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
star-formation
radio-continuum survey
sky survey
shock fronts
ionized-gas
wide-field
m51
magnetic-fields
Author
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- A74Subject Categories
Astronomy, Astrophysics and Cosmology
DOI
10.1051/0004-6361/201424187