J/A+A/588/A114 M101 radio polarization & magnetic structure (Berkhuijsen+, 2016)
Radio polarization and magnetic field structure in M 101.
Berkhuijsen E.M., Urbanik M., Beck R., Han J.L.
<Astron. Astrophys. 588, A114 (2016)>
=2016A&A...588A.114B 2016A&A...588A.114B (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Magnetic fields ; Radio sources
Keywords: galaxies: individual: M 101 - galaxies: magnetic fields -
galaxies: star formation - radio continuum: galaxies -
polarization - radiation mechanisms: non-thermal
Abstract:
We observed total and polarized radio continuum emission from the
spiral galaxy M 101 at λλ6.2cm and 11.1cm with the
Effelsberg telescope. The angular resolutions are 2.5' (=5.4kpc) and
4.4' (=9.5kpc), respectively. We use these data to study various
emission components in M 101 and properties of the magnetic field.
Separation of thermal and non-thermal emission shows that the thermal
emission is closely correlated with the spiral arms, while the
non-thermal emission is more smoothly distributed indicating di ff
usion of cosmic ray electrons away from their places of origin. The
radial distribution of both emissions has a break near R=16kpc
(=7.4'), where it steepens to an exponential scale length of L≃5kpc,
which is about 2.5 times smaller than at R<16kpc. The distribution of
the polarized emission has a broad maximum near R=12kpc and beyond
R=16kpc also decreases with L≃5kpc. It seems that near R=16kpc a
major change in the structure of M 101 takes place, which also a ff
ects the distributions of the strength of the random and ordered
magnetic field. Beyond R=16kpc the radial scale length of both fields
is about 20kpc, which implies that they decrease to about 0.3uG at
R=70kpc, which is the largest optical extent. The equipartition
strength of the total field ranges from nearly 10uG at R<2kpc to 4uG
at R=22-24kpc. As the random field dominates in M 101
(Bran/Bord≃2.4), wavelength-independent polarization is the main
polarization mechanism. We show that energetic events causing H i
shells of mean diameter <625pc could partly be responsible for this.
At radii <24kpc, the random magnetic field depends on the star
formation rate/area, ΣSFR, with a power-law exponent of
b=0.28±0.02. The ordered magnetic field is generally aligned with
the spiral arms with pitch angles that are about 8° larger than
those of HI filaments.
Description:
M 101 was observed at the frequencies 2.7GHz (λ11.1cm) and
4.85GHz (λ6.2cm) with receiver systems in the 100m Effelsberg
telescope between July and December 1997.
Objects:
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RA (2000) DE Designation(s)
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14 03 12.58 +54 20 55.5 M101 = NGC 5457
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 132 8 List of fits images
fits/* . 8 Individual fits images
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 23 I4 --- Nx Number of pixels along X-axis
25- 28 I4 --- Ny Number of pixels along Y-axis
30- 40 A11 "date" Obs.Date Observation date
42- 53 E12.6 Hz Freq ? Observed frequency
55- 58 I4 Kibyte size Size of FITS file
60- 80 A21 --- FileName Name of FITS file, in subdirectory fits
82-132 A51 --- Title Title of the FITS file
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Acknowledgements:
Rainer Beck, rbeck(at)mpifr-bonn.mpg.de
(End) Patricia Vannier [CDS] 17-Feb-2016