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12 pages, 260 KiB

Open AccessEditorial

Workshop Summary “The Power of Faraday Tomography”

by Marijke Haverkorn, Mami Machida and Takuya Akahori

Galaxies 2019, 7(1), 26; https://doi.org/10.3390/galaxies7010026 - 24 Jan 2019

Cited by 4 | Viewed by 2764

Abstract

This article summarizes the work presented at the workshop “The Power of Faraday Tomography: towards 3D mapping of cosmic magnetic fields”, held in Miyazaki, Japan, in Spring 2018. We place the various oral and poster presentations given at the workshop in a broader [...] Read more.

This article summarizes the work presented at the workshop “The Power of Faraday Tomography: towards 3D mapping of cosmic magnetic fields”, held in Miyazaki, Japan, in Spring 2018. We place the various oral and poster presentations given at the workshop in a broader perspective and present some highlight results from every presenter. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

Research

Jump to: Editorial, Other

10 pages, 397 KiB

Open AccessArticle

Performance Test of QU-Fitting

by Yoshimitsu Miyashita

Galaxies 2019, 7(3), 69; https://doi.org/10.3390/galaxies7030069 - 23 Jul 2019

Cited by 1 | Viewed by 2802

Abstract

QU-fitting is a model-fit method to reproduce the model of the Faraday Dispersion Function (FDF or Faraday spectrum), which is a probability distribution function of polarized intensity in Faraday depth space. In order to find the best-fit parameters of the model FDF, we [...] Read more.

QU-fitting is a model-fit method to reproduce the model of the Faraday Dispersion Function (FDF or Faraday spectrum), which is a probability distribution function of polarized intensity in Faraday depth space. In order to find the best-fit parameters of the model FDF, we adopt the Markov Chain Monte Carlo (MCMC) algorithm using Geweke’s convergence diagnostics. Akaike and Bayesian Information Criteria (AIC and BIC, respectively) are used to select the best model from several FDF fitting models. In this paper, we investigate the performance of the standard QU-fitting algorithm quantitatively by simulating spectro-polarimetric observations of two Faraday complex sources located along the same Line Of Sight (LOS), varying the gap between two sources in Faraday depth space and their widths, systematically. We fix the frequency bandwidth in 700–1800 MHz and make mock polarized spectra with a high Signal-to-Noise ratio (S/N). We prepare four FDF models for the fitting by changing the number of model parameters and test the correctness of MCMC and AIC/BIC. We find that the combination of MCMC and AIC/BIC works well for parameter estimation and model selection in the cases where the sources have widths smaller than 1/4 Full Width at Half Maximum (FWHM) and a gap larger than one FWHM in Faraday depth space. We note that when two sources have a gap of five FWHM in Faraday depth space, MCMC tends to be trapped in a local maximum likelihood compared to other situations. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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10 pages, 3090 KiB

Open AccessArticle

Magnetic Field Vector Structure of NGC6946

by Kohei Kurahara and Hiroyuki Nakanishi

Galaxies 2019, 7(2), 59; https://doi.org/10.3390/galaxies7020059 - 27 May 2019

Cited by 4 | Viewed by 3703

Abstract

We studied large-scale magnetic field reversals of a galaxy based on a magnetic vector map of NGC6946. The magnetic vector map was constructed based on the polarization maps in the C and X bands after the determination of the geometrical orientation of a [...] Read more.

We studied large-scale magnetic field reversals of a galaxy based on a magnetic vector map of NGC6946. The magnetic vector map was constructed based on the polarization maps in the C and X bands after the determination of the geometrical orientation of a disk with the use of an infrared image and the velocity field, according to the trailing spiral arm assumption. We examined the azimuthal variation of the magnetic vector and found that the magnetic pitch angle changes continually as a function of the azimuthal angle in the inter-arm region. However, the direction of the magnetic field had

180^{\circ}

jumps at the azimuthal angles of

20^{\circ}, 110^{\circ}, 140^{\circ}, 220^{\circ}, 280^{\circ}

, and

330^{\circ}

. These reversals seem to be related to the spiral arms since the locations of the jumps are coincident with those of the spiral arms. These six reversals of the magnetic field were seen only in the inner region of NGC6946 whereas four reversals can be identified in the outer region. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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7 pages, 249 KiB

Open AccessArticle

Tracing Primordial Magnetic Fields with 21 cm Line Observations

by Kerstin E. Kunze

Galaxies 2019, 7(1), 37; https://doi.org/10.3390/galaxies7010037 - 13 Mar 2019

Cited by 4 | Viewed by 2826

Abstract

Magnetic fields are observed on a large range of scales in the universe. Up until recently, the evidence always pointed to magnetic fields associated with some kind of structure, from planets to clusters of galaxies. Blazar observations have been used to posit the [...] Read more.

