The infrared spectral energy distribution of normal star-forming galaxies

We present a new phenomenological model for the spectral energy distributio n of normal star-forming galaxies between 3 and 1100 mum. A sequence of rea listic galaxy spectra are constructed from a family of dust emission curves assuming a power-law distribution of dust mass over a wide range of inters tellar radiation fields. For each interstellar radiation field heating inte nsity, we combine emission curves for large and very small grains and aroma tic feature carriers. The model is constrained by IRAS and ISOCAM broadband photometric and ISOPHOT spectrophotometric observations for our sample of 69 normal galaxies; the model reproduces well the empirical spectra and inf rared color trends. These model spectra allow us to determine the infrared energy budget for normal galaxies and in particular to translate far-infrar ed fluxes into total (bolometric) infrared fluxes. The 20-42 mum range appe ars to show the most significant growth in relative terms as the activity l evel increases, suggesting that the 20-42 mum continuum may be the best dus t emission tracer of current star formation in galaxies. The redshift depen dence of infrared color-color diagrams and the far-infrared-to-radio correl ation for galaxies are also explored.