Far infrared and radio emission in dusty starburst galaxies

We revisit the nature of the far infrared (FIR)/radio correlation by means of the most recent models of star forming galaxies, focusing in particular on the case of obscured starbursts. We model the IR emission with our population synthesis code, GRASIL (Silva et al. 1998). For the radio emission, we revisit the simple model of Condon & Yin (1990). We find that a tight FIR/radio correlation is natural when the synchrotron mechanism dominates over the inverse Compton, and the electron cooling time is shorter than the fading time of the supernova (SN) rate. Observations indicate that both these conditions are met in star forming galaxies, from normal spirals to obscured starbursts. However, since the radio non-thermal (NT) emission is delayed, deviations are expected both in the early phases of a starburst, when the radio thermal component dominates, and in the post-starburst phase, when the bulk of the NT component originates from less massive stars. We show that this delay allows the analysis of obscured starbursts with a time resolution of a few tens of Myrs, unreachable with other star formation (SF) indicators. We suggest a strategy to complement the analysis of the deviations from the FIR/radio correlation with the radio slope (q-radio slope diagram) to obtain characteristic parameters of the burst, e.g. its intensity, age and fading time scale. The analysis of a sample of compact ULIRGs shows that they are intense but transient starbursts, to which one should not apply usual SF indicators devised for constant SF rates. We also discuss the possibility of using the q-radio slope diagram to assess the presence of obscured AGN. A firm prediction of the models is an apparent radio excess during the post-starburst phase, which seems to be typical of a class of star forming galaxies in rich cluster cores. Finally we discuss how deviations from the correlation, due to the evolutionary status of the starburst, affect the technique of photometric redshift determination widely used for high-z sources.