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Initial LOFAR observations of Epoch of Reionization windows: II. Diffuse polarized emission in the ELAIS-N1 field
Abstract and Figures
This study aims to characterise the polarized foreground emission in the
ELAIS-N1 field and to address its possible implications for the extraction of
the cosmological 21-cm signal from the Low-Frequency Array - Epoch of
Reionization (LOFAR-EoR) data. We use the high band antennas of LOFAR to image
this region and RM-synthesis to unravel structures of polarized emission at
high Galactic latitudes. The brightness temperature of the detected Galactic
emission is on average 4 K in polarized intensity and covers the range from -10
to +13rad 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. The wide frequency range, good angular resolution and good
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
polarised 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 polarised 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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... Instances such as the observations by the Australia Telescope Compact Array (ATCA) at 1.4 GHz (Gaensler et al. 2011), which lacked single-dish measurements, and the Westerbork Synthesis Radio Telescope (WSRT) observations of the 3C 196 field at 350 MHz (Jelić et al. 2015), where single-dish measurements at the same frequency were unfeasible, underscore this issue. Additionally, data from the Low-Frequency Array (LOFAR) also experience the loss of low-spatial frequencies (Jelić et al. 2014). ...
... The Galactic foregrounds comprising of diffused synchrotron radiation and free-free emission from the Milky Way dominate at angular scales greater than a degree (Bernardi et al. 2009;Lian et al. 2020) and the extragalactic foregrounds of radio emission from Active Galactic Nuclei and star-forming galaxies dominate at smaller angular scales (Di Matteo et al. 2002. Besides, these foregrounds also exhibit polarization properties contaminating the cosmological signal, thus making the extraction and analysis challenging (Jelić et al. 2010;Moore et al. 2013;Jelić et al. 2014;Spinelli et al. 2018;Byrne et al. 2022). For a successful detection of the cosmological signal, these foregrounds must be removed from the EoR datasets. ...
Exploration of the 21cm signal during the Cosmic Dawn and the Epoch of Reionisation (EoR) can unravel the mysteries of the early Universe when the first stars and galaxies were born and ionised, respectively. However, the 21cm signal is exceptionally weak, and thus, the detection amidst the bright foregrounds is extremely challenging. The MurchisonWidefield Array (MWA) aims to measure the brightness temperature fluctuations of neutral hydrogen from the early Universe. The MWA telescope observes the radio sky with a large Field of View (FoV) that causes the bright galaxies, especially near the horizon, to contaminate the measurements. These foregrounds contaminating the EoR datasets must be meticulously removed or treated to detect the signal successfully. The Central Redundant Array Mega-tile (CRAM) is a zenith-pointing new instrument, installed at the centre of the MWA Phase II southern hexagonal configuration, comprising of 64 dipoles in an 8 × 8 configuration with a FoV half the width of the MWA’s at every frequency under consideration. The primary objective of this new instrument is to mitigate the impact of bright radio sources near the field centre in accordance with the reduced primary beamshape and to reduce the contamination of foreground sources near the horizon with the reduced sidelobe response of the larger array configuration. In this paper, we introduce the new instrument to the community and present the system architecture and characteristics of the instrument. Using the first light observations, we determine the CRAM system temperature and system performance.
... These temporal variations encapsulate diurnal, seasonal and Solar cycle periodicities, and can be as high as 4 rad/m 2 in amplitude (Oberoi and Lonsdale 2012). The uncompensated ionosphere is a common source of errors in radioastronomy at lower frequencies (Jelić et al. 2014;Mevius et al. 2016;Beser et al. 2022). In the context of pulsar observations, temporal changes in the RM due to the terrestrial plasma have been seen evidently in multiple studies (see, e.g. ...