Magnetic fields are observed on a large range of scales in the universe. Up until recently, the evidence always pointed to magnetic fields associated with some kind of structure, from planets to clusters of galaxies. Blazar observations have been used to posit the first evidence of truly cosmological magnetic fields or void magnetic fields. A cosmological magnetic field generated in the very early universe before recombination has implications for the cosmic microwave background (CMB), large scale structure as well as the 21 cm line signal. In particular, the Lorentz term causes a change in the linear matter power spectrum. Its implication on the 21 cm line signal was the focus of our recent simulations which will be summarised here. Modelling the cosmological magnetic field as a gaussian random field numerical solutions were found for magnetic fields with present day amplitudes of 5 nG and negative spectral indices which are within the range of observational constraints imposed by the cosmic microwave background (CMB). Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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9 pages, 4145 KiB

Open AccessArticle

Magnetic-Field Vector Maps of Nearby Spiral Galaxies

by Hiroyuki Nakanishi, Kohei Kurahara and Kenta Anraku

Galaxies 2019, 7(1), 32; https://doi.org/10.3390/galaxies7010032 - 11 Feb 2019

Cited by 2 | Viewed by 3729

Abstract

We present a method for determining the directions of magnetic-field vectors in a spiral galaxy using two synchrotron polarization maps, an optical image, and a velocity field. The orientation of the transverse magnetic field is determined with a synchrotron polarization map of a [...] Read more.

We present a method for determining the directions of magnetic-field vectors in a spiral galaxy using two synchrotron polarization maps, an optical image, and a velocity field. The orientation of the transverse magnetic field is determined with a synchrotron polarization map of a higher-frequency band, and the 180°-ambiguity is solved by using a sign of Rotation Measure (RM) after determining the geometrical orientation of a disk based on an assumption of trailing spiral arms. The advantage of this method is that the direction of a magnetic vector for each line of sight throughout the galaxy can inexpensively be determined, with easily available data and simple assumptions. We applied this method to three nearby spiral galaxies using archival data obtained with a Very Large Array (VLA) to demonstrate how it works. The three galaxies have both clockwise and counterclockwise magnetic fields, which implies that none of the three galaxies is classified in a simple Axis-Symmetric type, but types of higher modes, and that magnetic reversals commonly exist. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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13 pages, 2815 KiB

Open AccessArticle

2D MHD Simulations of the State Transitions of X-Ray Binaries Taking into Account Thermal Conduction

by Kenji E. Nakamura, Mami Machida and Ryoji Matsumoto

Galaxies 2019, 7(1), 22; https://doi.org/10.3390/galaxies7010022 - 21 Jan 2019

Cited by 1 | Viewed by 3425

Abstract

Thermal conduction plays an important role in bimodal accretion flows consisting of high-temperature flow and cool flow, especially when the temperature is high and/or has a steep gradient. For example, in hard-to-soft transitions of black hole accretion flows, thermal conduction between the high-temperature [...] Read more.

Thermal conduction plays an important role in bimodal accretion flows consisting of high-temperature flow and cool flow, especially when the temperature is high and/or has a steep gradient. For example, in hard-to-soft transitions of black hole accretion flows, thermal conduction between the high-temperature region and the low-temperature region is appropriately considered. We conducted two-dimensional magnetohydrodynamic (MHD) numerical simulations considering anisotropic heat conduction to study condensation of geometrically thick hot accretion flows driven by radiative cooling during state transitions. Numerical results show that the intermediate region appears between the hot corona and the cool accretion disk when we consider heat conduction. The typical temperature and number density of the intermediate region of the 10

M_{⊙}

black hole at

10 R_{g}

(

R_{g} = 3.0 \times 10^{6} cm

is the Schwarzschild radius) are

4 \times 10^{10} < T [K] < 4 \times 10^{12}

and

5 \times 10^{15} < n [{cm}^{- 3}] < 5 \times 10^{17}

, respectively. The thickness of intermediate region is about half of the radius. By comparing two models with or without thermal conduction, we demonstrate the effects of thermal conduction. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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9 pages, 541 KiB

Open AccessArticle

IMAGINE: Modeling the Galactic Magnetic Field

by Marijke Haverkorn, François Boulanger, Torsten Enßlin, Jörg R. Hörandel, Tess Jaffe, Jens Jasche, Jörg P. Rachen and Anvar Shukurov

Galaxies 2019, 7(1), 17; https://doi.org/10.3390/galaxies7010017 - 14 Jan 2019

Cited by 10 | Viewed by 5435

Abstract

The IMAGINE Consortium aims to bring modeling of the magnetic field of the Milky Way to the next level by using Bayesian inference. IMAGINE includes an open-source modular software pipeline that optimizes parameters in a user-defined galactic magnetic field model against various selected [...] Read more.

