ON THE LOW-FREQUENCY RADIO SPECTRUM OF SPIRAL GALAXIES

In this short note I reanalyse the low frequency radio continuum data on spiral galaxies published by Israel & Mahoney (1990, IM). I conclude that most spiral galaxies do indeed show a change in spectral index with frequency but that, contrary to IM, the amount of the change is not significantly dependent on the inclination of the galaxies. The latter makes the free-free absorption model proposed by IM to explain the flattening at low frequencies less compelling. A steepening of the spectrum at high frequencies (> 1 GHz) due to propagation effects of the relativistic electrons is at least equally plausible.