Galaxies: Structure and Kinematics; Magnetic fields: Interstellar and Intergalactic >> Projects overview

Interstellar and Intergalactic magnetic fields

Magnetic fields are present in almost every place in the Universe. Most of the luminous matter is tightly coupled to magnetic fields. Large-scale fields intersperse the gas in galaxies and galaxy clusters, they contribute to the nonlinear interplay of turbulent motions in the intracluster and interstellar medium. The magnetic energy content affects the evolution of galaxies and galaxy clusters, contributes significantly to the total pressure of interstellar gas, is essential for the onset of star formation, and controls the density and distribution of cosmic rays in the ISM. In spite of their importance, the evolution, structure and origin of magnetic fields are still open problems in fundamental physics and astrophysics. Most of what we know about cosmic magnetic fields has been detected via radio astronomical observations, as extragalactic radio emission is mainly synchrotron radiation by relativistic leptons.

The observed synchrotron luminosity is determined by the total field strength, while its polarisation yields the orientation of the regular field in the plane of the sky and also gives the field's degree of ordering. Moreover, Faraday rotation provides information on the field component along the line of sight. During the last 30 years, radio continuum observations in the Milky Way, of nearby galaxies and galaxy clusters have enormously increased our knowledge about the structure and strength of interstellar magnetic fields in galaxies and intergalactic magnetic fields in clusters.

The large-scale field structure in the Milky Way and almost all nearby galaxies was found to have a spiral structure, with pitch angles similar to those of the optical spiral arms. However, spiral fields were also detected in galaxies without any optical spiral structure. Faraday rotation revealed large-scale coherence (same direction) of the spiral fields in many galaxies. This indicates the action of alpha-omega dynamos in galaxies, where shearing motions driven by differential rotation act together with the Coriolis force of turbulent gas motions to amplify the weak seed fields exponentially and thus generate large-scale fields. The estimated growth time for large-scale fields is a few Gyr, but the ordering timescale until full coherence of the regular field is longer and may exceed the galaxy age for the largest galaxies.

Cosmic magnetic fields recently attracted special attention of the AUGER team who claimed that some of the arrival directions of detected ultrahigh-energy cosmic rays (UHECRs, > 10¹⁹ eV) are coincident with positions of known active galaxies. However, the interpretation is hampered by our lack of knowledge of the structure and strength of the magnetic field in the halo of our Milky Way and beyond. Only measurements of the Faraday rotation towards a large number of background sources can bring progress, and LOFAR is specially suited to measure low Faraday rotation in galactic haloes. Considering galactic winds and the frequent interactions between galaxies, a significant fraction of the intergalactic space may be magnetised. Observational tests of the connection between disk fields and intergalactic fields have to await more sensitive radio telescopes like LOFAR. It has been known for some time that the hot and dilute gas in clusters of galaxies hosts significant magnetic fields, with estimated typical magnetic field strengths of 5 ?G. Evidence for cluster-wide magnetic fields comes mainly from radio observations, for example from studies of the rotation measure of polarised radio galaxies and the synchrotron emission of diffuse sources, such as radio haloes. The study of cluster magnetic fields is relevant to understand the physical conditions and energetics of the intracluster medium. Cluster magnetic fields provide an additional term of pressure and may play a role in the cluster dynamics. Thus they also affect cluster mass estimates that are important for dark energy studies. They couple cosmic ray particles to the intracluster gas, and they are able to inhibit transport processes such as heat conduction, spatial mixing of gas, and propagation of cosmic rays. They are essential for the acceleration of cosmic rays and allow the cosmic-ray electron population to be observed by the synchrotron radiation. Key questions that are addressed in this proposal concern the role of magnetic fields in the dynamics and evolution of clusters and the question of the origin of cluster-wide magnetic fields.

Structure formation is still going on and merging of galaxy clusters is observed in the local Universe. Such non-relaxed structures are evident in the distribution of the hot gas (seen in X-rays) as well as in images of strong gravitational lensing. Probably the most sensitive tool to disclose such large-scale mergers is the measurent of (low-frequency) radio continuum emission. Synchrotron radiation is emitted in regions of large-scale cosmological shocks, where clusters of galaxies or their substructures compress the ubiquitous magnetic fields and re-accelerate cooled pools of formerly highly relativistic particles. The extended radio emission is characterised by strong polarisation. Other sources of diffuse radio emission are found in the centres of some clusters of galaxies. These radio haloes lack polarisation and the processes creating them are less obvious. Finally, there may be large numbers of exhausted radio sources, with 'starved' AGN. All of these structures are detectable and can be studied at low radio frequencies.

LOFAR, the Low Frequency Array, is a new-generation radio telescope under construction in the Netherlands, Germany and other European countries. It is currently becoming operational incrementally Its unprecedented sensitivity and angular resolution will open a new window to the low-frequency sky that has been largely neglected in the past, owing to the quest for investigations at higher and higher radio frequencies. With one LOFAR station already operational and four more funded stations, Germany took the lead in the European expansion of LOFAR and in the development of long-baseline observations. 'Cosmic Magnetism' is one of six Key Science Projects of LOFAR and is led by German scientists. Synchrotron polarisation is one of the main tools of this key project. Low-frequency polarisation observations are challenging and require careful tools for observation, calibration and data analysis. The expected data from the commissioning phase (beginning of 2010) and from the full LOFAR array (end of 2010) will allow to investigate the distribution and origin of weak, extended magnetic fields in the Milky Way, nearby galaxies and galaxy clusters, and possibly detect magnetic fields from intergalactic filaments for the first time. This application is to establish a network of experienced German scientists to prepare LOFAR for polarisation observations and for studies of cosmic magnetism.