Faraday tomography of LoTSS-DR2 data: I. Faraday moments in the high-latitude outer Galaxy and revealing Loop III in polarisation
Ana Erceg, Vibor Jelić, Marijke Haverkorn, Andrea Bracco, Timothy W. Shimwell, Cyril Tasse, John M. Dickey, Lana Ceraj, Alexander Drabent, Martin J. Hardcastle, Luka Turić
备注：
20 pages, 25 figures, accepted for publication in A&A
Observations of synchrotron emission at low radio frequencies reveal a labyrinth of polarised Galactic structures. However, the explanation for the wealth of structures remains uncertain due to the complex interactions between the interstellar medium and the magnetic field. A multi-tracer approach to the analysis of large sky areas is needed. This paper aims to use polarimetric images from the LOFAR Two metre Sky Survey (LoTSS) to produce the biggest mosaic of polarised emission in the northern sky at low radio frequencies (150 MHz) to date. The large area this mosaic covers allows for detailed morphological and statistical studies of polarised structures in the high-latitude outer Galaxy, including the well-known Loop III region. We produced a 3100 square degree Faraday tomographic cube using a rotation measure synthesis tool. We calculated the statistical moments of Faraday spectra and compared them with data sets at higher frequencies (1.4 GHz) and with a map of a rotation measure derived from extragalactic sources. The mosaic is dominated by polarised emission connected to Loop III. Additionally, the mosaic reveals an abundance of other morphological structures, mainly {narrow and extended} depolarisation canals, which are found to be ubiquitous. We find a correlation between the map of an extragalactic rotation measure and the LoTSS first Faraday moment image. The ratio of the two deviates from a simple model of a Burn slab (Burn 1966) along the line of sight, which highlights the high level of complexity in the magnetoionic medium that can be studied at these frequencies.
Faraday depth similarities across scales with LoTSS & DRAGONS
University of Western Ontario（韦仕兰大学）；University of British Columbia Okanagan（不列颠哥伦比亚大学奥肯那根分校）；Dominion Radio Astrophysical Observatory, Herzberg Research Centre for Astronomy and Astrophysics, National Research Council Canada（加拿大国家研究委员会赫茨伯格天文学与天体物理学研究中心多明尼奥射电天文台）；University of Calgary（卡尔加里大学）；Ruđer Bošković Institute（鲁德·博什科维奇研究所）；Radboud University（拉德堡德大学）
Faraday tomography allows us to map diffuse polarized synchrotron emission from our Galaxy and use it to interpret the magnetic field in the interstellar medium (ISM). We have applied Faraday tomography to 60 observations from the LOFAR Two-meter Sky Survey (LOTSS) and produced a Faraday depth cube mosaic covering 568 square degrees at high Galactic latitudes, at 4.3' angular resolution and 1 rad m$^{-2}$ Faraday depth resolution, with a typical noise level of 50--100 $μ$Jy per point spread function (PSF) per rotation measure spread function (RMSF) (40-80 mK RMSF$^{-1}$). While parts of the im…
We present full-polarisation, broadband observations of the radio galaxy NGC 612 (PKS B0131-637) from 1.3 to 3.1 GHz using the Australia Telescope Compact Array. The relatively large angular scale of the radio galaxy makes it a good candidate with which to investigate the polarisation mechanisms responsible for the observed Faraday depth structure. By fitting complex polarisation models to the polarised spectrum of each pixel, we find that a single polarisation component can adequately describe the observed signal for the majority of the radio galaxy. While we cannot definitively rule out inte…
We present observations of linear polarisation in the southern radio lobe of Centaurus A, conducted during commissioning of the Australian Square Kilometre Array Pathfinder (ASKAP) telescope. We used 16 antennas to observe a 30 square degree region in a single 12 hour pointing over a 240 MHz band centred on 913 MHz. Our observations achieve an angular resolution of $26\times33$ arcseconds (480 parsecs), a maximum recoverable angular scale of 30 arcminutes, and a full-band sensitivity of 85 $\muup$Jy beam$^{-1}$. The resulting maps of polarisation and Faraday rotation are amongst the most detai…