The IMAGINE Consortium aims to bring modeling of the magnetic field of the Milky Way to the next level by using Bayesian inference. IMAGINE includes an open-source modular software pipeline that optimizes parameters in a user-defined galactic magnetic field model against various selected observational datasets. Bayesian priors can be added as external probabilistic constraints of the model parameters. These conference proceedings describe the science goals of the IMAGINE consortium, the software pipeline and its inputs, namely observational data sets, galactic magnetic field models, and Bayesian priors. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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7 pages, 1996 KiB

Open AccessArticle

Faraday Depolarization Effects in Spiral Galaxies

by Mami Machida, Takuya Akahori, Kenji E. Nakamura, Hiroyuki Nakanishi and Marijke Haverkorn

Galaxies 2019, 7(1), 15; https://doi.org/10.3390/galaxies7010015 - 12 Jan 2019

Cited by 1 | Viewed by 3404

Abstract

Magnetic fields in the universe play an essential role in observations of the radio synchrotron continuum; however, we do not know enough about them, either observationally or theoretically. We are interested in galactic magnetic fields because they affect the structural formation of galaxies [...] Read more.

Magnetic fields in the universe play an essential role in observations of the radio synchrotron continuum; however, we do not know enough about them, either observationally or theoretically. We are interested in galactic magnetic fields because they affect the structural formation of galaxies in terms of star-forming regions, spiral arms, and threads at the galactic center. To clarify the importance of magnetic fields, we carried out numerical simulations of the galactic gaseous disk with magnetic fields. We also calculated observables, such as the rotation measure and Stokes parameters, from the results of numerical simulation. FD maps and intensity maps have been reported, and the relation between azimuthal angle and FD has been shown to depend on the inclination of the observer. Furthermore, it has been shown that a polarized intensity below 800 MHz reflects field structure in the halo region, although the intensity is weak. The present paper summarizes the effects of Faraday depolarization and the relation between magnetic-field structure and Stokes parameters. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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14 pages, 5020 KiB

Open AccessArticle

Two-Temperature Magnetohydrodynamics Simulations of Propagation of Semi-Relativistic Jets

by Takumi Ohmura, Mami Machida, Kenji Nakamura, Yuki Kudoh, Yuta Asahina and Ryoji Matsumoto

Galaxies 2019, 7(1), 14; https://doi.org/10.3390/galaxies7010014 - 11 Jan 2019

Cited by 8 | Viewed by 4334

Abstract

In astrophysical jets observed in active galactic nuclei and in microquasars, the energy exchange rate by Coulomb collision is insufficient for thermal equilibrium between ions and electrons. Therefore, it is necessary to consider the difference between the ion temperature and the electron temperature. [...] Read more.

In astrophysical jets observed in active galactic nuclei and in microquasars, the energy exchange rate by Coulomb collision is insufficient for thermal equilibrium between ions and electrons. Therefore, it is necessary to consider the difference between the ion temperature and the electron temperature. We present the results of two-temperature magnetohydrodynamics(MHD) simulations to demonstrate the effects of Coulomb coupling. It is assumed that the thermal dissipation heats only ions. We find that the ion and electron temperatures are separated through shocks. Since the ion entropy is increased by energy dissipation at shocks and the Coulomb collisions are inefficient, electron temperature becomes about 10 times lower than the ion temperature in the hotspot ahead of the jet terminal shock. In the cocoon, electron temperature decreases by gas mixing between high temperature cocoon gas and low temperature shocked-ambient gas even when we neglect radiative cooling, but electrons can be heated through collisions with ions. Radiation intensity maps are produced by post processing numerical results. Distributions of the thermal bremsstrahlung radiation computed from electron temperature have bright filament and cavity around the jet terminal shock. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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10 pages, 5668 KiB

Open AccessFeature PaperArticle

The Disk-Halo Distinction of Galaxies Using Faraday Tomography

by Shinsuke Ideguchi

Galaxies 2019, 7(1), 1; https://doi.org/10.3390/galaxies7010001 - 21 Dec 2018

Cited by 2 | Viewed by 2471

Abstract

Faraday tomography allows us to study the distribution and properties of the magnetoionic medium of galaxies through the Faraday effect. However, this can be achieved only after the Faraday spectrum is interpreted. One approach is using galactic ISM/magnetism models to investigate how characteristic [...] Read more.

Faraday tomography allows us to study the distribution and properties of the magnetoionic medium of galaxies through the Faraday effect. However, this can be achieved only after the Faraday spectrum is interpreted. One approach is using galactic ISM/magnetism models to investigate how characteristic properties of the galaxies in the physical depth space are reflected to the Faraday spectrum. In this paper, I employ a realistic Galactic ISM/magnetism model and calculate the intrinsic Faraday spectrum of face-on galaxies, especially focusing on the galactic disk-halo structure and on the presence of the coherent, vertical magnetic field. I also calculate the Faraday depth cubes of the Milky Way from the model. I discuss the possibility of studying the disk and halo structures by means of Faraday tomography. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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13 pages, 4794 KiB

Open AccessArticle

Magnetic Fields in Galaxy Clusters and in the Large-Scale Structure of the Universe

by Valentina Vacca, Matteo Murgia, Federica Govoni, Torsten Enßlin, Niels Oppermann, Luigina Feretti, Gabriele Giovannini and Francesca Loi

Galaxies 2018, 6(4), 142; https://doi.org/10.3390/galaxies6040142 - 17 Dec 2018

Cited by 24 | Viewed by 5302

Abstract

The formation and history of cosmic magnetism is still widely unknown. Significant progress can be made through the study of magnetic fields properties in the large-scale structure of the Universe: galaxy clusters, filaments, and voids of the cosmic web. A powerful tool to [...] Read more.