Broad band pulsar radiation can be effectively used to monitor the properties of the magneto-ionic media through which it propagates. Faraday rotation calculated from polarised pulsar observations provides an integrated product of electron densities and the line-of-sight component of the magnetic field in the intervening plasma. In particular, a time-variable effect mainly associated with the rapidly changing column density of the Earth’s ionosphere and plasmasphere heavily dominates the observed Faraday rotation of pulsar radiation. In this work, we aim to carry out a performance test of three GNSS-based models of the ionosphere using observations of PSR J0332+5434 taken with the LOw Frequency ARray (LOFAR). As it was shown in Porayko et al. (Month Not Roy Astron Soc 483(3):4100–4113, 2019. https://doi.org/10.1093/mnras/sty3324. arXiv:1812.01463), the conventional single layer model (SLM), which assumes that the ionosphere is a thin slab at a fixed effective height, is not capable of fully accounting for the ionospheric Faraday rotation in pulsar data. The simplified physics of the SLM is upgraded within IRI-Plas (International Reference Ionosphere and Plasmasphere) extended SLM and the dual-layer voxel TOmographic Model of the Ionosphere (TOMION), both of which partially account for the thickness and vertical dynamics of the terrestrial plasma. Although the last two improve the reconstruction of the ionospheric Faraday rotation, none of the considered models completely purge the observed residual variations. With this study, we show that the long term LOFAR observations of Faraday rotation of pulsars provide an excellent tool to test and improve models of the magneto-ionic content of the Earth’s atmosphere.
... Some of the most exciting reionization-era signals will be measured using surveys that span large swaths of the sky. This includes IM efforts observing 21 cm radiation such as MWA, LOFAR, HERA, and Square Kilometre Array (SKA; Jelić et al. 2014;Koopmans et al. 2015;DeBoer et al. 2017; Barry et al. 2022). It also includes IM with lines such Lyα, Hα, CO, and [CII] with instruments like SPHEREx , CDIM (Cooray et al. 2019), FYST (Karoumpis et al. 2022), COMAP (Cleary et al. 2022), TIME (Sun et al. 2021), and CONCERTO (CONCERTO Collaboration et al. 2020). ...
Line-intensity mapping (IM) experiments seek to perform statistical measurements of large-scale structure with spectral lines such as 21 cm, CO, and Ly α . A challenge in these observations is to ensure that astrophysical foregrounds, such as galactic synchrotron emission in 21 cm measurements, are properly removed. One method that has the potential to reduce foreground contamination is to cross correlate with a galaxy survey that overlaps with the IM volume. However, telescopes sensitive to high-redshift galaxies typically have small field of views compared to IM surveys. Thus, a galaxy survey for cross correlation would necessarily consist of pencil beams that sparsely fill the IM volume. In this paper, we develop the formalism to forecast the sensitivity of cross correlations between IM experiments and pencil-beam galaxy surveys. We find that a random distribution of pencil beams leads to very similar overall sensitivity as a lattice spaced across the IM survey and derive a simple formula for random configurations that agrees with the Fisher matrix formalism. We explore examples of combining high-redshift James Webb Space Telescope (JWST) observations with both an SPHEREx-like Ly α IM survey and a 21 cm experiment based on the Hydrogen Epoch of Reionization Array (HERA). We find that the JWST-SPHEREx case is promising, leading to a total signal-to-noise ratio of ∼5 after 100 total hours of JWST (at z = 7). We find that HERA is not well-suited for this approach owing to its drift-scan strategy, but that a similar experiment that can integrate down on one field could be.
... ELAIS-N1 has an ideal declination (+55 deg) for LOFAR observations, and is also a target field for LOFAR's Epoch of Reionisation studies (Jelić et al. 2014), providing a combined motivation for the observations. ELAIS-N1 benefits from some of the deepest widefield optical, near-IR and mid-IR imaging. ...
Source classifications, stellar masses and star formation rates are presented for 80,000 radio sources from the first data release of the Low Frequency Array Two-metre Sky Survey (LoTSS) Deep Fields, which represents the widest deep radio survey ever undertaken. Using deep multi-wavelength data spanning from the ultraviolet to the far-infrared, spectral energy distribution (SED) fitting is carried out for all of the LoTSS-Deep host galaxies using four different SED codes, two of which include modelling of the contributions from an active galactic nucleus (AGN). Comparing the results of the four codes, galaxies that host a radiative AGN are identified, and an optimised consensus estimate of the stellar mass and star-formation rate for each galaxy is derived. Those galaxies with an excess of radio emission over that expected from star formation are then identified, and the LoTSS-Deep sources are divided into four classes: star-forming galaxies, radio-quiet AGN, and radio-loud high-excitation and low-excitation AGN. Ninety-five per cent of the sources can be reliably classified, of which more than two-thirds are star-forming galaxies, ranging from normal galaxies in the nearby Universe to highly-starbursting systems at z>4. Star-forming galaxies become the dominant population below 150-MHz flux densities of about 1 mJy, accounting for 90 per cent of sources at a 150-MHz flux density of 100 microJy. Radio-quiet AGN comprise around 10 per cent of the overall population. Results are compared against the predictions of the SKADS and T-RECS radio sky simulations, and improvements to the simulations are suggested.
