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

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

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, Galaxies: statistics, Radio continuum: galaxies
0004-6361
Chyzy, K. T.
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Jurusik, W.
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Piotrowska, J.
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Nikiel-Wroczyński, B.
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Heesen, V.
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Paladino, R.
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Surma, P.
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Beck, R.
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Conway, J.
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Sendlinger, K.
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Curyło, M.
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Callingham, J. R.
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Gürkan, G.
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Iacobelli, M.
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Röttgering, H. J.A.
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Adebahr, B.
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Shulevski, A.
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Dettmar, R. J.
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Clarke, A. O.
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Farnes, J. S.
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Orrú, E.
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Pandey, V. N.
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Riseley, C. J.
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Rowlinson, A.
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Scaife, A. M.M.
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Stewart, A. J.
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Van Der Horst, A. J.
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Chyzy, K. T.
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Jurusik, W.
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Piotrowska, J.
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Nikiel-Wroczyński, B.
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Heesen, V.
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Vacca, V.
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Nowak, N.
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Paladino, R.
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Surma, P.
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Heald, G.
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Beck, R.
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Conway, J.
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Callingham, J. R.
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Gürkan, G.
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Iacobelli, M.
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Adebahr, B.
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Shulevski, A.
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Farnes, J. S.
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Pizzo, R.
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Riseley, C. J.
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Rowlinson, A.
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Scaife, A. M.M.
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Stewart, A. J.
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Van Der Horst, A. J.
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Van Weeren, R. J.
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Chyzy, K. T., Jurusik, W., Piotrowska, J., Nikiel-Wroczyński, B., Heesen, V., Vacca, V., Nowak, N., Paladino, R., Surma, P., Sridhar, S. S., Heald, G., Beck, R., Conway, J., Sendlinger, K., Curyło, M., Mulcahy, D., Broderick, J. W., Hardcastle, M. J., Callingham, J. R., Gürkan, G., Iacobelli, M., Röttgering, H. J.A., Adebahr, B., Shulevski, A., Dettmar, R. J., Breton, R. P., Clarke, A. O., Farnes, J. S., Orrú, E., Pandey, V. N., Pandey-Pommier, M., Pizzo, R., Riseley, C. J., Rowlinson, A., Scaife, A. M.M., Stewart, A. J., Van Der Horst, A. J. and Van Weeren, R. J. (2018) LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies. Astronomy & Astrophysics, 619, [A36]. (doi:10.1051/0004-6361/201833133).

Record type: Article

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 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.

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LOFAR MSSS- Flattening low-frequency radio continuum spectra of nearby galaxies - Accepted Manuscript
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Accepted/In Press date: 9 August 2018
e-pub ahead of print date: 7 November 2018
Published date: November 2018
Additional Information: Author, Heeson, confirmed that the Arxiv record is AM
Keywords: Galaxies: evolution, Galaxies: statistics, Radio continuum: galaxies

Identifiers

Local EPrints ID: 426811
URI: http://eprints.soton.ac.uk/id/eprint/426811
ISSN: 0004-6361
PURE UUID: e696c075-53ac-4b28-b915-df2a3e83b74d

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Date deposited: 12 Dec 2018 17:32
Last modified: 09 Dec 2019 17:50

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Contributors

Author: K. T. Chyzy
Author: W. Jurusik
Author: J. Piotrowska
Author: B. Nikiel-Wroczyński
Author: V. Heesen
Author: V. Vacca
Author: N. Nowak
Author: R. Paladino
Author: P. Surma
Author: S. S. Sridhar
Author: G. Heald
Author: R. Beck
Author: J. Conway
Author: K. Sendlinger
Author: M. Curyło
Author: D. Mulcahy
Author: J. W. Broderick
Author: M. J. Hardcastle
Author: J. R. Callingham
Author: G. Gürkan
Author: M. Iacobelli
Author: H. J.A. Röttgering
Author: B. Adebahr
Author: A. Shulevski
Author: R. J. Dettmar
Author: R. P. Breton
Author: A. O. Clarke
Author: J. S. Farnes
Author: E. Orrú
Author: V. N. Pandey
Author: M. Pandey-Pommier
Author: R. Pizzo
Author: C. J. Riseley
Author: A. Rowlinson
Author: A. M.M. Scaife
Author: A. J. Stewart
Author: A. J. Van Der Horst
Author: R. J. Van Weeren

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