The FIR / radio correlation in starburst galaxies – constraints on starburst models

This paper presents an analysis of the correlation between the far-infrared (FIR) and the radio emission of starburst galaxies. Data for interacting galaxies, many of which are undergoing a starburst, and for normal galaxies have been analysed and compared in order to test for any influence of the star-formation activity on the ratio between the FIR and the radio emission at 2.4 GHz, q 2.4GHz. No statistically significant indication for such an influence has been found: There is neither a significant difference between q 2.4GHz of the two samples nor a dependence of this ratio on the starburst strength. This observational fact is unexpected because of the different physical conditions and the short timescale of the star-formation activity in a starburst. In order to interprete the observations the FIR and the radio emission during a starburst have been modeled. The following conclusions could be drawn from the observed constancy of q 2.4GHz : A strong and fast (≈ 10 7 yr) increase of the magnetic field at the beginning of the starburst is required in order to maintain a constant q 2.4GHz. Otherwise the strong Inverse Compton losses that are due to the intense radiation field in a starburst would lower the synchrotron emission drastically resulting in a value of q 2.4GHz significantly higher than the observed one. Furthermore, the timescale of the variation of the star-formation rate has to be longer than some 10 7 yr. For lower values the different timescales of the FIR and the radio emission produce large fluctuations of q 2.4GHz .