The formation and history of cosmic magnetism is still widely unknown. Significant progress can be made through the study of magnetic fields properties in the large-scale structure of the Universe: galaxy clusters, filaments, and voids of the cosmic web. A powerful tool to study magnetization of these environments is represented by radio observations of diffuse synchrotron sources and background or embedded radio galaxies. To draw a detailed picture of cosmic magnetism, high-quality data of these sources need to be used in conjunction with sophisticated tools of analysis. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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9 pages, 4561 KiB

Open AccessArticle

Magneto Rotational Instability in Magnetized AGN Tori

by Yuki Kudoh and Keiichi Wada

Galaxies 2018, 6(4), 139; https://doi.org/10.3390/galaxies6040139 - 11 Dec 2018

Cited by 2 | Viewed by 3305

Abstract

It is widely believed that, in active galactic nuclei (AGNs), a supermassive black hole with an accretion disk is surrounded by an optically and geometrically thick torus at sub-parsec scale. However, it is not clear how the mass supply is toward the central [...] Read more.

It is widely believed that, in active galactic nuclei (AGNs), a supermassive black hole with an accretion disk is surrounded by an optically and geometrically thick torus at sub-parsec scale. However, it is not clear how the mass supply is toward the central engine caused and how it is related with the internal structures of the tori. The magnetic field in the tori may contribute to the accretion process via the magneto-rotational instability (MRI). Using global three-dimensional magnetohydrodynamic (MHD) simulations taking the effects of X-ray heating and radiative cooling into account studied the numerical resolution for azimuthal direction for MRI driving. We found that a strongly magnetized disk consisted of a cold (<

10^{3}

K) and warm (

10^{4}

K) gas is developed in about 30 rotational periods. We also found in a high resolution model that the mean azimuthal magnetic fields reverse their direction quasi-periodically. We confirmed that the typical wave length of the MRI should be resolved with a least 20 azimuthal grid cells. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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8 pages, 1775 KiB

Open AccessArticle

Faraday Tomography of the SS433 Jet Termination Region

by Haruka Sakemi, Mami Machida, Takumi Ohmura, Shinsuke Ideguchi, Yoshimitsu Miyashita, Keitaro Takahashi, Takuya Akahori, Hiroki Akamatsu, Hiroyuki Nakanishi, Kohei Kurahara and Jamie Farnes

Galaxies 2018, 6(4), 137; https://doi.org/10.3390/galaxies6040137 - 9 Dec 2018

Cited by 4 | Viewed by 3595

Abstract

A jet termination region provides us with useful information about how a jet interacts with the interstellar medium. Identifying the strength and orientation of magnetic fields at the terminal is crucially important to understanding the mechanism of cosmic-ray acceleration. In this article, we [...] Read more.

A jet termination region provides us with useful information about how a jet interacts with the interstellar medium. Identifying the strength and orientation of magnetic fields at the terminal is crucially important to understanding the mechanism of cosmic-ray acceleration. In this article, we report results of our Faraday-tomography analysis of the eastern region of the radio nebula W50, where a jet from the microquasar SS433 seems to terminate. We apply QU-fitting, a method of Faraday-tomography, to data from the Australia Telescope Compact Array (ATCA) at 1.3–3.0 GHz. In the analysis, we distinguish multiple polarized sources along the line of sight. We identify Galactic emission candidates at Faraday depths around 0 rad m⁻² and 300 rad m⁻². The Galactic emission around 0 rad m⁻² is possibly located in front of W50. We also find emission from W50 with Faraday depths between φ = −112 to 228 rad m⁻². Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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8 pages, 1695 KiB

Open AccessArticle

Simulations of the Polarized Sky for the SKA: How to Constrain Intracluster Magnetic Fields

by Francesca Loi, Matteo Murgia, Federica Govoni, Valentina Vacca, Isabella Prandoni, Hui Li, Luigina Feretti and Gabriele Giovannini

Galaxies 2018, 6(4), 133; https://doi.org/10.3390/galaxies6040133 - 4 Dec 2018

Cited by 4 | Viewed by 2720

Abstract

The advent of the Square Kilometer Array (SKA) will have unprecedented impact on the study of magnetic fields in galaxy clusters. This instrument will be able to perform all-sky surveys in polarization, allowing us to build a rotation-measure (RM) grid based on an [...] Read more.

