Magnetic fields and ionized gas in nearby late type galaxies ∗

Aims. In order to analyze the importance of the star formation rate in generating and amplifying magnetic fields in the interstellar medium we perform a deep continuum polarization study of three angularly large, late type spiral galaxies. Methods. We obtained deep total power and polarization maps at 4.85 GHz of NGC 4236, NGC 4656 and IC 2574 using the 100-m Effelsberg radio telescope. This was accompanied by imaging in the Hα line. We also observed these objects at 1.4 GHz to obtain their integrated fluxes at this frequency and to determine their radio spectra. Results. All galaxies were found to possess weak but detectable total power emission at 4.85 GHz, coincident with regions of recent star formation as traced by bright Hα regions. The surface brightness of the radio-strongest object of our sample (NGC 4656) is comparable to the radio-weakest objects in a sample of more than 50 normally star-forming spiral galaxies for which measurements at 4.8 GHz with the Effelsberg radio telescope are available. The surface brightness of the two other objects is even three times smaller. The fractional polarization of the 2 galaxies of our sample is less than 2%, significantly lower than for spiral galaxies of intermediate types, suggesting that the magnetic fields are not only weaker, but also less ordered than in spiral galaxies. The radio spectra of galaxies in our small sample are indicative of a substantial fraction of thermal emission, with a higher thermal fraction than in spirals with high star formation rates (SFR), while the nonthermal emission in our sample is relatively weak compared to spiral galaxies. We propose an equipartition model where the nonthermal emission increases ∝ SF R ≈1.4 and the ratio of nonthermal to thermal emission increases ∝ SF R ≈0.4. The objects of our sample still follow the radio-FIR correlation of surface brightness of the total emission, extending it towards the lowest values measured so far.