The nature of the low-frequency emission of M 51 First observations of a nearby galaxy with LOFAR
Journal article, 2014

Context. Low frequency radio continuum observations (<300 MHz) can provide valuable information propagation loss energy cosmic ray electrons (CRE). Nearby spiral galaxies have hardly been studied in this frequency range because of the technical challenges of low frequency radio mterferometry. This is now changing with the start of operations of LOFAR. Aims. We aim to study the propagation of low energy CRE in the interarm regions and the extended disk of the nearly face on sp galaxy Messier 51. We also search for polarisation in M 51 and other extragalactic sources in the held. Methods. The grand-design spiral galaxy M 51 was observed with the LOFAR High Frequency Antennas (H BA) and imaged in total y and polarisation. This observation covered the frequencies between 115 MHz and 175 MHz with 244 subbands of 8 channels each, resulting in 1952 channels, This allowed us to use RM synthesis to search for polarisation, Results. We produced an image of total emission of M 51 at the mean frequency of 151 MHz with 20" resolution and 0.3 mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300 MHz so far. The integrated spectrum of total radio on is described well by a power law while flat spectral indices in the central region indicate thermal absorption. We observe at the disk extends out to 16 kpc and see a break in the radial profile near the optical radius of the disk. The radial scale lengths in the inner and outer disks are greater at 151 MHz, and the break is smoother at 151 MHz than those observed at 1.4 GHz. The arm-interarm contrast is lower at 151 MHz than at 1400 MHz, indicating propagation of CRE from spiral arms into interarm regions. The correlations between the images of radio emission at 151 MHz and 1400 MHz and the FIR emission at 70 pm reveal breaks on scales of 1.4 and 0.7 kpc, respectively'. The total (equipartition) magnetic held strength decreases from about 28 mu G in the central region to about 10 mu G at 10 kpc radius. No significant polarisation was detected from M51. owing to severe Faraday depolarisation. Six extragalactic sources are detected in polarisation in the M51 field of 4.1 degrees x 4.1 degrees size. Two sources show complex structures in Faraday space. Conclusions. Our main results, the scale lengths of the inner and outer disks at 151 MHz and 1.4 GHz, arm-interann contrast, and the break scales of the radio-FIR correlations, can be explained consistently by CRE diffusion, leading to a longer propagation length of CRE of lower energy. The distribution of CRE sources drops sharply at about 10 kpc radius, where the star formation rate also decreases sharply. We find evidence that thermal absorption is primarily caused by HI! regions. The non detection of polarisation from M 51 at 151 MHz is consistent with the estimates of Faraday depolarisation. Future searches for polarised emission in this frequency range should concentrate on regions with low star formation rates.

cosmic-ray propagation

spiral galaxy


radio-continuum survey

sky survey

shock fronts






D. D. Mulcahy

Max Planck Society

University of Southampton

A. Horneffer

Max Planck Society

R. Beck

Max Planck Society

G. Heald

University of Groningen

Netherlands Institute for Radio Astronomy (ASTRON)

A. Fletcher

Newcastle University

A. Scaife

University of Southampton

B. Adebahr

Max Planck Society

J. M. Anderson

Leibniz Institute for Astrophysics Potsdam

Max Planck Society

A. Bonafede

University of Hamburg

M. Brueggen

University of Hamburg

G. Brunetti

Istituto nazionale di astrofisica (INAF)

K.T. Chyz̊y

Jagiellonian University in Kraków

John Conway

Chalmers, Earth and Space Sciences, Onsala Space Observatory

R. -J. Dettmar

Ruhr-Universität Bochum

T.A. Enßlin

Max Planck Society

M. Haverkorn

Leiden University

Radboud University

Cathy Horellou

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

M. Iacobelli

Netherlands Institute for Radio Astronomy (ASTRON)

Leiden University

F. P. Israel

Leiden University

H. Junklewitz

Argelander-Institut für Astronomie

W. Jurusik

Jagiellonian University in Kraków

J. Koehler

Max Planck Society

M. Kuniyoshi

Max Planck Society

E. Orru

Netherlands Institute for Radio Astronomy (ASTRON)

R. Paladino

Istituto nazionale di astrofisica (INAF)

University of Bologna

R. Pizzo

Netherlands Institute for Radio Astronomy (ASTRON)

W. Reich

Max Planck Society

H. Rottgering

Leiden University

Astronomy and Astrophysics

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

Vol. 568 Article no. A74- A74

Subject Categories

Astronomy, Astrophysics and Cosmology



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