The advent of the Square Kilometer Array (SKA) will have unprecedented impact on the study of magnetic fields in galaxy clusters. This instrument will be able to perform all-sky surveys in polarization, allowing us to build a rotation-measure (RM) grid based on an enormous number of sources. However, it is not always obvious how to extract correct information about the strength and the structure of magnetic fields from the RM grid. The simulations presented here help us to investigate this topic as they consist of full-Stokes idealized (because we did not add thermal noise) images of a pair of galaxy clusters between 950–1760 GHz, i.e., the SKA1-MID band 2. These images include not just cluster-embedded radio sources but also foreground and background discrete radio sources populating the simulated portion of the universe. To study the magnetic fields of the simulated galaxy clusters, we applied the RM synthesis technique on the simulated images and compared the “true” cluster RM values with those inferred from RM synthesis. The accuracy of our methodology is guarantee by the excellent agreement that we observed when we considered only the signal from the background radio sources. The presence of a Faraday screen, foreground, and cluster sources, introduces degeneracies and/or ambiguities that make the interpretation of the results more difficult. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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13 pages, 5504 KiB

Open AccessArticle

Is There a Polarization Horizon?

by Alex S. Hill

Galaxies 2018, 6(4), 129; https://doi.org/10.3390/galaxies6040129 - 30 Nov 2018

Cited by 8 | Viewed by 2770

Abstract

Modern radio spectrometers make measurement of polarized intensity as a function of Faraday depth possible. I investigate the effect of depolarization along a model line of sight. I model sightlines with two components informed by observations: a warm ionized medium with a lognormal [...] Read more.

Modern radio spectrometers make measurement of polarized intensity as a function of Faraday depth possible. I investigate the effect of depolarization along a model line of sight. I model sightlines with two components informed by observations: a warm ionized medium with a lognormal electron density distribution and a narrow, denser component simulating a spiral arm or Hii region, all with synchrotron-emitting gas mixed in. I then calculate the polarized intensity from 300–1800 MHz and calculate the resulting Faraday depth spectrum. The idealized synthetic observations show far more Faraday complexity than is observed in Global Magneto-Ionic Medium Survey observations. In a model with a very nearby Hii region observed at low frequencies, most of the effects of a “depolarization wall” are evident: the Hii region depolarizes background emission, and less (but not zero) information from beyond the Hii region reaches the observer. In other cases, the effects are not so clear, as significant amounts of information reach the observer even through significant depolarization, and it is not clear that low-frequency observations sample largely different volumes of the interstellar medium than high-frequency observations. The observed Faraday depth can be randomized such that it does not always have any correlation with the true Faraday depth. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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17 pages, 3137 KiB

Open AccessArticle

The Challenge of Detecting Intracluster Filaments with Faraday Rotation

by Nicola Locatelli, Franco Vazza and Paola Domínguez-Fernández

Galaxies 2018, 6(4), 128; https://doi.org/10.3390/galaxies6040128 - 30 Nov 2018

Cited by 14 | Viewed by 3391

Abstract

The detection of filaments in the cosmic web will be crucial to distinguish between the possible magnetogenesis scenarios, and future large polarization surveys will be able to shed light on their magnetization level. In this work, we use numerical simulations of galaxy clusters [...] Read more.

The detection of filaments in the cosmic web will be crucial to distinguish between the possible magnetogenesis scenarios, and future large polarization surveys will be able to shed light on their magnetization level. In this work, we use numerical simulations of galaxy clusters to investigate their possible detection. We compute the Faraday Rotation signal in intracluster filaments and compare it to its surrounding environment. We find that the expected big improvement in sensitivity with the SKA-MID will in principle allow the detection of a large fraction of filaments surrounding galaxy clusters. However, the contamination of the intrinsic Faraday Rotation of background polarized sources will represent a big limiter to the number of objects that can be significantly detected. We discuss possible strategies to minimize this effect and increase the chances of detection of the cosmic web with the large statistics expected from future surveys. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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14 pages, 2186 KiB

Open AccessArticle

The Extraordinary Linear Polarisation Structure of the Southern Centaurus A Lobe Revealed by ASKAP

by Craig S. Anderson, George Heald, Shane P. O’Sullivan, John D. Bunton, Ettore Carretti, Aaron P. Chippendale, Jordan D. Collier, Jamie S. Farnes, Bryan M. Gaensler, Lisa Harvey-Smith, Bärbel S. Koribalski, Tom L. Landecker, Emil Lenc, Naomi M. McClure-Griffiths, Daniel Mitchell, Lawrence Rudnick and Jennifer West

Galaxies 2018, 6(4), 127; https://doi.org/10.3390/galaxies6040127 - 29 Nov 2018

Cited by 10 | Viewed by 3630

Abstract

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-h pointing over [...] Read more.

