Dust and nebular emission. I. Models for normal galaxies

We present a model for nebular emission in star forming galaxies, which takes into account the effects of dust reprocessing. The nebular emissions (continuum emission, 54 H and He recombination lines and 60 nebular lines from UV to IR) have been computed with CLOUDY and then included into GRASIL, our spectrophotometric code specifically developed for dusty galaxies. The interface between nebular emission and population synthesis is based on a set of pre-computed H II region emission models covering a wide range of physical quantities (metallicity, density, geometry and number of H I, He I and O II ionizing photons). These quantities are fully adequate to describe the emission properties of the majority of star-forming and starburst galaxies. Concerning the extinction properties of normal star forming galaxies, we are able to interpret the observed lack of correlation between the attenuation measured at Halpha and in the UV band as a consequence of age selective extinction. We also find that, for these galaxies with modest SFR, the ratio FIR/UV provides the best constraints on the UV attenuation. The accurate treatment of lines and continuum in dusty galaxies also allows to deal with different SFR estimators in a consistent way, from the UV to radio wavelengths, and to discuss the uncertainties arising from the different physical conditions encountered in star forming galaxies. We provide our best estimates of SFR/luminosity calibrations, together with their expected range of variation. It results that SFR derived through Halpha, even when corrected for extinction using the Balmer decrement, is affected by important uncertainties due to age selective extinction. Another remarkable result is that SFR from UV luminosity corrected by means of the ratio FIR/UV has a small uncertainty. Finally, our model provides a calibration of SFR from radio luminosity; its value differs from estimates from other works, but we are able to reproduce the observed FIR/radio ratio. These results are relevant to estimates of the contribution of disk galaxies to the cosmic SFR at z less than or equal to 1.