LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby
  galaxies
  The shape of low-frequency radio continuum spectra of normal galaxies is 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 LOFAR
Multifrequency 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 LOFAR MSSS 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 galaxy
radio emission, and radiation transfer dependent on the galaxy viewing angle
and absorption processes. Spectra of our galaxies are generally flatter at
lower compared to higher frequencies but as there is no tendency for the highly
inclined galaxies to have more flattened low-frequency spectra, we argue that
the observed flattening is 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 flattening due 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 galaxies as
spectral flattening around 100-200MHz in the face-on objects, and as turnovers
in the edge-on ones, while in M82-like galaxies there are strong turnovers at
frequencies above 700MHz, regardless of viewing angle. Our modelling suggests
that the weak spectral flattening observed in the nearby galaxies studied here
results principally from synchrotron spectral curvature due to cosmic ray
energy losses and propagation effects.