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-h pointing over a 240 MHz band centred on 913 MHz. Our observations achieve an angular resolution of

26 \times 33

arcseconds (480 parsecs), a maximum recoverable angular scale of 30 arcminutes, and a full-band sensitivity of 85

μ

Jy beam

^{- 1}

. The resulting maps of polarisation and Faraday rotation are amongst the most detailed ever made for radio lobes, with order 10

^{5}

resolution elements covering the source. We describe several as-yet unreported observational features of the lobe, including its detailed peak Faraday depth structure, and intricate networks of depolarised filaments. These results demonstrate the exciting capabilities of ASKAP for widefield radio polarimetry. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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13 pages, 1222 KiB

Open AccessArticle

Untangling Cosmic Magnetic Fields: Faraday Tomography at Metre Wavelengths with LOFAR

by Shane P. O’Sullivan, Marcus Brüggen, Cameron L. Van Eck, Martin J. Hardcastle, Marijke Haverkorn, Timothy W. Shimwell, Cyril Tasse, Valentina Vacca, Cathy Horellou and George Heald

Galaxies 2018, 6(4), 126; https://doi.org/10.3390/galaxies6040126 - 29 Nov 2018

Cited by 16 | Viewed by 3599

Abstract

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 centimetre-wavelength observations. However, the effect [...] Read more.

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 centimetre-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 Survey (LoTSS). Of the 76 extragalactic polarised sources analysed here, we find that all host a radio-loud AGN (Active Galactic Nucleus). The majority of the sources (∼64%) are large FRII radio galaxies with a median projected linear size of 710 kpc and median radio luminosity at 144 MHz of

4 \times 10^{26}

W Hz

^{- 1}

(with ∼13% of all sources having a linear size >1 Mpc). In several cases, both hotspots are detected in polarisation at an angular resolution of ∼20

^{″}

. One such case allowed a study of intergalactic magnetic fields on scales of 3.4 Mpc. Other detected source types include an FRI radio galaxy and at least eight blazars. Most sources display simple Faraday spectra, but we highlight one blazar that displays a complex Faraday spectrum, with two close peaks in the Faraday dispersion function. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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7 pages, 2525 KiB

Open AccessArticle

Combining Faraday Tomography and Wavelet Analysis

by Dmitry Sokoloff, Rainer Beck, Anton Chupin, Peter Frick, George Heald and Rodion Stepanov

Galaxies 2018, 6(4), 121; https://doi.org/10.3390/galaxies6040121 - 22 Nov 2018

Cited by 4 | Viewed by 3604

Abstract

We present a concept for using long-wavelength broadband radio continuum observations of spiral galaxies to isolate magnetic structures that were only previously accessible from short-wavelength observations. The approach is based on combining the RM Synthesis technique with the 2D continuous wavelet transform. Wavelet [...] Read more.

We present a concept for using long-wavelength broadband radio continuum observations of spiral galaxies to isolate magnetic structures that were only previously accessible from short-wavelength observations. The approach is based on combining the RM Synthesis technique with the 2D continuous wavelet transform. Wavelet analysis helps to isolate and recognize small-scale structures which are produced by Faraday dispersion. We find that these structures can trace galactic magnetic arms as illustrated by the case of the galaxy NGC 6946 observed at

λ = 17 - 22

cm. We support this interpretation through the analysis of a synthetic observation obtained using a realistic model of a galactic magnetic field. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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9 pages, 5402 KiB

Open AccessArticle

Science Pipelines for the Square Kilometre Array

by Jamie Farnes, Ben Mort, Fred Dulwich, Stef Salvini and Wes Armour

Galaxies 2018, 6(4), 120; https://doi.org/10.3390/galaxies6040120 - 20 Nov 2018

Cited by 15 | Viewed by 4048

Abstract

The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and the largest Big Data project in the known Universe. The first phase of the project will generate on the order of five zettabytes of data per year. A [...] Read more.

The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and the largest Big Data project in the known Universe. The first phase of the project will generate on the order of five zettabytes of data per year. A critical task for the SKA will be its ability to process data for science, which will need to be conducted by science pipelines. Together with polarization data from the LOFAR Multifrequency Snapshot Sky Survey (MSSS), we have been developing a realistic SKA-like science pipeline that can handle the large data volumes generated by LOFAR at 150 MHz. The pipeline uses task-based parallelism to image, detect sources and perform Faraday tomography across the entire LOFAR sky. The project thereby provides a unique opportunity to contribute to the technological development of the SKA telescope, while simultaneously enabling cutting-edge scientific results. In this paper, we provide an update on current efforts to develop a science pipeline that can enable tight constraints on the magnetised large-scale structure of the Universe. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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10 pages, 1248 KiB

Open AccessArticle

Strategy to Explore Magnetized Cosmic Web with Forthcoming Large Surveys of Rotation Measure

by Takuya Akahori

Galaxies 2018, 6(4), 118; https://doi.org/10.3390/galaxies6040118 - 16 Nov 2018

Cited by 2 | Viewed by 3159

Abstract

The warm-hot intergalactic medium (WHIM) is a candidate for the missing baryons in the Universe. If the WHIM is permeated with the intergalactic magnetic field (IGMF), the Faraday rotation measure (RM) of the WHIM is imprinted in linearly-polarized emission from extragalactic objects. In [...] Read more.

