J/A+A/597/A98 Faraday tomography of foreground towards IC342 (Van Eck+, 2017)
Faraday tomography of the Local interstellar medium with LOFAR:
galactic foregrounds towards IC342.
Van Eck C.L., Haverkorn M., Alves M.I.R., Beck R., de Bruyn A.G.,
Ensslin T., Farnes J.S., Ferriere K., Heald G., Horellou C.,
Horneffer A., Iacobelli M., Jelic V., Marti-Vidal I., Mulcahy D.D.,
Reich W., Rottgering H.J.A., Scaife A.M.M, Schnitzeler D.H.F.M., Sobey C.,
Sridhar S.S.
<Astron. Astrophys. 597, A98 (2017)>
=2017A&A...597A..98V 2017A&A...597A..98V (SIMBAD/NED BibCode)
ADC_Keywords: Interstellar medium ; Polarization ; Magnetic fields
Keywords: ISM: magnetic fields - ISM: clouds - polarization -
local interstellar matter - radio continuum: ISM
Abstract:
Magnetic fields pervade the interstellar medium (ISM), but are
difficult to detect and characterize. The new generation of
low-frequency radio telescopes, such as the Low Frequency Array
(LOFAR: a Square Kilometre Array-low pathfinder), provides
advancements in our capability of probing Galactic magnetism through
low-frequency polarimetry. Maps of diffuse polarized radio emission
and the associated Faraday rotation can be used to infer properties
of, and trace structure in, the magnetic fields in the ISM. However,
to date very little of the sky has been probed at high angular and
Faraday depth resolution. We observed a 5 by 5 degree region centred
on the nearby galaxy IC 342 (l=138.2, b=+10.6) using the LOFAR High
Band Antennas in the frequency range 115-178MHz. We imaged this
region at 4.5x3.8arcmin2 resolution and performed Faraday tomography
to detect foreground Galactic polarized synchrotron emission separated
by Faraday depth (different amounts of Faraday rotation). Our Faraday
depth cube shows rich polarized structure, with up to 30K of polarized
emission at 150MHz. We clearly detect two polarized features that
extend over most of the field but are clearly separated in Faraday
depth. Simulations of the behaviour of the depolarization of
Faraday-thick structures at such low frequencies show that such
structures would be too strongly depolarized to explain the
observations. These structures are therefore rejected as the source of
the observed polarized features. Only Faraday thin structures will not
be strongly depolarized at low frequencies; producing such structures
requires localized variations in the ratio of synchrotron emissivity
to Faraday depth per unit distance. Such variations can arise from
several physical phenomena, such as a transition between regions of
ionized and (mostly) neutral gas. We conclude that the observed
polarized emission is Faraday thin, and propose that the emission
originates from two mostly neutral clouds in the local ISM. Using maps
of the local ISM to estimate distances to these clouds, we have
modelled the Faraday rotation for this line of sight and estimated
that the strength of the line of sight component of magnetic field of
the local ISM for this direction varies between -0.86 and +0.12uG
(where positive is towards the Earth). We propose that this may be a
useful method for mapping magnetic fields within the local ISM in all
directions towards nearby neutral clouds.
Description:
The Faraday depth cube of the IC342 field in polarized intensity,
produced from LOFAR HBA observations as part of LOFAR proposal
LC0_043. The cube is approximately 5x5 degrees in size, with 4-arcmin
resolution, and covers Faraday depths from -25 to +25rad/m2. The
detailed specifications are given in the table and in the FITS header.
Selected frames from this cubes are shown in the paper in Figures 2
through 5. An extended description of the data processing leading to
this cube is included in the paper.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table.dat 116 1 Table of RM cube properties
PIcube.fit 2880 2222226 RM cube
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Byte-by-byte Description of file: table.dat
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Bytes Format Units Label Explanations
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1- 9 F9.6 deg RAdeg Right ascension of image centre (J2000)
11- 19 F9.6 deg DEdeg Declination of image centre (J2000)
21- 22 I2 arcsec/pix Scale Pixel scale
24- 27 I4 --- Nx Number of pixels in along X-axis
29- 32 I4 --- Ny Number of pixels in along Y-axis
34- 36 I3 --- Nz Number of pixels in Faraday depth
38- 44 I7 Kibyte size Size of FITS file
46- 55 A10 --- FileName Name of the fits file (1)
57-116 A60 --- Title Title of the file
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Note (1): The RM cube is given in units of Jansky/PSF/RMSF, where PSF is
Point Spread Function (4.5x3.8arcmin2) and RMSF is Rotation Measure
Spread Function (0.9rad/m2).
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Acknowledgements:
Cameron Van Eck, c.vaneck (at) astro.ru.nl
(End) Cameron Van Eck [Radboud University], Patricia Vannier [CDS] 02-Dec-2016