LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies
Journal article, 2018

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 αlow 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 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.

Radio continuum: galaxies

Galaxies: evolution

Galaxies: statistics

Author

K.T. Chyz̊y

Jagiellonian University in Kraków

W. Jurusik

Jagiellonian University in Kraków

J. Piotrowska

Jagiellonian University in Kraków

B. Nikiel-Wroczyński

Jagiellonian University in Kraków

V. Heesen

Universität Hamburg

University of Southampton

V. Vacca

Osservatorio Astronomico di Cagliari

N. Nowak

Jagiellonian University in Kraków

R. Paladino

INAF OAS Bologna

P. Surma

Jagiellonian University in Kraków

S. S. Sridhar

Netherlands Institute for Radio Astronomy (ASTRON)

University of Groningen

G. Heald

CSIRO Astronomy and Space Science

R. Beck

Max Planck Institute

John Conway

Chalmers, Space, Earth and Environment, Onsala Space Observatory

K. Sendlinger

Ruhr-Universität Bochum

M. Curyło

Jagiellonian University in Kraków

D. D. Mulcahy

University of Manchester

Max Planck Institute

J.W. Broderick

Netherlands Institute for Radio Astronomy (ASTRON)

M. J. Hardcastle

University of Hertfordshire

J. R. Callingham

Netherlands Institute for Radio Astronomy (ASTRON)

G. Gurkan

CSIRO Astronomy and Space Science

M. Iacobelli

Netherlands Institute for Radio Astronomy (ASTRON)

H. Rottgering

Leiden University

B. Adebahr

Ruhr-Universität Bochum

A. Shulevski

University of Amsterdam

R. -J. Dettmar

Ruhr-Universität Bochum

R. P. Breton

University of Manchester

A. O. Clarke

University of Manchester

J.S. Farnes

University of Oxford

E. Orru

Netherlands Institute for Radio Astronomy (ASTRON)

V. N. Pandey

Netherlands Institute for Radio Astronomy (ASTRON)

M. Pandey-Pommier

CRAL Le Centre de Recherche Astronomique de Lyon

R. Pizzo

Netherlands Institute for Radio Astronomy (ASTRON)

C. J. Riseley

CSIRO Astronomy and Space Science

A. Rowlinson

Netherlands Institute for Radio Astronomy (ASTRON)

A. M. M. Scaife

University of Manchester

A. Stewart

The University of Sydney

A. J. van der Horst

George Washington University

R. van Weeren

Leiden University

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 619 A36

Subject Categories

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics

Other Physics Topics

DOI

10.1051/0004-6361/201833133

More information

Latest update

4/4/2019 1