The warm-hot intergalactic medium (WHIM) is a candidate for the missing baryons in the Universe. If the WHIM is permeated with the intergalactic magnetic field (IGMF), the Faraday rotation measure (RM) of the WHIM is imprinted in linearly-polarized emission from extragalactic objects. In this article, we discuss strategies to explore the WHIM’s RM from forthcoming radio broadband and wide-field polarization sky surveys. There will be two observational breakthroughs in the coming decades; the RM grid and Faraday tomography. They will allow us to find ideal RM sources for the study of the IGMF and give us unique information of the WHIM along the line of sight. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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14 pages, 4462 KiB

Open AccessArticle

The Power of Low Frequencies: Faraday Tomography in the Sub-GHz Regime

by Cameron L. Van Eck

Galaxies 2018, 6(4), 112; https://doi.org/10.3390/galaxies6040112 - 23 Oct 2018

Cited by 9 | Viewed by 2857

Abstract

Faraday tomography, the study of the distribution of extended polarized emission by strength of Faraday rotation, is a powerful tool for studying magnetic fields in the interstellar medium of our Galaxy and nearby galaxies. The strong frequency dependence of Faraday rotation results in [...] Read more.

Faraday tomography, the study of the distribution of extended polarized emission by strength of Faraday rotation, is a powerful tool for studying magnetic fields in the interstellar medium of our Galaxy and nearby galaxies. The strong frequency dependence of Faraday rotation results in very different observational strengths and limitations for different frequency regimes. I discuss the role these effects take in Faraday tomography below 1 GHz, emphasizing the 100–200 MHz band observed by the Low Frequency Array and the Murchison Widefield Array. With that theoretical context, I review recent Faraday tomography results in this frequency regime, and discuss expectations for future observations. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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9 pages, 826 KiB

Open AccessArticle

Towards Exascale Simulations of the ICM Dynamo with WENO-Wombat

by Julius Donnert, Hanbyul Jang, Peter Mendygral, Gianfranco Brunetti, Dongsu Ryu and Thomas Jones

Galaxies 2018, 6(4), 104; https://doi.org/10.3390/galaxies6040104 - 29 Sep 2018

Cited by 2 | Viewed by 2572

Abstract

In galaxy clusters, modern radio interferometers observe non-thermal radio sources with unprecedented spatial and spectral resolution. For the first time, the new data allows to infer the structure of the intra-cluster magnetic fields on small scales via Faraday tomography. This leap forward demands [...] Read more.

In galaxy clusters, modern radio interferometers observe non-thermal radio sources with unprecedented spatial and spectral resolution. For the first time, the new data allows to infer the structure of the intra-cluster magnetic fields on small scales via Faraday tomography. This leap forward demands new numerical models for the amplification of magnetic fields in cosmic structure formation—the cosmological magnetic dynamo. Here we present a novel numerical approach to astrophyiscal MHD simulations aimed to resolve this small-scale dynamo in future cosmological simulations. As a first step, we implement a fifth order WENO scheme in the new code WOMBAT. We show that this scheme doubles the effective resolution of the simulation and is thus less expensive than common second order schemes. WOMBAT uses a novel approach to parallelization and load balancing developed in collaboration with performance engineers at Cray Inc. This will allow us scale simulation to the exaflop regime and achieve kpc resolution in future cosmological simulations of galaxy clusters. Here we demonstrate the excellent scaling properties of the code and argue that resolved simulations of the cosmological small scale dynamo within the whole virial radius are possible in the next years. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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7 pages, 1183 KiB

Open AccessConference Report

X-ray and Radio Observations of the Radio Relic Galaxy Clusters 1RXS J0603.3+4214 and RXC J1053.7+5453

by Motokazu Takizawa

Galaxies 2019, 7(1), 2; https://doi.org/10.3390/galaxies7010002 - 22 Dec 2018

Cited by 2 | Viewed by 2629

Abstract

We study two galaxy clusters with radio relics, 1RXS J0603.3+4214 and RXC J1053.7+5453, through X-ray and radio observations. Radio relics are diffuse non-thermal radio sources found in outskirts of galaxy clusters. Because of their shape and location, they are thought to be related [...] Read more.

