LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies
Artikel i vetenskaplig tidskrift, 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.
Galaxies: evolution
Radio continuum: galaxies
Galaxies: statistics
Författare
K.T. Chyz̊y
Uniwersytet Jagiellonski w Krakowie
W. Jurusik
Uniwersytet Jagiellonski w Krakowie
J. Piotrowska
Uniwersytet Jagiellonski w Krakowie
B. Nikiel-Wroczyński
Uniwersytet Jagiellonski w Krakowie
V. Heesen
Universität Hamburg
University of Southampton
V. Vacca
Istituto nazionale di astrofisica (INAF)
N. Nowak
Uniwersytet Jagiellonski w Krakowie
R. Paladino
Istituto nazionale di astrofisica (INAF)
P. Surma
Uniwersytet Jagiellonski w Krakowie
S. S. Sridhar
Netherlands Institute for Radio Astronomy (ASTRON)
Rijksuniversiteit Groningen
G. Heald
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
R. Beck
Max-Planck-Gesellschaft
John Conway
Chalmers, Rymd-, geo- och miljövetenskap, Onsala rymdobservatorium
K. Sendlinger
Ruhr-Universität Bochum
M. Curyło
Uniwersytet Jagiellonski w Krakowie
D. D. Mulcahy
University of Manchester
Max-Planck-Gesellschaft
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
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
M. Iacobelli
Netherlands Institute for Radio Astronomy (ASTRON)
H. Rottgering
Universiteit Leiden
B. Adebahr
Ruhr-Universität Bochum
A. Shulevski
Universiteit Van 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
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
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
Universiteit Leiden
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 619 A36Ämneskategorier
Astronomi, astrofysik och kosmologi
Atom- och molekylfysik och optik
Annan fysik
DOI
10.1051/0004-6361/201833133