FOR 1254: Magnetisation of Interstellar and Intergalactic Media: The Prospects of Low-Frequency Radio Observations
Final Report Abstract
We summarize the scientific achievements made over the past (second) three-year funding period of the DFG Research Unit FOR 1254. On the observational side, this progress was rendered feasible by utlilizing a routinely operational LOFAR in the newly explored low-frequency regime and the upgraded VLA, the so-called JVLA in the classical regime. The latter now has continuous frequency coverage from L- through X-band and produces high-fidelity images after less than one hour of integration time. The higher demands on planning and performing the observations, and on reducing the data and producing high-quality images require intense training of students and young scientists, who are thus also well prepared for future work with upcoming instrumentation such as the SKA. This also encompasses Bayesian imaging of diffuse radio emission, with the aim to perform three-dimensional tomography of the Milky Way, and to study extragalactic magnetic fields. Our research has thus been focussing on the relativistic plasma in galaxies and in cluster of galaxies, also tackling the issue of existence of magnetic fields on still larger scales, viz. cosmic filaments. The main questions to be tackled in the framework of the Research Unit concerned: (i) the extent of radio haloes around spiral and dwarf galaxies and, along with this, cosmicray propagation and the structure of ordered magnetic fields; (ii) the connection between magnetic-field structure and the galactic dynamo; (iii) the origin of magnetization of the IGM, viz. starburst galaxies vs. AGN; (iv) the nature of diffuse radio relics haloes in galaxy clusters. The prime astrophysical tools used to shed light on these questions were: (i) the total synchrotron emission, related to the strength of the total magnetic field, the cosmic-ray number density and the energy spectrum of the relativistic particles; (ii) the polarized synchrotron emission, which measures the strength of ordered magnetic fields and their orientation; (iii) Faraday rotation, which is the product of the strength of the regular magnetic field and the number density of thermal electrons, also delivering the field direction; (iv) Faraday depolarization, which reflects the turbulence of the magnetic fields and the fluctuation of the electron density. Observationally, these observables have been extensively explored using LOFAR, the GMRT, and the JVLA, thus covering the frequency range between about 150 MHz and 8 GHz. The Effelsberg 100-m telescope was involved to (i) explore the characteristics of relics in galaxy clusters at high frequencies and to (ii) provide missing zero-spacing (i.e. large angular scale) information for the interferometric data. With its overall topic, the Research Unit had a clear focus, while bringing together essentially all German experts in the field. With the low-frequency regime largely neglected over many decades in the past, this venture made many advances and was very successful. Since all participating institutes are also organized within GLOW, the German Long-Wavelength Consortium and since essentially all of the members of the Research Unit were/are also leaders and members of the LOFAR Key Science Projects, privileged access to this instrument has been guaranteed throughout the funding period and beyond. Members of the Research Unit played an important role to develop the required new techniques to calibrate LOFAR data from galaxies and galaxy clusters, resulting in some of the deepest images at low frequencies obtained so far. The images of nearby galaxies allowed them to determine that cosmic-ray electrons propagate mainly via diffusion. No diffuse polarized emission from galaxies could be detected with LOFAR, demonstrating that Faraday depolarization is stronger than previously predicted. Thanks to LOFAR’s huge field of view, many polarized background sources were found, which open a promising method to measure regular magnetic fields in the foreground galaxy. In the course of this funding period, observing programs to subject known diffuse radio structures in galaxy clusters to scrutiny or or to discover new ones have been intensified. These efforts made by the Research Unit resulted in both, a significant improvement of the statistics of such diffuse structures, and a large number of individual spectral studies. A cardinal aspect of this Research Unit has been the frequent get-together of its members, with one large conference held every year, and dedicated workshops on the pertinent astrophysics of galaxies and clusters of galaxies taking place at regular intervals. With a considerable number of renowned guests and experts in the field, persistent collaborations, and continued training of students and young scientists, this Research Unit is considered to have been very successful - also as far as synergy is concerned. This holds true for both, observational radio astronomy as well as theoretical astrophysics. In the course of this funding period, observing programs to subject known or to discover new diffuse radio structures in galaxy clusters have been intensified. The Research Unit has developed novel techniques to analyse radio data with respect to magnetic fields. Bayesian techniques to statistically separate the galactic and extragalactic Faraday rotation signals were developed. The therewith generated possibility to search for magnetic field in cosmic structures like galaxy filaments in now used in the LOFAR Magnetism Key science project. A method to reconstruct the 3D electron distribution in the Milky Way has been established and its potential for future pulsar datasets delivered by the Square Kilometre Array (SKA) demonstrated. The tomography code itself, has also been adapted for 3D matter tomography based on gravitational lensing data. The RESOLVE code for Bayesian radio interferometric imaging has been accelerated by two to three orders of magnitude via algorithmic advancements. Theoretical work on how to fuse calibration and imaging into a single, coherent step following information field theoretical standards have been performed.