We study two galaxy clusters with radio relics, 1RXS J0603.3+4214 and RXC J1053.7+5453, through X-ray and radio observations. Radio relics are diffuse non-thermal radio sources found in outskirts of galaxy clusters. Because of their shape and location, they are thought to be related to cluster merger shocks. The galaxy cluster 1RXS J0603.3+4214 has a well-known linear-shape “toothbrush” radio relic. We investigate the temperature structure across the relic to constrain the Mach number of the associated shock. The results are compared with radio spectral results, which suggest that a simple diffusive shock acceleration model does not hold for this relic. The RXC J1053.7+5453 harbors a standard arc-like relic. We also get the Mach number from the temperature profile. In addition, we found an edge-like structure in the X-ray image between the X-ray peak and relic. We investigate the density and temperature profiles across the edge and found that the structure is likely relevant to not a shock but a contact discontinuity. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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7 pages, 468 KiB

Open AccessConference Report

Thermal Sunyaev–Zel’dovich Effect in the IGM due to Primordial Magnetic Fields

by Teppei Minoda, Kenji Hasegawa, Hiroyuki Tashiro, Kiyotomo Ichiki and Naoshi Sugiyama

Galaxies 2018, 6(4), 143; https://doi.org/10.3390/galaxies6040143 - 18 Dec 2018

Cited by 6 | Viewed by 3062

Abstract

In the present universe, magnetic fields exist with various strengths and on various scales. One possible origin of these cosmic magnetic fields is the primordial magnetic fields (PMFs) generated in the early universe. PMFs are considered to contribute to matter density evolution via [...] Read more.

In the present universe, magnetic fields exist with various strengths and on various scales. One possible origin of these cosmic magnetic fields is the primordial magnetic fields (PMFs) generated in the early universe. PMFs are considered to contribute to matter density evolution via Lorentz force and the thermal history of intergalactic medium (IGM) gas due to ambipolar diffusion. Therefore, information about PMFs should be included in the temperature anisotropy of the Cosmic Microwave Background through the thermal Sunyaev–Zel’dovich (tSZ) effect in IGM. In this article, given an initial power spectrum of PMFs, we show the spatial fluctuation of mass density and temperature of the IGM and tSZ angular power spectrum created by the PMFs. Finally, we find that the tSZ angular power spectrum induced by PMFs becomes significant on small scales, even with PMFs below the observational upper limit. Therefore, we conclude that the measurement of tSZ anisotropy on small scales will provide the most stringent constraint on PMFs. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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13 pages, 1228 KiB

Open AccessConference Report

Faraday Tomography Tutorial

by Shinsuke Ideguchi, Yoshimitsu Miyashita and George Heald

Galaxies 2018, 6(4), 140; https://doi.org/10.3390/galaxies6040140 - 14 Dec 2018

Cited by 6 | Viewed by 3880

Abstract

The capabilities of wide-band polarization datasets that are now becoming available from precursors/pathfinders to the Square Kilometre Array (SKA), and eventually from the SKA itself, make it possible to use the Faraday tomography technique to facilitate the study of cosmic magnetism. While many [...] Read more.

The capabilities of wide-band polarization datasets that are now becoming available from precursors/pathfinders to the Square Kilometre Array (SKA), and eventually from the SKA itself, make it possible to use the Faraday tomography technique to facilitate the study of cosmic magnetism. While many programs enabling Faraday tomography have been developed by various authors and it is now becoming easier to apply the required techniques, the interpretation of the results is not straightforward. This is not only because of the lack of a one-to-one relation between the Faraday depth and the physical depth, and observational artifacts such as instrumental polarization, but also because the choice of the method that is used and its settings can be reflected in the results. Thus, it is essential to understand how the various methods enabling Faraday tomography are suited for the efficient application of the technique. In the workshop “The Power of Faraday Tomography”, we organized a Faraday tomography tutorial to help the participants understand the required tools. In this article, we summarize the basics of the techniques, and provide an overview of the tutorial. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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11 pages, 1165 KiB

Open AccessConference Report

Magnetizing the Cosmic Web during Reionization

by Mathieu Langer and Jean-Baptiste Durrive

Galaxies 2018, 6(4), 124; https://doi.org/10.3390/galaxies6040124 - 23 Nov 2018

Cited by 5 | Viewed by 3741

Abstract

Increasing evidence suggests that cosmological sheets, filaments, and voids may be substantially magnetized today. The origin of magnetic fields in the intergalactic medium (IGM) is, however, currently uncertain. It seems well known that non-standard extensions to the physics of the standard model can [...] Read more.

Increasing evidence suggests that cosmological sheets, filaments, and voids may be substantially magnetized today. The origin of magnetic fields in the intergalactic medium (IGM) is, however, currently uncertain. It seems well known that non-standard extensions to the physics of the standard model can provide mechanisms susceptible of magnetizing the universe at large. Perhaps less well known is the fact that standard, classical physics of matter–radiation interactions actually possesses the same potential. We discuss a magnetogenesis mechanism based on the exchange of momentum between hard photons and electrons in an inhomogeneous IGM. Operating in the neighborhood of ionizing sources during the epoch of reionization, this mechanism is capable of generating magnetic seeds of relevant strengths over scales comparable to the distance between ionizing sources. In addition, summing up the contributions of all ionizing sources and taking into account the distribution of gas inhomogeneities, we show that this mechanism leaves the IGM, at the end of reionization, with a level of magnetization that might account, when amplification mechanisms take over, for the magnetic fields strengths in the current cosmic web. Full article

(This article belongs to the Special Issue The Power of Faraday Tomography)

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