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

The shape of low-frequency radio continuum spectra of normal galaxiesis not well understood, the key question being the role of physical processes such as thermal absorption in shaping them. In this work we take advantage of the LOFARMultifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for a large sample of nearby star-forming galaxies. Using the measured 150MHz flux densities from the LOFARMSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies. The spectra 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 galaxiesare generally flatter at lower compared to higher frequencies but as there is no tendency for the highly inclined galaxiesto have more flattenedlow-frequency spectra, we argue that the observed flatteningis not due to thermal absorption, contradicting the suggestion of Israel & Mahoney (1990). According to our modelled radio maps for M51-like galaxies, the free-free absorption effects can be seen only below 30MHz and in the global spectra just below 20MHz, while in the spectra of starburst galaxies, like M82, the flatteningdue to absorption is instead visible up to higher frequencies of about 150MHz. Locally, within galactic disks, the absorption effects are distinctly visible in M51-like galaxiesas spectral flatteningaround 100-200MHz in the face-on objects, and as turnovers in the edge-on ones, while in M82-like galaxiesthere are strong turnovers at frequencies above 700MHz, regardless of viewing angle. Our modelling suggests that the weak spectral flatteningobserved in the nearby galaxiesstudied here results principally from synchrotron spectral curvature due to cosmic ray energy losses and propagation effects.