The Westerbork SINGS survey

Heald, G.; Braun, Róbert; Edmonds, Robert

doi:10.1051/0004-6361/200912240

Cited by 217 publications

(311 citation statements)

References 45 publications

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“…2) shows a corresponding asymmetry in the northern part while the radio emission in the southern half is smoothed with the emission of the background source at this low angular resolution of 147 HPBW. The northern asymmetric distribution looks similar also at λ 20 cm by Hummel et al (1991) and in the L-band regime by Heald et al (2009). However, a second larger extent of the radio emission is visible at λ 3.6 cm ( Fig.…”

Section: Distribution Of the Total Intensity Emissionsupporting

confidence: 55%

“…The obtained rotation measures are biased by the Galactic foreground component (RM fg ), which is quite small in the direction of the sky in which NGC 4631 is located. Nevertheless, we corrected for this by subtracting the value of RM fg = (−4 ± 3) rad/m 2 (Heald et al 2009) from our rotation measures.…”

Section: Rotation Measure Distribution and The Magnetic Field Along Tmentioning

confidence: 99%

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Magnetic field structure and halo in NGC 4631

Mora

Krause

2013

A&A

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Context. All of the edge-on spiral galaxies observed so far present a similar magnetic field configuration, which consists of a planeparallel field in the disk and an X-shaped field at larger z-distances from the plane of the galaxy. Only NGC 4631 seems to have a different field orientation in its disk. Along the eastern and western halves of the disk of NGC 4631 the magnetic field orientation is parallel to the galactic plane, but in the central region of the disk a vertical field seems to dominate. Aims. In order to clarify whether NGC 4631 has a unique magnetic field configuration in the central region along its disk, we present high-resolution Faraday-corrected polarization data. Methods. Radio continuum observations of NGC 4631 at 4.85 GHz were performed with the VLA. In addition, observations were made with the Effelsberg telescope at 4.85 GHz and at 8.35 GHz. These were analyzed together with archival VLA-data at 8.35 GHz. The single-dish and interferometer data were combined to recover the missing zero-spacings. Results. We determined an integrated total spectral index of α tot = −0.78 ± 0.04 and a nonthermal integrated spectral index of α nth = −0.87 ± 0.03. The vertical scale heights in NGC 4631 vary significantly in different regions within the galaxy and their mean values at 4.85 GHz are with 2.3 kpc (370 pc) for the thick (thin) disk higher than the mean values found so far in six other edge-on spiral galaxies. This may originate in the tidal interaction of NGC 4631 with its neighbouring galaxies. The rotation measures are characterized by a smooth large-scale distribution. Along the galactic plane the degree of Faraday depolarization is significantly high. We estimated a total magnetic field strength in the disk of NGC 4631 of B t ≈ 9 ± 2 μG and an ordered field strength of B ord ≈ 2 ± 1 μG. The total field strengths in the halo are of the order of the total magnetic field strength in the disk, whereas the ordered field strengths in the halo seem to be higher than the value in the disk. Conclusions. The derived distribution of rotation measures implies that NGC 4631 has a large-scale regular magnetic field configuration. Despite the strong Faraday depolarization along the galactic plane and the strong beam depolarization in the transition zone between the disk and halo, our research strongly indicates that the magnetic field orientation along the central 5-7 kpc of the disk is also plane-parallel. Therefore, we claim that NGC 4631 also has a magnetic field structure plane-parallel along its entire disk, similar to all other edge-on galaxies observed up to now.

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Section: Distribution Of the Total Intensity Emissionsupporting

confidence: 55%

Section: Rotation Measure Distribution and The Magnetic Field Along Tmentioning

confidence: 99%

Magnetic field structure and halo in NGC 4631

Mora

Krause

2013

A&A

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“…where B is the area under the restoring beam of RMCLEAN (Heald et al 2009) divided by ∆Φ = |Φ i + 1 − Φ i | and n PI is the Fig. 4.…”

