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

¹ Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
e-mail: krzysztof.chyzy@uj.edu.pl
² Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
³ School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
⁴ INAF–Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁵ INAF-Istituto di Radioastronomia, via P. Gobetti, 101, 40129 Bologna, Italy
⁶ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁷ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁸ CSIRO Astronomy and Space Science, PO Box 1130 Bentley, WA, 6102, Australia
⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁰ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992, Sweden
¹¹ Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany
¹² Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹³ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹⁴ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
¹⁵ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
¹⁶ University of Manchester, Jodrell Bank Centre for Astrophysics, Manchester M13 9PL, UK
¹⁷ Oxford e-Research Centre, University of Oxford, Keble Road, Oxford OX1 3QG, England
¹⁸ Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon, UMR5574, 69230 Saint-Genis-Laval, France
¹⁹ Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
²⁰ Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC 20052, USA

Received: 29 March 2018
Accepted: 9 August 2018

Abstract

Aims. 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.

Methods. Using the measured 150 MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies characterised by different morphology and star formation rate. 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.

Results. Our galaxies’ spectra are generally flatter at lower compared to higher frequencies: the median spectral index α_low measured between ≈50 MHz and 1.5 GHz is −0.57 ± 0.01 while the high-frequency one α_high, calculated between 1.3 GHz and 5 GHz, is −0.77 ± 0.03. 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, 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 flattening due 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 galaxies of our sample. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxies as spectral flattening around 100–200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxies there are strong turnovers at frequencies above 700 MHz, regardless of viewing angle.

Conclusions. Our modelling of galaxy spectra 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. We predict much stronger effects of thermal absorption in more distant galaxies with high star formation rates. Some influence exerted by the Milky Way’s foreground on the spectra of all external galaxies is also expected at very low frequencies.