... ELAIS-N1 has an ideal declination (+55 deg) for LOFAR observations, and is also a target field for LOFAR's Epoch of Reionisation studies (Jelić et al. 2014), providing a combined motivation for the observations. ELAIS-N1 benefits from some of the deepest widefield optical, near-IR and mid-IR imaging. ...
Source classifications, stellar masses and star formation rates are presented for ≈80,000 radio sources from the first data release of the Low Frequency Array Two-metre Sky Survey (LoTSS) Deep Fields, which represents the widest deep radio survey ever undertaken. Using deep multi-wavelength data spanning from the ultraviolet to the far-infrared, spectral energy distribution (SED) fitting is carried out for all of the LoTSS-Deep host galaxies using four different SED codes, two of which include modelling of the contributions from an active galactic nucleus (AGN). Comparing the results of the four codes, galaxies that host a radiative AGN are identified, and an optimised consensus estimate of the stellar mass and star-formation rate for each galaxy is derived. Those galaxies with an excess of radio emission over that expected from star formation are then identified, and the LoTSS-Deep sources are divided into four classes: star-forming galaxies, radio-quiet AGN, and radio-loud high-excitation and low-excitation AGN. Ninety-five per cent of the sources can be reliably classified, more than two-thirds of which are star-forming galaxies, ranging from normal galaxies in the nearby Universe to highly-starbursting systems at z > 4. Star-forming galaxies become the dominant population below 150-MHz flux densities of ≈1 mJy, accounting for 90 per cent of sources at S150MHz ∼ 100μJy. Radio-quiet AGN comprise ≈10 per cent of the overall population. Results are compared against the predictions of the SKADS and T-RECS radio sky simulations, and improvements to the simulations are suggested.
... E-mail: damien.korber@protonmail.ch Modern telescopes such as LOFAR 1 are able to measure the global statistics of the 21-cm signal (Paciga et al. 2013 ;Yatawatta et al. 2013 ;Jeli ć et al. 2014 ;Parsons et al. 2014 ;Jacobs et al. 2015 ;Patil et al. 2017 ;Mertens et al. 2020 ;Trott et al. 2020 ). Ho we ver, during reionization the signal fluctuations are expected to be non-Gaussian (Mondal, Bharadwaj & Majumdar 2017 ;Shimabukuro et al. 2017 ;Majumdar et al. 2018 ;Gorce & Pritchard 2019 ;Hutter et al. 2019 ;Watkinson, Greig & Mesinger 2022 ), and therefore cannot be fully depicted by the power spectra only. ...
With the advent of the Square Kilometre Array Observatory (SKAO), scientists will be able to directly observe the Epoch of Reionization by mapping the distribution of neutral hydrogen at different redshifts. While physically motivated results can be simulated with radiative transfer codes, these simulations are computationally expensive and can not readily produce the required scale and resolution simultaneously. Here we introduce the Physics-Informed neural Network for reIONization (PINION), which can accurately and swiftly predict the complete 4-D hydrogen fraction evolution from the smoothed gas and mass density fields from pre-computed N-body simulation. We trained PINION on the C2-Ray simulation outputs and a physics constraint on the reionization chemistry equation is enforced. With only five redshift snapshots, PINION can accurately predict the entire reionization history between z = 6 and 12. We evaluate the accuracy of our predictions by analysing the dimensionless power spectra and morphology statistics estimations against C2-Ray results. We show that while the network’s predictions are in very good agreement with simulation to redshift z > 7, the network’s accuracy suffers for z < 7. We motivate how PINION performance could be improved using additional inputs and potentially generalized to large-scale simulations.
