J/A+A/571/A61 M31 polarization & magnetic structure (Giessuebel+, 2014)
The magnetic field structure of the central region in M 31.
Giessuebel R., Beck R.
<Astron. Astrophys., 571, A61-61 (2014)>
=2014A&A...571A..61G 2014A&A...571A..61G (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Magnetic fields ; Radio sources
Keywords: techniques: polarimetric - galaxies: individual: M 31 -
galaxies: magnetic fields - radio continuum: galaxies - polarization
Abstract:
The Andromeda Galaxy (M31) is the nearest grand-design spiral galaxy.
Thus far, most studies in the radio regime concentrated on the 10kpc
ring. The central region of M31 has significantly different
properties than the outer parts: The star formation rate is low, and
inclination and position angle are largely different from the outer
disk. The existing model of the magnetic field in the radial range
6≤r≤14kpc is extended to the innermost part r≤0.5kpc to ultimately
achieve a picture of the entire magnetic field in M 31. We combined
observations taken with the VLA at 3.6cm and 6.2cm with data from the
Effelsberg 100-m telescope to fill the missing spacings of the
synthesis data. The resulting polarization maps were averaged in
sectors to analyse the azimuthal behaviour of the polarized intensity
(PI), rotation measure (RM), and apparent pitch angle (φobs). We
developed a simplified 3D model for the magnetic field in the central
region to explain the azimuthal behaviour of the three observables.
Our 3D model of a quadrupolar or dipolar dynamo field can explain the
observed patterns in PI, RM, and φobs, while a 2D configuration
is not sufficient to explain the azimuthal behaviour. In addition and
independent of our model, the RM pattern shows that the spiral
magnetic field in the inner 0.5kpc points outward, which is opposite
to that in the outer disk, and has a pitch angle of ≃33°, which
is much larger than that of 8°-19° in the outer disk. The
physical conditions in the central region differ significantly from
those in the 10kpc ring. In addition, the orientation of this region
with respect to the outer disk is completely different. The opposite
magnetic field directions suggest that the central region is decoupled
from the outer disk, and we propose that an independent dynamo is
active in the central region.
Description:
The M31 galaxy has been observed with the Very Large Array (VLA) at
3.6cm (8.46GHz) on 23 December 2005 and at 6.2cm (4.86GHz) on 26
July 1992. Both observations were made using the D-Array. The 6.2cm
data has already been presented in Hoernes (1997, IAU Symp., 184, 351)
but was reduced again from the raw UV data for this analysis, since
the images were contaminated by side-lobes from a bright off-centre
source .
The Effelsberg data was taken over several years (1999 (8.35GHz,
3.6cm); 2001-2005 (4.85GHz, 6.2cm)) and will be presented in a
separate paper (Giessuebel et al., in prep.). The 3.6cm receiver is a
single-horn receiver with a bandwidth of 1.1GHz centred on 8.35GHz;
the 6.2cm receiver is a dual-horn receiver with a bandwidth of 0.5GHz
centred on 4.85GHz. Both are installed in the secondary focus of the
Effelsberg 100 m Telescope. Both maps were observed using on-the-fly
mapping, alternating in two perpendicular directions.
Objects:
-----------------------------------------
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 102 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- 22 I3 --- Nx Number of pixels along X-axis
24- 26 I3 --- Ny Number of pixels along Y-axis
28- 31 I4 Kibyte size Size of FITS file
33- 52 A20 --- FileName Name of FITS file, in subdirectory fits
54-102 A49 --- Title Title of the FITS file
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Acknowledgements:
Rainer Beck, rbeck(at)mpifr-bonn.mpg.de
(End) Patricia Vannier [CDS] 18-Feb-2016