LOFAR MSSS reveals low-frequency spectra of nearby galaxies

© Krzysztof T. Chyzy et al.

LOFAR's Multifrequency Snapshot Sky Survey (MSSS) has resulted in extensive studies of a large sample of nearby star-forming galaxies. Using the measured 150-MHz flux densities from MSSS and flux densities at various frequencies from the literature, we have obtained integrated radio spectra for 106 galaxies. This image illustrates examples of such spectra, showing that they are generally flatter at lower compared to higher frequencies. However, as there is no tendency for the highly inclined galaxies to have more flattened low-frequency spectra, we came to the conclusion that the observed flattening is not due to thermal absorption, contradicting previous suggestions.

The interpretation of the observed spectra was performed with a three-dimensional numerical model of galaxy radio emission, and radiation transfer dependent on the galaxy viewing angle and absorption processes. Our modelling suggests that the weak spectral flattening observed in the nearby galaxies 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 galaxies with high star formation rates. We also show that integrated spectra alone cannot be properly interpreted without supplementary data on the properties of the local interstellar medium within the galaxies.

If you wish to learn more about MSSS results and modelling of galaxy spectra, please look at our recent article - https://arxiv.org/abs/1808.10374 (K. T. Chyzy et al. 2018) - which is now accepted for publication in Astronomy & Astrophysics.