Radio continuum emission and magnetic fields

We observed the non-interacting edge-on galaxy NGC 5907 in total power and polarized radio continuum emission at wavelengths of λλ 2.8, 6.2, and 20 cm, using the VLA and the Effelsberg 100-m telescope. The total power emission of the galaxy shows (after subtraction of a double background source in the southern half) its maximum intensity slightly shifted to the north with respect to the optical and dynamical centre. The high spatial resolution of the 20 cm data allows to distinguish two emission components, a thin and a thick disk, with scale-heights of 0.34 and 1.5 kpc respectively at this wavelength. The spectral indices over the observed frequency range suggest a thermal fraction of the emission much larger than usually assumed for normal galaxies, more than 70 % at λ 2.8 cm. This finding is further supported by the spectral index distribution in the disk and the very weak polarization, which does not exceed ∼ 15%. From the estimated thermal (free-free) emission we calculated an averaged thermal electron density in the ISM of NGC 5907 of ne ∼ 0.03 cm−3 (for a clumping factor f = 20), a value similar to the local interstellar medium. From the nonthermal and the polarized intensity on the major axis we could estimate the total magnetic field strength in NGC 5907 to Btot ∼ 5μG. We found some evidence that the large-scale component of the total field is aligned with the galactic plane, as in most other galaxies, but is relatively weak (Bu ∼ 1.1μG). This weakness of the large-scale magnetic field and the small nonthermal fraction of the radio continuum emission at high frequencies can easily explain the non-detection of polarized emission at λ 2.8 cm. When we compare our results with previous ones on the radio continuum emission in nearby galaxies, we find that the quiescent galaxy NGC 5907 (in terms of star formation) is characterized by a deficiency of nonthermal radio emission. Hence we argue that in general a higher star formation activity, as observed in some other galaxies, may lead to a proportional enhancement of the thermal free-free emission from H ii regions, but – with some time lag – to a much more significant increase of the number density of relativistic electrons and a more effective amplification of the large-scale magnetic field, and therefore to a higher amount of nonthermal (synchrotron) radiation from Send offprint requests to: M.Dumke ([email protected]) those more actively star-forming galaxies than from the rather inactive galaxy NGC 5907.