Detection of magnetic fields in the circumgalactic medium of nearby galaxies using Faraday rotation

Context. The existence of magnetic fields in the circumgalactic medium (CGM) is largely unconstrained. Their detection is important as magnetic fields can have a significant impact on the evolution of the CGM and, in turn, the fields can serve as tracers for dynamical processes in the CGM. Aims. With Faraday rotation of polarised background sources, we aim to detect a possible excess of the rotation measure in the surrounding area of nearby galaxies. Methods. We use 2,461 residual rotation measures (RRMs) observed with the LOw Frequency ARray (LOFAR), where the foreground contribution from the Milky Way is subtracted. The RRMs are then studied around a subset of 183 nearby galaxies that was selected by apparent $B$-band magnitude. Results. We find that, in general, the RRMs show no significant excess for small impact parameters (i.e. the perpendicular distance to the line of sight). However, if we only consider galaxies at higher inclination angles and sight lines that pass close to the minor axis of the galaxies, we find significant excess at impact parameters of less than 100 kpc. The excess in |RRM| is 3.7 $\rm rad\,m^{-2}$ with an uncertainty between $\pm 0.9~\rm rad\,m^{-2}$ and $\pm 1.3~\rm rad\,m^{-2}$ depending on the statistical properties of the background (2.8$\sigma$-4.1$\sigma$). With electron densities of ~$10^{-4}~\rm cm^{-3}$ this suggests magnetic field strengths of a few tenths of a micro Gauss. Conclusions. Our results suggest a slow decrease of the magnetic field strength with distance from the galactic disc such as expected if the CGM is magnetised by galactic winds and outflows.Comment: 5 pages, 4 figures, accepted as Letter to Astronomy and Astrophysic

Synthetic observations of spiral arm tracers of a simulated Milky Way analog

Context. The Faraday rotation measure (RM) is often used to study the magnetic field strength and orientation within the ionized medium of the Milky Way. Recent observations indicate an RM magnitude in the spiral arms that exceeds the commonly assumed range. This raises the question of how and under what conditions spiral arms create such strong Faraday rotation. Aims. We investigate the effect of spiral arms on Galactic Faraday rotation through shock compression of the interstellar medium. It has recently been suggested that the Sagittarius spiral arm creates a strong peak in Faraday rotation where the line of sight is tangent to the arm, and that enhanced Faraday rotation follows along side lines which intersect the arm. Here our aim is to understand the physical conditions that may give rise to this effect and the role of viewing geometry. Methods. We apply a magnetohydrodynamic simulation of the multi-phase interstellar medium in a Milky Way-type spiral galaxy disk in combination with radiative transfer in order to evaluate different tracers of spiral arm structures. For observers embedded in the disk, dust intensity, synchrotron emission, and the kinematics of molecular gas observations are derived to identify which spiral arm tangents are observable. Faraday rotation measures are calculated through the disk and evaluated in the context of different observer positions. The observer’s perspectives are related to the parameters of the local bubbles surrounding the observer and their contribution to the total Faraday rotation measure along the line of sight. Results. We reproduce a scattering of tangent points for the different tracers of about 6◦ per spiral arm similar to the Milky Way. For the RM, the model shows that compression of the interstellar medium and associated amplification of the magnetic field in spiral arms enhances Faraday rotation by a few hundred rad m−2 in addition to the mean contribution of the disk. The arm–interarm contrast in Faraday rotation per unit distance along the line of sight is approximately ∼ 10 in the inner Galaxy, fading to ∼ 2 in the outer Galaxy in tandem with the waning contrast of other tracers of spiral arms. We identify a shark fin pattern in the RM Milky Way observations and in the synthetic data that is characteristic for a galaxy with spiral arms

The Galactic Faraday rotation sky 2020

Galaxie

The Galactic Faraday depth sky revisited

Context. The Galactic Faraday depth sky is a tracer for both the Galactic magnetic field and the thermal electron distribution. It was previously reconstructed from polarimetric measurements of extra-Galactic point sources. Aims. Here we improve on these works by using an updated inference algorithm and by taking into account the electron emission measure as traced by free–free emission measured by the Planck survey. In the future the data situation will improve drastically thanks to the next generation Faraday rotation measurements from the SKA and its pathfinders. Anticipating this, a further aim of this paper is to update the map reconstruction method with some of the latest developments in Bayesian imaging. Methods. To this end we made use of information field theory, an inference scheme that is particularly powerful in cases of noisy and incomplete data. Results. We demonstrate the validity of the new algorithm by applying it to an existing data compilation. Even though we used exactly the same data set, a number of novel findings are made; for example, a non-parametric reconstruction of an overall amplitude field resembles the free–free emission measure map of the Galaxy. Folding this emission measure map into the analysis provides more detailed predictions. The joint inference enables us to identify regions with deviations from the assumed correlations between the emission measure and Faraday data, thereby pointing us to Galactic structures with distinguishably different physics. We find evidence for an alignment of the magnetic field within the lines of sight along both directions of the Orion arm

Erratum: Determining the composition of radio plasma via circular polarization: the prospects of the Cygnus A hot spots

Erratu

Determining the composition of radio plasma via circular polarization: the prospects of the Cygnus A hot spots