... However, making threedimensional (3D) 21 cm maps requires high sensitivity and spatial resolution, so it is technically extremely difficult. Instead, ongoing large radio interferometer array experiments, e.g., the Giant Metrewave Radio Telescope (GMRT) 4 , the LOw Frequency Array (LOFAR) 5 , the Murchison Widefield Array (MWA) 6 and the Precision Array for Probing the Epoch of Reionization (PAPER) 7 , have first attempted to detect the 21 cm power spectrum from the EOR, a two-point statistic of 21 cm brightness temperature fluctuations (e.g., Furlanetto et al. 2006;Pritchard & Loeb 2012), and have put upper limits on the 21 cm power spectrum (Paciga et al. 2013;Yatawatta et al. 2013;Tingay et al. 2013;Patil et al. 2014Patil et al. , 2017Parsons et al. 2014;Jelić et al. 2014;Ali et al. 2015;Dillon et al. 2015;Jacobs et al. 2015;Pober et al. 2015;Mertens et al. 2020). Furthermore, upcoming experiments such as the Square Kilometre Array (SKA) 8 (Mellema et al. 2013;Koopmans et al. 2015) and the Hydrogen Epoch of Reionization Array (HERA) 9 (DeBoer et al. 2017) promise to measure the 21 cm power spectrum from the EOR for the first time and with high sensitivity (The HERA Collaboration et al. 2021). ...
The bubble size distribution of ionized hydrogen regions probes information about the morphology of H II bubbles during reionization. Conventionally, the H II bubble size distribution can be derived from the tomographic imaging data of the redshifted 21 cm signal from the epoch of reionization, which, however, is observationally challenging even for upcoming large radio interferometer arrays. Given that these interferometers promise to measure the 21 cm power spectrum accurately, we propose a new method, which is based on artificial neural networks, to reconstruct the H II bubble size distribution from the 21 cm power spectrum. We demonstrate that reconstruction from the 21 cm power spectrum can be almost as accurate as being directly measured from the imaging data with fractional error ≲10%, even with thermal noise at the sensitivity level of the Square Kilometre Array. Nevertheless, the reconstruction implicitly exploits the modeling in reionization simulations, and hence the recovered H II bubble size distribution is not an independent summary statistic from the power spectrum, and should be used only as an indicator for understanding H II bubble morphology and its evolution.
We present the first deep polarimetric study of Galactic synchrotron emission at low radio frequencies. Our study is based on 21 observations of the European Large Area Infrared Space Observatory Survey-North 1 (ELAIS-N1) field using the Low-Frequency Array (LOFAR) at frequencies from 114.9 to 177.4 MHz. These data are a part of the LOFAR Two-metre Sky Survey Deep Fields Data Release 1. We used very low-resolution (4.3′) Stokes QU data cubes of this release. We applied rotation measure (RM) synthesis to decompose the distribution of polarised structures in Faraday depth, and cross-correlation RM synthesis to align different observations in Faraday depth. We stacked images of about 150 h of the ELAIS-N1 observations to produce the deepest Faraday cube at low radio frequencies to date, tailored to studies of Galactic synchrotron emission and the intervening magneto-ionic interstellar medium. This Faraday cube covers ~36 deg ² of the sky and has a noise of 27 µJy PSF ⁻¹ RMSF ⁻¹ in polarised intensity. This is an improvement in noise by a factor of approximately the square root of the number of stacked data cubes (~√20), as expected, compared to the one in a single data cube based on five-to-eight-hour observations. We detect a faint component of diffuse polarised emission in the stacked cube, which was not detected previously. Additionally, we verify the reliability of the ionospheric Faraday rotation corrections estimated from the satellite-based total electron content measurements to be of ~0.05 гad m ⁻² . We also demonstrate that diffuse polarised emission itself can be used to account for the relative ionospheric Faraday rotation corrections with respect to a reference observation.
Aims. The bubble size distribution is a summary statistics that can be computed from the observed 21-cm signal from the Epoch of Reionization. As it depends only on the ionization field and is not limited to Gaussian information, it is an interesting probe that is complementary to the power spectrum of the full 21-cm signal. Devising a flexible and reliable theoretical model for the bubble size distribution paves the way for its use in astrophysical parameter inference.
Methods. The proposed model was built from the excursion set theory and a functional relation between the bubble volume and the collapsed mass in the bubble. Unlike previous models, it can accommodate any functional relation or distribution. The use of parameterized relations allows us to test the predictive power of the model by performing a minimization best-fit to the bubble size distribution obtained from a high-resolution, fully coupled radiative hydrodynamics simulation known as HIRRAH-21.
Results. Our model is able to provide a better fit to the numerical bubble size distribution at an ionization fraction of x H II ∼ 1% and 3%, as compared to other existing models. Moreover, we compare the relation between the bubble volume and the collapsed mass corresponding to the best-fit parameters, which is not an observable, to the numerical simulation data. A good match is obtained, confirming the possibility of inferring this relation from an observed bubble size distribution using our model. Finally, we present a simple algorithm that empirically implements the process of percolation. We show that it extends the usability of our bubble size distribution model up to x H II ∼ 30%.
