A 189 MHz, 2400 deg<sup>2</sup>POLARIZATION SURVEY WITH THE MURCHISON WIDEFIELD ARRAY 32-ELEMENT PROTOTYPE

Bernardi, G.; Greenhill, L. J.; Mitchell, D. A.; Ord, S. M.; Hazelton, B. J.; Gaensler, B. M.; Oliveira-Costa, A. de; Morales, M. F.; Shankar, N. Udaya; Subrahmanyan, R.; Wayth, R. B.; Lenc, E.; Williams, Christopher; Arcus, W.; Arora, B. S.; Barnes, David G.; Bowman, Judd D.; Briggs, F.; Bunton, John D.; Cappallo, R. J.; Corey, B. E.; Deshpande, A. A.; deSouza, L.; Emrich, D.; Goeke, R.; Herne, D.; Hewitt, Jacqueline N.; Johnston‐Hollitt, M.; Kaplan, D. L.; Kasper, J. C.; Kincaid, B. B.; Koenig, R.; Kratzenberg, E.; Lonsdale, Colin J.; Lynch, M. J.; McWhirter, S. R.; Morgan, E.; Oberoi, D.; Pathikulangara, J.; Prabu, T.; Remillard, Ron; Rogers, A. E. E.; Roshi, A.; Salah, J. E.; Sault, R. J.; Srivani, K. S.; Stevens, J.; Tingay, S. J.; Waterson, M.; Webster, R. L.; Whitney, A. R.; Williams, A.; Wyithe, J. Stuart B.

doi:10.1088/0004-637x/771/2/105

Cited by 102 publications

(154 citation statements)

References 92 publications

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“…The higher spatial and RM resolution of the LOFAR observations reveal canals and filaments (i.e. at φ = −2.5 rad m −2 ) more pronounced than previous observations of fields at the same frequency and at high Galactic latitude ( Bernardi et al 2010) and in a large fraction of the Southern sky (Bernardi et al 2013).…”

Section: Properties Of Galactic Polarized Emissioncontrasting

confidence: 55%

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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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Section: Properties Of Galactic Polarized Emissioncontrasting

confidence: 55%

“…Pen et al 2009) in the same frequency band. First results from MWA were ambiguous in the observed intensity and morphology (Bernardi et al 2013). The wide range of morphological features detected in ELAIS-N1 field at LOFAR frequencies are reminiscent of those observed in the Galactic polarized emission at 350 MHz (e.g.…”

Section: Discussionmentioning

confidence: 97%

Initial LOFAR observations of epoch of reionization windows

Jelić

Bruyn

Mevius

et al. 2014

A&A

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“…With a sensitivity in total intensity of 200 mJy/beam, the authors detected polarized emission only for one source among all of the sources brighter than 4 Jy, implying at these frequencies a fractional polarization below 2% for the remaining sources. The low number density derived by Bernardi et al (2013) may be due to beam depolarization. The number of polarized sources in this frequency band is not yet clear and different assumptions on the density of sources translate to different predictions on the capabilities to infer properties of extragalactic magnetic fields.…”

Section: Low-frequency Arraymentioning

confidence: 97%

“…A survey of the sky at similar frequencies (170-200 MHz) but with lower spatial resolution (15.6 ) was conducted with the Murchison Widefield Array over an area of 2400 deg 2 (Bernardi et al 2013). With a sensitivity in total intensity of 200 mJy/beam, the authors detected polarized emission only for one source among all of the sources brighter than 4 Jy, implying at these frequencies a fractional polarization below 2% for the remaining sources.…”

Section: Low-frequency Arraymentioning

confidence: 99%

Using rotation measure grids to detect cosmological magnetic fields: A Bayesian approach

Vacca

Oppermann

Enßlin

et al. 2016

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Determining magnetic field properties in different environments of the cosmic large-scale structure as well as their evolution over redshift is a fundamental step toward uncovering the origin of cosmic magnetic fields. Radio observations permit the study of extragalactic magnetic fields via measurements of the Faraday depth of extragalactic radio sources. Our aim is to investigate how much different extragalactic environments contribute to the Faraday depth variance of these sources. We develop a Bayesian algorithm to distinguish statistically Faraday depth variance contributions intrinsic to the source from those due to the medium between the source and the observer. In our algorithm the Galactic foreground and measurement noise are taken into account as the uncertainty correlations of the Galactic model. Additionally, our algorithm allows for the investigation of possible redshift evolution of the extragalactic contribution. This work presents the derivation of the algorithm and tests performed on mock observations. Because cosmic magnetism is one of the key science projects of the new generation of radio interferometers, we have predicted the performance of our algorithm on mock data collected with these instruments. According to our tests, high-quality catalogs of a few thousands of sources should already enable us to investigate magnetic fields in the cosmic structure.

