The Galactic Faraday depth sky revisited

The Galactic Faraday depth sky is a tracer for both the Galactic magnetic field and the thermal electron distribution. It has been previously reconstructed from polarimetric measurements of extra-galactic point sources. Here, we improve on these works by using an updated inference algorithm as well as by taking into account the free-free emission measure map from the Planck survey. In the future, the data situation will improve drastically with the next generation Faraday rotation measurements from SKA and its pathfinders. Anticipating this, the aim of this paper is to update the map reconstruction method with the latest development in imaging based on information field theory. We demonstrate the validity of the new algorithm by applying it to the Oppermann et al. (2012) data compilation and compare our results to the previous map.\\ Despite using exactly the previous data set, a number of novel findings are made: A non-parametric reconstruction of an overall amplitude field resembles the free-free emission measure map of the Galaxy. Folding this free-free map into the analysis allows for more detailed predictions. The joint inference enables us to identify regions with deviations from the assumed correlations between the free-free and Faraday data, thereby pointing us to Galactic structures with distinguishably different physics. We e.g. find evidence for an alignment of the magnetic field within the line of sights along both directions of the Orion arm.Comment: 16 pages, 15 figure

Magnetic fields in the Local Universe

This thesis comprises several research efforts centering around cosmological and astrophysical magnetic fields. In the following summary, these are shortly outlined. References and acknowledgments to the respective works are put in front of each chapter. The first chapter entails the first prediction of today's remnants of a primordial large scale magnetic field both in strength and in three dimensional morphology within a comoving box with edge length of 600 Mpc/h. The general idea here is to translate the matter density field inferred from large scale structure data into the radiation dominated epoch up to the point where the horizon scale is much smaller than the smallest scale resolvable by the data. The density field obtained this way is used as initial conditions for the so called Harrison effect, which results in a magnetic field being generated up to recombination. From there on, magnetic field and matter evolution are simulated via a Magnetohydrodynamics solver up to red-shift z=0, revealing the magnetic field structure today. In chapters two to four several analyses of the Galactic Faraday depth sky are presented. Here, rotation measures of extra-Galactic point sources are used to constrain the Galactic component of the Faraday rotation sky. In a first simple inference model a full sky estimate is build from the scattered data points. A component of the inference, which is intended to model the sky amplitude, is found to have strong resemblance with the Galactic free-free emission measure sky. Hence, building on the simple model, additional data is used to disentangle the Faraday sky into its components. In a first phenomenological model, the signature of the local Galactic arm is discovered with the help of emission measure data. In further attempts, dispersion measure data from Galactic pulsars is additionally used to give a quantitative prediction of the line-of-sight averaged Galactic magnetic field sky. In the last chapter, two research projects revolving around circular polarization in the radio regime are summarized. In the first work, the Faraday depth sky and synchrotron intensity data are used to give a prediction on the Galactic synchrotron circular polarization sky. Due to the sensitivity of circular polarization to the charge of the synchrotron light emitting medium, statements on the leptonic content of the Milky Way can be made. The very same property of circular polarization is used in the second paper in order to show that observations of Stokes V may help to decide whether the content of radio jets is hadronic or leptonic

Magnetic fields in the Local Universe

This thesis comprises several research efforts centering around cosmological and astrophysical magnetic fields. In the following summary, these are shortly outlined. References and acknowledgments to the respective works are put in front of each chapter. The first chapter entails the first prediction of today's remnants of a primordial large scale magnetic field both in strength and in three dimensional morphology within a comoving box with edge length of 600 Mpc/h. The general idea here is to translate the matter density field inferred from large scale structure data into the radiation dominated epoch up to the point where the horizon scale is much smaller than the smallest scale resolvable by the data. The density field obtained this way is used as initial conditions for the so called Harrison effect, which results in a magnetic field being generated up to recombination. From there on, magnetic field and matter evolution are simulated via a Magnetohydrodynamics solver up to red-shift z=0, revealing the magnetic field structure today. In chapters two to four several analyses of the Galactic Faraday depth sky are presented. Here, rotation measures of extra-Galactic point sources are used to constrain the Galactic component of the Faraday rotation sky. In a first simple inference model a full sky estimate is build from the scattered data points. A component of the inference, which is intended to model the sky amplitude, is found to have strong resemblance with the Galactic free-free emission measure sky. Hence, building on the simple model, additional data is used to disentangle the Faraday sky into its components. In a first phenomenological model, the signature of the local Galactic arm is discovered with the help of emission measure data. In further attempts, dispersion measure data from Galactic pulsars is additionally used to give a quantitative prediction of the line-of-sight averaged Galactic magnetic field sky. In the last chapter, two research projects revolving around circular polarization in the radio regime are summarized. In the first work, the Faraday depth sky and synchrotron intensity data are used to give a prediction on the Galactic synchrotron circular polarization sky. Due to the sensitivity of circular polarization to the charge of the synchrotron light emitting medium, statements on the leptonic content of the Milky Way can be made. The very same property of circular polarization is used in the second paper in order to show that observations of Stokes V may help to decide whether the content of radio jets is hadronic or leptonic

The Galaxy in circular polarization: all-sky radio prediction, detection strategy, and the charge of the leptonic cosmic rays

The diffuse Galactic synchrotron emission should exhibit a low level of diffuse circular polarization (CP) due to the circular motions of the emitting relativistic electrons. This probes the Galactic magnetic field in a similar way as the product of total Galactic synchrotron intensity times Faraday depth. We use this to construct an all sky prediction of the so far unexplored Galactic CP from existing measurements. This map can be used to search for this CP signal in low frequency radio data even prior to imaging. If detected as predicted, it would confirm the expectation that relativistic electrons, and not positrons, are responsible for the Galactic radio emission. Furthermore, the strength of real to predicted circular polarization would provide statistical information on magnetic structures along the line-of-sights.Comment: 11 pages, 5 figures, revise

The primordial magnetic field in our cosmic backyard

International audienceWe reconstruct for the first time the three dimensional structure of magnetic fields on cosmological scales, which were seeded by density perturbations during the radiation dominated epoch of the Universe and later on were evolved by structure formation. To achieve this goal, we rely on three dimensional initial density fields inferred from the 2M++ galaxy compilation via the Bayesian BORG algorithm. Using those, we estimate the magnetogenesis by the so called Harrison mechanism. This effect produced magnetic fields exploiting the different photon drag on electrons and ions in vortical motions, which are exited due to second order perturbation effects in the Early Universe. Subsequently we study the evolution of these seed fields through the non-linear cosmic structure formation by virtue of a magneto-hydrodynamics simulation to obtain a 3D estimate for the structure of this primordial magnetic field component today. At recombination we obtain large scale magnetic field strengths around , with a power spectrum peaking at about in comoving scales. At present we expect this evolved primordial field to have strengths above  ≈10 and  ≈10 in clusters of galaxies and voids, respectively. We also calculate the corresponding Faraday rotation measure map and show the magnetic field morphology and strength for specific objects of the Local Universe. These results provide a reliable lower limit on the primordial component of the magnetic fields in these structures