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The shape of low-frequency radio continuum spectra of normal galaxies is notwell understood, the key question being the role of physical processes such asthermal absorption in shaping them. In this work we take advantage of the LOFARMultifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for alarge sample of nearby star-forming galaxies. Using the measured 150MHz fluxdensities from the LOFAR MSSS survey and literature flux densities at variousfrequencies we have obtained integrated radio spectra for 106 galaxies. Thespectra are explained through the use of a three-dimensional model of galaxyradio emission, and radiation transfer dependent on the galaxy viewing angleand absorption processes. Spectra of our galaxies are generally flatter atlower compared to higher frequencies but as there is no tendency for the highlyinclined galaxies to have more flattened low-frequency spectra, we argue thatthe observed flattening is not due to thermal absorption, contradicting thesuggestion of Israel & Mahoney (1990). According to our modelled radio maps forM51-like galaxies, the free-free absorption effects can be seen only below30MHz and in the global spectra just below 20MHz, while in the spectra ofstarburst galaxies, like M82, the flattening due to absorption is insteadvisible up to higher frequencies of about 150MHz. Locally, within galacticdisks, the absorption effects are distinctly visible in M51-like galaxies asspectral flattening around 100-200MHz in the face-on objects, and as turnoversin the edge-on ones, while in M82-like galaxies there are strong turnovers atfrequencies above 700MHz, regardless of viewing angle. Our modelling suggeststhat the weak spectral flattening observed in the nearby galaxies studied hereresults principally from synchrotron spectral curvature due to cosmic rayenergy losses and propagation effects.

LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies