J/A+A/633/A5 M 31 Stokes images (Beck+, 2020)
Magnetic fields and cosmic rays in M 31. I.
Spectral indices, scale lengths, Faraday rotation, and magnetic field pattern.
Beck R., Berkhuijsen E.M., Giessuebel R., Mulcahy D.D.
<Astron. Astrophys. 633, A5 (2020)>
=2020A&A...633A...5B 2020A&A...633A...5B (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Polarization
Keywords: galaxies: spiral - galaxies: magnetic fields - galaxies: ISM -
galaxies: individual: M 31 - radio continuum: galaxies -
radio continuum: ISM
Abstract:
Magnetic fields play an important role in the dynamics and evolution
of galaxies; however, the amplification and ordering of the initial
seed fields are not fully understood. The nearby spiral galaxy M 31 is
an ideal laboratory for extensive studies of magnetic fields. Our aim
was to measure the intrinsic structure of the magnetic fields in M 31
and compare them with dynamo models of field amplification. The
intensity of polarized synchrotron emission and its orientation are
used to measure the orientations of the magnetic field components in
the plane of the sky. The Faraday rotation measure gives information
about the field components along the line of sight. With the
Effelsberg 100-m telescope three deep radio continuum surveys of the
Andromeda galaxy, M 31, were performed at 2.645, 4.85, and 8.35 GHz
(wavelengths of 11.3, 6.2, and 3.6 cm). The λ 3.6 cm survey is
the first radio survey of M 31 at such small wavelengths. Maps of the
Faraday rotation measures (RMs) are calculated from the distributions
of the polarization angle. At all wavelengths the total and polarized
emission is concentrated in a ring-like structure of about 7-13 kpc in
radius from the centre. Propagation of cosmic rays away from the
star-forming regions is evident. The ring of synchrotron emission is
wider than the ring of the thermal radio emission, and the radial
scale length of synchrotron emission is larger than that of thermal
emission. The polarized intensity from the ring in the plane of the
sky varies double-periodically with azimuthal angle, indicating that
the ordered magnetic field is oriented almost along the ring, with a
pitch angle of -14°±2° at λ6.2 cm. The RM varies
systematically along the ring. The analysis shows a large-scale
sinusoidal variation with azimuthal angle, signature of an
axisymmetric spiral (ASS) regular magnetic field, plus a superimposed
double-periodic variation of a bisymmetric spiral (BSS) regular field
with about six times smaller amplitude. The RM amplitude of
(118±3) rad/m2 between λ6.2 cm and λ3.6 cm is
about 50% larger than between λ11.3 cm and λ6.2 cm,
indicating that Faraday depolarization at λ11.3 cm is stronger
(i.e. with a larger Faraday thickness) than at λ6.2 cm and
λ3.6 cm. The phase of the sinusoidal RM variation of
-7°±1° is interpreted as the average spiral pitch angle of
the regular field. The average pitch angle of the ordered field, as
derived from the intrinsic orientation of the polarized emission
(corrected for Faraday rotation), is significantly smaller:
-26°±3°. The dominating ASS plus the weaker BSS field of M 31
is the most compelling case so far of a field generated by the action
of a mean-field dynamo. The difference in pitch angle of the regular
and the ordered fields indicates that the ordered field contains a
significant fraction of an anisotropic turbulent field that has a
different pattern than the regular (ASS + BSS) magnetic field.
Description:
This catalogue contains three radio continuum surveys of M 31
performed with the Effelsberg 100-m telescope at central frequencies
of 2.645, 4.85, and 8.35GHz (λ11.33 cm, λ6.18 cm, and
λ3.59 cm).
Objects:
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RA (2000) DE Designation(s)
-----------------------------------------
00 42 44.33 +41 16 07.5 M 31 = NGC 224
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 155 26 List of fits files
fits/* . 26 Individual fits files
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See also:
J/A+A/559/A27 : RM Synthesis of 33 sources around M31 (Giessubel+, 2013)
J/A+A/571/A61 : M31 polarization + magnetic structure (Giessuebel+, 2014)
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- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 33 F6.4 GHz Freq ? Observed frequency
35- 38 I4 Kibyte size Size of FITS file
40- 65 A26 --- FileName Name of FITS file, in subdirectory fits
67-155 A89 --- Title Title of the FITS file
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Acknowledgements:
Rainer Beck, rbeck(at)mpifr-bonn.mpg.de
(End) Patricia Vannier [CDS] 24-Oct-2019