A Faraday rotation measure (RM) catalogue, or RM Grid, is a valuable resource for the study of cosmic magnetism. Using the second data release (DR2) from the LOFAR Two-metre Sky Survey (LoTSS), we have produced a catalogue of 2461 extragalactic high-precision RM values across 5720 deg$^{2}$ of sky (corresponding to a polarized source areal number density of $\sim$0.43 deg$^{-2}$). The linear polarization and RM properties were derived using RM synthesis from the Stokes $Q$ and $U$ channel images at an angular resolution of 20'' across a frequency range of 120 to 168 MHz with a channel bandwidt…
We discuss the existence of elliptical polarisation in rotational spectral lines of CO and other molecules within the context of the Anisotropic Resonant Scattering (ARS) model. We show that the effect of ARS on the radiation field can lead to not only the previously predicted transformation of background linear polarisation into circular polarisation (i.e., Faraday conversion), but also the occurrence of Faraday rotation and the generation of elliptically polarised signals in an otherwise initially unpolarised radiation field. This is due to a collective behaviour between the large number of …
The polarization properties of radio sources at very low frequencies (<200 MHz) have not been widely measured, but the new generation of low-frequency radio telescopes, including the Low Frequency Array (LOFAR: a Square Kilometre Array Low pathfinder), now gives us the opportunity to investigate these properties. In this paper, we report on the preliminary development of a data reduction pipeline to carry out polarization processing and Faraday tomography for data from the LOFAR Two-meter Sky Survey (LOTSS) and present the results of this pipeline from the LOTSS preliminary data release region…
We present the detection of 68 sources from the most sensitive radio survey in circular polarisation conducted to date. We use the second data release of the 144 MHz LOFAR Two-metre Sky Survey to produce circularly-polarised maps with median 140 $μ$Jy beam$^{-1}$ noise and resolution of 20$''$ for $\approx$27% of the northern sky (5634 deg$^{2}$). The leakage of total intensity into circular polarisation is measured to be $\approx$0.06%, and our survey is complete at flux densities $\geq1$ mJy. A detection is considered reliable when the circularly-polarised fraction exceeds 1%. We find the po…
The technique of Faraday tomography is a key tool for the study of magnetised plasmas in the new era of broadband radio polarisation observations. In particular, observations at metre-wavelengths provide significantly better Faraday depth accuracies compared to traditional cm-wavelength observations. However, the effect of Faraday depolarisation makes the polarised signal very challenging to detect at metre wavelengths (MHz frequencies). In this work, Faraday tomography is used to characterise the Faraday rotation properties of polarised sources found in data from the LOFAR Two-Metre Sky Surve…
We present an updated version of POLARIS, a well established code designated for dust polarisation and line radiative transfer (RT) in arbitrary astrophysical environments. We extend the already available capabilities with a synchrotron feature for polarised emission. Here, we combine state-of-the-art solutions of the synchrotron RT coefficients with numerical methods for solving the complete system of equations of the RT problem, including Faraday rotation (FR) as well as Faraday conversion (FC). We validate the code against Galactic and extragalactic observations by performing a statistical …
We have developed a C++ implementation of the Maximum Entropy Method (MEM) suitable for deconvolving VLBI polarisation data. The first results of this implementation are presented and compared with CLEAN-based deconvolutions of the same data. We present Faraday rotation measure and intrinsic polarisation maps of AGN which have been made from MEM deconvolutions of multi-wavelength observations of Stokes parameters I, Q and U. The advantages of using MEM are demonstrated, in particular its enhanced resolution over the CLEAN algorithm.