Radio Interferometry is an essential method for astronomical observations. Self-calibration techniques have increased the quality of the radio astronomical observations (and hence the science) by orders of magnitude. Recently, there is a drive towards sensor arrays built using inexpensive hardware and distributed over a wide area acting as radio interferometers. Calibration of such arrays poses new problems in terms of computational cost as well as in performance of existing calibration algorithms. We consider the application of the Space Alternating Generalized Expectation Maximization (SAGE) \cite{Fess94} algorithm for calibration of radio interferometric arrays. Application to real data shows that this is an improvement over existing calibration algorithms that are based on direct, deterministic non linear optimization. As presented in this paper, we can improve the computational cost as well as the quality of the calibration using this algorithm.
DustEM computes the extinction and the emission of interstellar dust
grains heated by photons. It is written in Fortran 95 and is jointly
developed by IAS and CESR. The dust emission is calculated in the
optically thin limit (no radiative transfer) and the default spectral
range is 40 to 108 nm. The code is designed so dust properties can
easily be changed and mixed and to allow for the inclusion of new grain
physics.
The accurate and precise removal of 21-cm foregrounds from Epoch of
Reionization (EoR) redshifted 21-cm emission data is essential if we are
to gain insight into an unexplored cosmological era. We apply a
non-parametric technique, Generalized Morphological Component Analysis
(gmca), to simulated Low Frequency Array (LOFAR)-EoR data and show that
it has the ability to clean the foregrounds with high accuracy. We
recover the 21-cm 1D, 2D and 3D power spectra with high accuracy across
an impressive range of frequencies and scales. We show that gmca
preserves the 21-cm phase information, especially when the smallest
spatial scale data is discarded. While it has been shown that LOFAR-EoR
image recovery is theoretically possible using image smoothing, we add
that wavelet decomposition is an efficient way of recovering 21-cm
signal maps to the same or greater order of accuracy with more
flexibility. By comparing the gmca output residual maps (equal to the
noise, 21-cm signal and any foreground fitting errors) with the 21-cm
maps at one frequency and discarding the smaller wavelet scale
information, we find a correlation coefficient of 0.689, compared to
0.588 for the equivalently smoothed image. Considering only the pixels
in a central patch covering 50 per cent of the total map area, these
coefficients improve to 0.905 and 0.605, respectively, and we conclude
that wavelet decomposition is a significantly more powerful method to
denoise reconstructed 21-cm maps than smoothing.
The characteristic outer scale of turbulence and the ratio of the random to
ordered components of the magnetic field are key parameters to characterise
magnetic turbulence in the interstellar gas, which affects the propagation of
cosmic rays within the Galaxy. We provide new constraints to those two
parameters. We use the LOw Frequency ARray (LOFAR) to image the diffuse
continuum emission in the Fan region at (l,b) (137.0,+7.0) at 80"x70"
resolution in the range [146,174] MHz. We detect multi-scale fluctuations in
the Galactic synchrotron emission and compute their power spectrum. Applying
theoretical estimates and derivations from the literature for the first time,
we derive the outer scale of turbulence and the ratio of random to ordered
magnetic field from the characteristics of these fluctuations . We obtain the
deepest image of the Fan region to date and find diffuse continuum emission
within the primary beam. The power spectrum of the foreground synchrotron
fluctuations displays a power law behaviour for scales between 100 and 8 arcmin
with a slope of (-1.84+/-0.19). We find an upper limit of about 20 pc for the
outer scale of the magnetic interstellar turbulence toward the Fan region. We
also find a variation of the ratio of random to ordered field as a function of
Galactic coordinates, supporting different turbulent regimes. We use power
spectra fluctuations from LOFAR as well as earlier GMRT and WSRT observations
to constrain the outer scale of turbulence of the Galactic synchrotron
foreground, finding a range of plausible values of 10-20 pc. Then, we use this
information to deduce lower limits of the ratio of ordered to random magnetic
field strength. These are found to be 0.3, 0.3, and 0.5 for the LOFAR, WSRT and
GMRT fields considered respectively. Both these constraints are in agreement
with previous estimates.
LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer
constructed in the north of the Netherlands and across europe. Utilizing a
novel phased-array design, LOFAR covers the largely unexplored low-frequency
range from 10-240 MHz and provides a number of unique observing capabilities.
