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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 (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:
FileName Lrecl Records Explanations
ReadMe 80 . This file table.dat 116 1 Table of RM cube properties 2880 2222226 RM cube
Byte-by-byte Description of file: table.dat
Bytes Format Units Label Explanations
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 %s Size of FITS file 46- 55 A10 --- FileName Name of the fits file (1) 57-116 A60 --- Title Title of the file
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).
Acknowledgements: Cameron Van Eck, c.vaneck (at)
(End) Cameron Van Eck [Radboud University], Patricia Vannier [CDS] 02-Dec-2016
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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