Observing the magnetic fields of low-mass interacting galaxies tells us how they have evolved over cosmic time and their importance in galaxy evolution. We have measured the Faraday rotation of 80 extra-galactic radio sources behind the Small Magellanic Cloud (SMC) using the CSIRO Australia Telescope Compact Array (ATCA) with a frequency range of 1.4 -- 3.0 GHz. Both the sensitivity of our observations and the source density are an order of magnitude improvement on previous Faraday rotation measurements of this galaxy. The SMC generally produces negative rotation measures (RMs) after accountin…
We identify a counterpart to a Galactic supershell in diffuse radio polarisation, and use this to determine the magnetic fields associated with this object. GSH 006$-$15$+$7 has perturbed the polarised emission at 2.3$\,$GHz, as observed in the S-band Polarisation All Sky Survey (S-PASS), acting as a Faraday screen. We model the Faraday rotation over the shell, and produce a map of Faraday depth over the area across it. Such models require information about the polarised emission behind the screen, which we obtain from the Wilkinson Microwave Anisotropy Probe (WMAP), scaled from 23$\,$GHz to 2…
(Abridged) We conducted new 6cm and 11cm observations of CTA 1 using the Urumqi 25-m and Effelsberg 100-m telescopes. Data at other wavelengths were included to investigate the spectrum and polarisation properties. We obtained new total intensity and polarisation maps at 6cm and 11cm with angular resolutions of 9.5 arcmin and 4.4 arcmin, respectively. We derived a spectral index of alpha=-0.63+/-0.05 based on the integrated flux densities at 408 MHz, 1420 MHz, 2639 MHz, and 4800 MHz. The spectral index map calculated from data at the four frequencies shows a clear steepening of the spectrum fr…
The Galactic magnetic field is an integral constituent of the interstellar medium (ISM), and knowledge of its structure is crucial to understanding Galactic dynamics. The Rotation Measures (RM) of extragalactic (EG) sources have been the basis of comprehensive Galactic magnetic field models. Polarised extended emission (XE) is also seen along lines of sight through the Galactic disk, and also displays the effects of Faraday rotation. Our aim is to investigate and understand the relationship between EG and XE RMs near the Galactic plane, and to determine how the XE RMs, a hitherto unused resour…
Magnetic fields are essential to fully understand the interstellar medium (ISM) and its role in the disk-halo interface of galaxies is still poorly understood. Star formation is known to expel hot gas vertically into the halo and these outflows have important consequences for mean-field dynamo theory in that they can be efficient in removing magnetic helicity. We perform new observations of the nearby face-on spiral galaxy NGC 628 with the Karl G. Jansky Very Large Array (JVLA) at S-band and the Effelsberg 100-m telescope at frequencies of 2.6 GHz and 8.35 GHz. We obtain some of the most sensi…
Measuring the properties of extragalactic magnetic fields through the effect of Faraday rotation provides a means to understand the origin and evolution of cosmic magnetism. Here we use data from the LOFAR Two-Metre Sky Survey (LoTSS) to calculate the Faraday rotation measure (RM) of close pairs of extragalactic radio sources. By considering the RM difference ($Δ$RM) between physical pairs (e.g. double-lobed radio galaxies) and non-physical pairs (i.e. close projected sources on the sky), we statistically isolate the contribution of extragalactic magnetic fields to $Δ$RM along the line of sigh…
Faraday tomography of a field centred on the extragalactic point source 3C 196 with the LOw Frequency ARray (LOFAR) revealed an intertwined structure of diffuse polarised emission with straight depolarisation canals and tracers of the magnetized and multi-phase interstellar medium (ISM), such as dust and line emission from atomic hydrogen (HI). This study aims at extending the multi-tracer analysis of LOFAR data to three additional fields in the surroundings of the 3C 196 field. For the first time, we study the three-dimensional structure of the LOFAR emission by determining the distance to th…
Faraday Rotation Measures (RM) should be interpreted with caution because there could be multiple magneto-ionized medium components that contribute to the net Faraday rotation along sight-lines. We introduce a simple test using Galactic diffuse polarised emission that evaluates whether structures evident in RM observations are associated with distant circumgalactic medium (CGM) or foreground interstellar medium (ISM). We focus on the Magellanic Leading Arm region where a clear excess of RM was previously reported. There are two gaseous objects standing out in this direction: the distant Magell…
