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

Aims. 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. Methods. Using the measured 150 MHz flux densities from the LOFARMSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxiescharacterised by different morphology and star formation rate. 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. Results. Our galaxies' spectra are generally flatter at lower compared to higher frequencies: the median spectral indexαlow measured between ≈ 50 MHz and 1.5 GHz is -0.57 ± 0.01 while the high-frequency one αhigh, calculated between 1.3 GHz and 5 GHz, is -0.77 ± 0.03. 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, ApJ, 352, 30). According to our modelled radio maps for M 51-like galaxies, the free-free absorption effects can be seen only below 30 MHz and in the global spectra just below 20 MHz, while in the spectra of starburst galaxies, like M 82, the flatteningdue to absorption is instead visible up to higher frequencies of about 150 MHz. Starbursts are however scarce in the local Universe, in accordance with the weak spectral curvature seen in the galaxiesof our sample. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxiesas spectral flatteningaround 100-200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxiesthere are strong turnovers at frequencies above 700 MHz, regardless of viewing angle. Conclusions. Our modelling of galaxyspectra 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. We predict much stronger effects of thermal absorption in more distant galaxieswith high star formation rates. Some influence exerted by the Milky Way's foreground on the spectra of all external galaxiesis also expected at very low frequencies.