Section: Diffuse Polarized Emissionmentioning

confidence: 99%

Initial LOFAR observations of epoch of reionization windows

Jelić

Bruyn

Mevius

et al. 2014

A&A

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Aims. This study aims to characterise the polarized foreground emission in the ELAIS-N1 field and to address its possible implications for extracting of the cosmological 21 cm signal from the LOw-Frequency ARray -Epoch of Reionization (LOFAR-EoR) data. Methods. We used the high band antennas of LOFAR to image this region and RM-synthesis to unravel structures of polarized emission at high Galactic latitudes. Results. The brightness temperature of the detected Galactic emission is on average ∼4 K in polarized intensity and covers the range from -10 to +13 rad m −2 in Faraday depth. The total polarized intensity and polarization angle show a wide range of morphological features. We have also used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. The LOFAR and WSRT images show a similar complex morphology at comparable brightness levels, but their spatial correlation is very low. The fractional polarization at 150 MHz, expressed as a percentage of the total intensity, amounts to ≈1.5%. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies Conclusions. The wide frequency range, high angular resolution, and high sensitivity make LOFAR an exquisite instrument for studying Galactic polarized emission at a resolution of ∼1-2 rad m −2 in Faraday depth. The different polarized patterns observed at 150 MHz and 350 MHz are consistent with different source distributions along the line of sight wring in a variety of Faraday thin regions of emission. The presence of polarized foregrounds is a serious complication for epoch of reionization experiments. To avoid the leakage of polarized emission into total intensity, which can depend on frequency, we need to calibrate the instrumental polarization across the field of view to a small fraction of 1%.

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“…RM synthesis has been applied to spectropolarimetric data from the Westerbork telescope Schnitzeler et al 2009;Heald et al 2009;Mao et al 2010;Brentjens 2011;Pizzo et al 2011), from the Parkes telescope and the Australia Telescope Compact Array (ATCA) (Feain et al 2009;Harvey-Smith et al 2010;O'Sullivan et al 2012), from the VLA (Van Eck et al 2011) and from the EVLA (Heesen et al 2011). The Δλ 2 coverages of these observations were small, so that the resolution in Faraday space was limited.…”

Section: Introductionmentioning

confidence: 99%

Recognizing magnetic structures by present and future radio telescopes with Faraday rotation measure synthesis

Beck

Frick

Stepanov

et al. 2012

A&A

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Context. Modern radio telescopes allow us to record a large number of spectral channels. The application of a Fourier transform to spectropolarimetric data in radio continuum, Faraday rotation measure (RM) synthesis, yields the "Faraday spectrum", which hosts valuable information about the magneto-ionic medium along the line of sight. Aims. We investigate whether the method of wavelet-based RM synthesis can help us to identify structures of regular and turbulent magnetic fields in extended magnetized objects, such as galaxies and galaxy clusters. Methods. The analysis of spectropolarimetric radio observations of multi-scale targets calls for a corresponding mathematical technique. Wavelets allow us to reformulate the RM synthesis method in a scale-dependent way and to visualize the data as a function of Faraday depth and scale.Results. We present observational tests to recognize magnetic field structures. A region with a regular magnetic field generates a broad "disk" in Faraday space, with two "horns" when the distribution of cosmic-ray electrons is broader than that of the thermal electrons. Each field reversal generates one asymmetric "horn" on top of the "disk". A region with a turbulent field can be recognized as a "Faraday forest" of many components. These tests are applied to the spectral ranges of various synthesis radio telescopes. We argue that the ratio of maximum to minimum wavelengths determines the range of scales that can be identified in Faraday space. Conclusions. A reliable recognition of magnetic field structures in spiral galaxies or galaxy clusters requires the analysis of data cubes in position-position-Faraday depth space ("PPF cubes"), observed over a wide and continuous frequency range, allowing the recognition of a wide range of scales as well as high resolution in Faraday space. The planned Square Kilometre Array (SKA) will fulfill this condition and will be close to representing a perfect "Faraday telescope". The combination of data from the Low Frequency Array (LOFAR, at low frequencies) and the Expanded Very Large Array (EVLA, at high frequencies) appears to be a promising approach for the recognition of magnetic structures on all scales. The addition of data at intermediate frequencies from the Westerbork Synthesis Radio Telescope (WSRT) or the Giant Meterwave Radio Telescope (GMRT) would fill the gap between the LOFAR and EVLA frequency ranges. The Global Magneto-Ionic Medium Survey (GMIMS), planned with several single-dish telescopes at low angular resolution, will also provide good scale recognition and high resolution in Faraday space.

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The Westerbork SINGS survey

Cited by 217 publications

References 45 publications

Magnetic field structure and halo in NGC 4631

Magnetic field structure and halo in NGC 4631

Initial LOFAR observations of epoch of reionization windows

Recognizing magnetic structures by present and future radio telescopes with Faraday rotation measure synthesis

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