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“…From previous observations at 350 MHz (Schnitzeler et al 2009), one would optimistically expect to detect one polarised source with a polarisation degree of a few percent for every four square degrees on the sky (Bernardi et al 2013). It is worth noting that for the observations of Schnitzeler et al (2009) at 350 MHz, the resolution was 2.7 × 4.7 arcmin with a noise of polarised intensity of 0.5 mJy/beam.…”

Section: Introductionmentioning

confidence: 95%

The nature of the low-frequency emission of M 51

Mulcahy

Horneffer²,

Beck³

et al. 2014

A&A

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Context. Low-frequency radio continuum observations (<300 MHz) can provide valuable information on the propagation of low-energy cosmic ray electrons (CRE). Nearby spiral galaxies have hardly been studied in this frequency range because of the technical challenges of low-frequency radio interferometry. This is now changing with the start of operations of LOFAR. Aims. We aim to study the propagation of low-energy CRE in the interarm regions and the extended disk of the nearly face-on spiral galaxy Messier 51. We also search for polarisation in M 51 and other extragalactic sources in the field. Methods. The grand-design spiral galaxy M 51 was observed with the LOFAR High Frequency Antennas (HBA) and imaged in total intensity and polarisation. This observation covered the frequencies between 115 MHz and 175 MHz with 244 subbands of 8 channels each, resulting in 1952 channels. This allowed us to use RM synthesis to search for polarisation. Results. We produced an image of total emission of M 51 at the mean frequency of 151 MHz with 20 resolution and 0.3 mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300 MHz so far. The integrated spectrum of total radio emission is described well by a power law, while flat spectral indices in the central region indicate thermal absorption. We observe that the disk extends out to 16 kpc and see a break in the radial profile near the optical radius of the disk. The radial scale lengths in the inner and outer disks are greater at 151 MHz, and the break is smoother at 151 MHz than those observed at 1.4 GHz. The arm-interarm contrast is lower at 151 MHz than at 1400 MHz, indicating propagation of CRE from spiral arms into interarm regions. The correlations between the images of radio emission at 151 MHz and 1400 MHz and the FIR emission at 70 μm reveal breaks on scales of 1.4 and 0.7 kpc, respectively. The total (equipartition) magnetic field strength decreases from about 28 μG in the central region to about 10 μG at 10 kpc radius. No significant polarisation was detected from M 51, owing to severe Faraday depolarisation. Six extragalactic sources are detected in polarisation in the M 51 field of 4.1 • × 4.1 • size. Two sources show complex structures in Faraday space. Conclusions. Our main results, the scale lengths of the inner and outer disks at 151 MHz and 1.4 GHz, arm-interarm contrast, and the break scales of the radio-FIR correlations, can be explained consistently by CRE diffusion, leading to a longer propagation length of CRE of lower energy. The distribution of CRE sources drops sharply at about 10 kpc radius, where the star formation rate also decreases sharply. We find evidence that thermal absorption is primarily caused by H ii regions. The non-detection of polarisation from M 51 at 151 MHz is consistent with the estimates of Faraday depolarisation. Future searches for polarised emission in this frequency range should concentrate on regions with low star formation rates.Key words. polarization -cosmic rays -galaxies: ISM -galaxies: ...

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A 189 MHz, 2400 deg²POLARIZATION SURVEY WITH THE MURCHISON WIDEFIELD ARRAY 32-ELEMENT PROTOTYPE

Cited by 102 publications

References 92 publications

Initial LOFAR observations of epoch of reionization windows

Initial LOFAR observations of epoch of reionization windows

Using rotation measure grids to detect cosmological magnetic fields: A Bayesian approach

The nature of the low-frequency emission of M 51

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A 189 MHz, 2400 deg2POLARIZATION SURVEY WITH THE MURCHISON WIDEFIELD ARRAY 32-ELEMENT PROTOTYPE

Cited by 102 publications

References 92 publications

Initial LOFAR observations of epoch of reionization windows

Initial LOFAR observations of epoch of reionization windows

Using rotation measure grids to detect cosmological magnetic fields: A Bayesian approach

The nature of the low-frequency emission of M 51

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A 189 MHz, 2400 deg²POLARIZATION SURVEY WITH THE MURCHISON WIDEFIELD ARRAY 32-ELEMENT PROTOTYPE