Spreading out from a core located near the village of Exloo in the northeast of
the Netherlands, a total of 40 LOFAR stations are nearing completion. A further
five stations have been deployed throughout Germany, and one station has been
built in each of France, Sweden, and the UK. Digital beam-forming techniques
make the LOFAR system agile and allow for rapid repointing of the telescope as
well as the potential for multiple simultaneous observations. With its dense
core array and long interferometric baselines, LOFAR achieves unparalleled
sensitivity and angular resolution in the low-frequency radio regime. The LOFAR
facilities are jointly operated by the International LOFAR Telescope (ILT)
foundation, as an observatory open to the global astronomical community. LOFAR
is one of the first radio observatories to feature automated processing
pipelines to deliver fully calibrated science products to its user community.
LOFAR's new capabilities, techniques and modus operandi make it an important
pathfinder for the Square Kilometre Array (SKA). We give an overview of the
LOFAR instrument, its major hardware and software components, and the core
science objectives that have driven its design. In addition, we present a
selection of new results from the commissioning phase of this new radio
observatory.
A technique is described that is used to improve the detection of radio-frequency interference in astronomical radio observatories. It is applied on a two-dimensional interference mask after regular detection in the time-frequency domain with existing techniques. The scale-invariant rank (SIR) operator is defined, which is a one-dimensional mathematical morphology technique that can be used to find adjacent intervals in the time or frequency domain that are likely to be affected by RFI. The technique might also be applicable in other areas in which morphological scale-invariant behaviour is desired, such as source detection. A new algorithm is described, that is shown to perform quite well, has linear time complexity and is fast enough to be applied in modern high resolution observatories. It is used in the default pipeline of the LOFAR observatory.
Bivariate Gaussian Probability DistributionPeriodic SamplingSampling with QuantizationAccuracy in Digital SamplingDigital Delay CircuitsQuadrature Phase Shift of a Digital SignalDigital Correlators
Aims: We investigate the properties of the Galactic ISM by applying Faraday tomography to a radio polarization data set in the direction of the Galactic anti-centre. Methods: We address the problem of missing large-scale structure in our data, and show that this does not play an important role for the results we present. Results: The main peak of the Faraday depth spectra in our data set is not measurably resolved for about 8% of the lines of sight. An unresolved peak indicates a separation between the regions with Faraday rotation and synchrotron emission. However, cosmic rays pervade the ISM, and synchrotron emission would therefore also be produced where there is Faraday rotation. We suggest that the orientation of the magnetic field can separate the two effects. By modelling the thermal electron contribution to the Faraday depth, we map the strength of the magnetic field component along the line of sight. Polarized point sources in our data set have rotation measures that are comparable to the Faraday depths of the diffuse emission in our data. Our Faraday depth maps show narrow canals of low polarized intensity. We conclude that depolarization over the telescope beam produces at least some of these canals. Finally, we investigate the properties of one conspicuous region in this data set and argue that it is created by a decrease in line-of-sight depolarization compared to its surroundings.
We present a Stokes I, Q and U survey at 189 MHz with the Murchison Widefield
Array 32-element prototype covering 2400 square degrees. The survey has a 15.6
arcmin angular resolution and achieves a noise level of 15 mJy/beam. We
demonstrate a novel interferometric data analysis that involves calibration of
drift scan data, integration through the co-addition of warped snapshot images
and deconvolution of the point spread function through forward modeling. We
present a point source catalogue down to a flux limit of 4 Jy. We detect
polarization from only one of the sources, PMN J0351-2744, at a level of 1.8
\pm 0.4%, whereas the remaining sources have a polarization fraction below 2%.
Compared to a reported average value of 7% at 1.4 GHz, the polarization
fraction of compact sources significantly decreases at low frequencies. We find
a wealth of diffuse polarized emission across a large area of the survey with a
maximum peak of ~13 K, primarily with positive rotation measure values smaller
than +10 rad/m^2. The small values observed indicate that the emission is
likely to have a local origin (closer than a few hundred parsecs). There is a
large sky area at 2^h30^m where the diffuse polarized emission rms is fainter
than 1 K. Within this area of low Galactic polarization we characterize the
foreground properties in a cold sky patch at $(\alpha,\delta) =
(4^h,-27^\circ.6)$ in terms of three dimensional power spectra