The low-frequency polarisation properties of radio sources are poorly studied, particularly in statistical samples. However, the new generation of low-frequency telescopes, such as the Murchison Widefield Array (MWA; the precursor for the low-frequency component of the Square Kilometre Array) offers an opportunity to probe the physics of radio sources at very low radio frequencies. In this paper, we present a catalogue of linearly-polarised sources detected at 216 MHz, using data from the Galactic and Extragalactic All-sky MWA (GLEAM) survey. Our catalogue covers the Declination range $-17^{\c…
We present high sensitivity and absolutely calibrated images of diffuse radio polarisation at a resolution of about 10 arcmin covering the range 10 degr <l<34 degr and |b|<5 degr at 2.3 GHz from the S-band Parkes All Sky Survey and at 4.8 GHz from the Sino-German 6 cm polarisation survey of the Galactic plane. Strong depolarisation near the Galactic plane is seen at 2.3 GHz, which correlates with strong Halpha emission. We ascribe the depolarisation to spatial Faraday rotation measure fluctuations of about 65 rad/m2 on scales smaller than 6-9 pc. We argue that most (about 90%) of the polarised…
Faraday rotation of polarised background sources is a unique probe of astrophysical magnetic fields in a diverse range of foreground objects. However, to understand the properties of the polarised sources themselves and of depolarising phenomena along the line of sight, we need to complement Faraday rotation data with polarisation observations over very broad bandwidths. Just as it is impossible to properly image a complex source with limited u-v coverage, we can only meaningfully understand the magneto-ionic properties of polarised sources if we have excellent coverage in $λ^2$-space. We here…
Radio emission in the form of giant radio relics is observed at the periphery of galaxy clusters. This non-thermal emission is an important tracer for cosmic-ray electrons and intracluster magnetic fields. One striking observational feature of these objects is their high degree of polarisation which provides information on the magnetic fields at the relics' positions. In this contribution, we test if state-of-the-art high resolution cosmological simulations are able to reproduce the polarisation features of radio relics. Therefore, we present a new analysis of high-resolution cosmological simu…
We present Q-U-I JOint TEnerife (QUIJOTE) intensity and polarisation maps at 10-20 GHz covering a region along the Galactic plane 24<l<45 deg, |b|<8 deg. These maps result from 210 h of data, have a sensitivity in polarisation of ~40 muK/beam and an angular resolution of ~1 deg. Our intensity data are crucial to confirm the presence of anomalous microwave emission (AME) towards the two molecular complexes W43 (22 sigma) and W47 (8 sigma). We also detect at high significance (6 sigma) AME associated with W44, the first clear detection of this emission towards a SNR. The new QUIJOTE polarisation…
(Abridged) We study the polarisation properties, magnetic field strength, and synchrotron emission scale-height of Milky-Way-like galaxies in comparison with other spiral galaxies. We use our 3D-emission model of the Milky Way Galaxy for viewing the Milky Way from outside at various inclinations as spiral galaxies are observed. When seen edge-on the synchrotron emission from the Milky Way has an exponential scale-height of about 0.74 kpc, which is much smaller than the values obtained from previous models. We find that current analysis methods overestimate the scale-height of synchrotron emiss…
Helical radio jets as probes of magnetised cluster environments: Periodic Faraday Rotation Revealed in the Corkscrew Galaxy by POSSUM
Australian National University（澳大利亚国立大学）；CSIRO Space & Astronomy（联邦科学与工业研究组织空间与天文部门）；SKA Observatory（平方公里阵列望远镜观测站）；Max-Planck-Institut für Radioastronomie（马克斯·普朗克射电天文研究所）；University of Minnesota（明尼苏达大学）；Universidad Complutense de Madrid（马德里康普顿斯大学）
研究利用ASKAP Band 2对螺旋尾状射电星系进行极化观测，通过检测法拉第旋转量振荡及相关分析，探究其能否揭示主导法拉第旋转介质，结果表明螺旋射流结构有望探测磁化星系团环境。
We investigate the previously proposed possibility that multi-epoch broadband polarimetry could act as a complement or limited proxy for VLBI observations of blazars, in that the number of polarised emission components in the jet, and some of their properties and those of the foreground environment, might be inferred from the object's time-varying 1D Faraday depth spectrum (FDS) alone. We report on a pilot-scale experiment designed to establish the basic plausibility and utility of this idea. We analyse temporal changes in the complex polarisation spectra of nine spatially unresolved (at arcse…
The Rapid ASKAP Continuum Survey VII: Spectra and Polarisation In Cutouts of Extragalactic Sources (SPICE-RACS) Second Data Release -- Unveiling the Magnetised Sky