The composition of the relativistic plasma produced in active galactic nuclei and ejected via powerful jets into the interstellar/intergalactic medium is still a major unsettled issue. It might be a positron-electron plasma in case the plasma was created by pair production in the intense photon fields near accreting super-massive black holes. Alternatively, it might be an electron-proton plasma in case magnetic fields lift and accelerate the thermal gas of accretion discs into relativistic jets as the recent detection of $\gamma$-rays from blazars indicates. Despite various attempts to unambiguously establish the composition of the relativistic jets, this remains a major unknown. Here, we propose a way to settle the question via sensitive measurements of circular polarization (CP) in the radio emission of the hot spots of bright radio galaxies like Cygnus A. The CP of synchrotron emission is determined by the circular motions of the radiating relativistic leptons. In case of charge symmetric energy spectra of a electron-positron plasma, it should be exactly zero. In case of an electron-proton plasma the electrons imprint their gyration onto the CP and we expect the hot spots of Cygnus A to exhibit a fractional CP at a level of 10^{-3}\,(\nu/\mbox{GHz})^{-{1}/{2}}, which is challenging to measure, but not completely unfeasible.Comment: 6 pages, no figures, accepted by JCAP, erratum with respect to accepted version applied, numerical corrections leading to a stronger effec

Studying bioluminescence flashes with the ANTARES deep-sea neutrino telescope

[EN] We develop a novel technique to exploit the extensive data sets provided by underwater neutrino telescopes to gain information on bioluminescence in the deep sea. The passive nature of the telescopes gives us the unique opportunity to infer information on bioluminescent organisms without actively interfering with them. We propose a statistical method that allows us to reconstruct the light emission of individual organisms, as well as their location and movement. A mathematical model is built to describe the measurement process of underwater neutrino telescopes and the signal generation of the biological organisms. The Metric Gaussian Variational Inference algorithm is used to reconstruct the model parameters using photon counts recorded by photomultiplier tubes. We apply this method to synthetic data sets and data collected by the ANTARES neutrino telescope. The telescope is located 40 km off the French coast and fixed to the sea floor at a depth of 2475 m. The runs with synthetic data reveal that we can model the emitted bioluminescent flashes of the organisms. Furthermore, we find that the spatial resolution of the localization of light sources highly depends on the configuration of the telescope. Precise measurements of the efficiencies of the detectors and the attenuation length of the water are crucial to reconstruct the light emission. Finally, the application to ANTARES data reveals the first localizations of bioluminescent organisms using neutrino telescope data.We thank our colleagues of the Information Field Theory Group who provided insight and expertise on the NIFTy framework and Statistical Inference. We would also like to show our gratitude to Thomas Eberl for sharing his expertise on the ANTARES experiment and the data set. SH acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No 772663). The authors acknowledge the financial support of the funding agencies: Centre National de la Recherche Scientifique (CNRS), Commissariat a l'energie atomique et aux energies alternatives (CEA), Commission Europeenne (FEDER fund and Marie Curie Program), Institut Universitaire de France (IUF), LabEx UnivEarthS (ANR-10-LABX-0023 and ANR-18-IDEX-0001), Region Ile-de-France (DIM-ACAV), Region Alsace (contrat CPER), Region Provence-Alpes-Cote d'Azur, Departement du Var and Ville de La Seyne-sur-Mer, France; Bundesministerium fur Bildung und Forschung (BMBF), Germany; Istituto Nazionale di Fisica Nucleare (INFN), Italy; Nederlandse organisatie voor Wetenschappelijk Onderzoek (NWO), the Netherlands; Council of the President of the Russian Federation for young scientists and leading scientific schools supporting grants, Russia; Executive Unit for Financing Higher Education, Research, Development and Innovation (UEFISCDI), Romania; Ministerio de Ciencia, Innovacion, Investigacion y Universidades (MCIU): Programa Estatal de Generacion de Conocimiento (refs. PGC2018-096663-B-C41, PGC2018-096663-A-C42, PGC2018-096663-B-C43, PGC2018-096663-B-C44) (MCIU/FEDER), Generalitat Valenciana: Prometeo (PROMETEO/2020/019), Grisolia (ref. GRISOLIA/2018/119), and GenT (refs. CIDEGENT/2018/034, CIDEGENT//2019/043, CIDEGENT//2020/049) programs, Junta de Andalucia (ref. A-FQM-053-UGR18), La Caixa Foundation (ref. LCF/BQ/IN17/11620019), EU: MSC program (ref. 101025085), Spain; Ministry of Higher Education, Scientific Research and Professional Training, Morocco. We also acknowledge the technical support of Ifremer, AIM, and Foselev Marine for the sea operation and the CC-IN2P3 for the computing facilities. Open Access funding enabled and organized by Projekt DEAL.Reeb, N.; Hutschenreuter, S.; Zehetner, P.; Ensslin, T.; Albert, A.; Alves, S.; Andre, M.... (2023). Studying bioluminescence flashes with the ANTARES deep-sea neutrino telescope. Limnology and Oceanography Methods. 21(11):734-760. https://doi.org/10.1002/lom3.10578734760211