., Comment: 2 pages, 4 EPS figures, uses asp2006.sty, to be published in the proceedings of the workshop 'Pathways through an eclectic Universe', J. H. Knapen, T. J. Mahoney, and A. Vazdekis (Eds.), ASP Conf. Ser., 2007