SKA Observatory, SKA-Low Science Operations Centre（SKA 天文台，SKA-Low 科学运营中心）；ATNF, CSIRO Space & Astronomy（CSIRO 空间与天文学部 ATNF）；CSIRO Information Management & Technology（CSIRO 信息管理与技术部）；Departamento de Física de la Tierra y Astrofísica & IPARCOS-UCM, Universidad Complutense de Madrid（马德里康普顿斯大学地球物理与天体物理学系及 IPARCOS-UCM）；Research School of Astronomy & Astrophysics, The Australian National University（澳大利亚国立大学天文学与天体物理学研究学院）；Dunlap Institute for Astronomy and Astrophysics, University of Toronto（多伦多大学邓普勒天文学与天体物理学研究所）
We present the highest-quality polarisation profiles to date of 16 non-recycled pulsars and four millisecond pulsars, observed below 200 MHz with the LOFAR high-band antennas. Based on the observed profiles, we perform an initial investigation of expected observational effects resulting from the propagation of polarised emission in the pulsar magnetosphere and the interstellar medium. The predictions of magnetospheric birefringence in pulsars have been tested using spectra of the pulse width and fractional polarisation from multifrequency data. The derived spectra offer only partial support fo…
We present the first Faraday rotation measure (RM) grid study of an individual low-mass cluster -- the Fornax cluster -- which is presently undergoing a series of mergers. Exploiting commissioning data for the POlarisation Sky Survey of the Universe's Magnetism (POSSUM) covering a $\sim34$ square degree sky area using the Australian Square Kilometre Array Pathfinder (ASKAP), we achieve an RM grid density of $\sim25$ RMs per square degree from a 280 MHz band centred at 887 MHz, which is similar to expectations for forthcoming GHz-frequency all-sky surveys. We thereby probe the extended magnetoi…
ViCTORIA project: A pilot study of the M49 region in the Virgo cluster in polarisation
Hamburger Sternwarte, Universität Hamburg（汉堡天文台，汉堡大学）；INAF - Istituto di Radioastronomia（意大利国家天体物理研究所射电天文研究所）；Dipartimento di Fisica e Astronomia, Università di Bologna（博洛尼亚大学物理与天文系）；ASTRON, Netherlands Institute for Radio Astronomy（荷兰射电天文研究所）；INAF - Osservatorio Astronomico di Cagliari（意大利国家天体物理研究所卡利亚里天文台）；Aix Marseille Univ, CNRS, CNES, LAM（艾克斯-马赛大学，法国国家科学研究中心，法国国家空间研究中心，拉格朗日实验室）；Department of Astronomy and Astrophysics, University of Chicago（芝加哥大学天文学与天体物理学系）
We analyse the observational signatures of galactic magnetic fields that are self-consistently generated in magnetohydrodynamic simulations of the interstellar medium through turbulence driven by supernova (SN) explosions and differential rotation. In particular, we study the time evolution of the Faraday rotation measure (RM), synchrotron radiation, and Stokes parameters by characterising the typical structures formed in the plane of observation. We do this by defining two distinct models for both thermal and cosmic ray (CR) electron distributions. Our results indicate that the maps of RM hav…
Interstellar magnetic fields play critical roles in many astrophysical processes. Yet despite their importance, our knowledge about magnetic fields in our Galaxy remains limited. For the field within the Milky Way, much of what we do know comes from observations of polarisation and Faraday rotation measures (RMs) of extragalactic sources and pulsars. A high angular density of RM measurements in several critical areas of the Galaxy is needed to clarify the Galactic magnetic field structure. Using observations made with the VLA, we have determined RMs for sources in regions of the Galactic plane…
The low-frequency linearly-polarised radio source population is largely unexplored. However, a renaissance in low-frequency polarimetry has been enabled by pathfinder and precursor instruments for the Square Kilometre Array. In this second paper from the POlarised GaLactic and Extragalactic All-Sky Murchison Widefield Array (MWA) Survey -- the POlarised GLEAM Survey, or POGS -- we present the results from our all-sky MWA Phase I Faraday Rotation Measure survey. Our survey covers nearly the entire Southern sky in the Declination range $-82^{\circ}$ to $+30^{\circ}$ at a resolution between aroun…
The grand-design spiral galaxy M51 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. We produced an image of total emission of M51 at the mean frequency of 151 MHz with 20 arcsec 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 t…
CHANG-ES XXXIX. Magnetic field structure in edge-on galaxies: Stacking Stokes parameters
Ruhr University Bochum（鲁尔大学波鸿）；Max-Planck-Institut für Radioastronomie（马克斯·普朗克射电天文研究所）；CSIRO Space and Astronomy（CSIRO空间与天文学）；University of Manitoba（曼尼托巴大学）；Instituto de Astrofísica de Andalucía (IAA-CSIC)（安达卢西亚天体物理研究所）；The University of Calgary（卡尔加里大学）；Hamburger Sternwar（汉堡天文台）
In this data release from the LOFAR Two-metre Sky Survey (LoTSS) we present 120-168MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44$^\circ$30' and 1h00m +28$^\circ$00' and spanning 4178 and 1457 square degrees respectively. The images were derived from 3,451hrs (7.6PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4,396,228 radio sources is…
