FNWI --- IMAPP Department of Astrophysics
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Galactic Magnetism

Fletcher et al (2011) M51 Galaxies, such as our Milky Way, consist of a number of components: stars are the most well known, but a comparable amount of gas is in the interstellar medium, from which stars are born. Galaxies are also pervaded by cosmic rays, highly energetic ionized particles moving at speeds close to the light speed. Lastly, magnetic fields thread through the interstellar gas. As these magnetic fields are mostly 'frozen into' the gas, these components cannot move independently but are closely interconnected.

Therefore, magnetic fields influence a wide range of physical processes such as the early stages of star formation, the creation of stellar or black hole jets, or acceleration and propagation of cosmic rays.

Most of what is known about cosmic magnetic fields comes from (polarized) radio emission, through a number of different physical mechanisms. The study of polarized radio emission in a wide range of frequencies captures all these mechanisms, enabling us to form a complete picture of the structure, strength and influence of magnetism in galaxies.

The Whirlpool Galaxy (M51) in the picture on the right (copyright MPIfR) shows regular magnetic field lines following the spiral arms. The Milky Way is believed to have a similar magnetic field structure.

Observing the magnetic field in the Milky Way

Although on large scales, the magnetic field in our home galaxy, the Milky Way, seem to nicely follow the Milky Way spiral arms, the situation on smaller scales is much more complex. The magnetic field is constantly pushed around, churned, amplified and diffused by supernova explosions, shock waves, jets and other dynamical processes in the Milky Way. This turbulent magnetic field also provides important feedback on these gas motions such as transport of gas and energy from the star forming disk to the Galactic halo, star formation or the propagation and acceleration of cosmic rays.

Modeling the magnetic field in the Milky Way

The Galactic magnetic field is a key ingredient to understand the physics of the interstellar medium and some aspects of cosmology. For instance, the study of polarization of the Cosmic Microwave Background radiation requirs dealing with its main polarized foregrounds: Galactic thermal dust and synchrotron emission. These emissions are intrinsically linked to the Galactic magnetic field. In order to reproduce their features, an accurate model of the Galactic magnetic field is needed. At large scales, the Galactic magnetic field is decomposed into two parts, the halo and disk components. The model of the disk component reproduces the structure in spiral arms of the Galaxy (large scale structure) and the turbulences of the field (small scale structure). The halo part is so far less known, but more accurate models are in development.

Observational methods

Synchrotron radiation and Faraday rotation

Synchrotron emission in interstellar gas in the Milky Way Polarized synchrotron emission in the same field in the Milky Way

Relativistic cosmic ray electrons circling in a magnetic field emit synchrotron radiation, which is intrinsically highly polarized. However, partial depolarization can occur due to small-scale structure in the magnetic field, or through Faraday rotation effects in the intervening medium. As clearly visible in the above synchrotron maps of a strip of sky, the polarized synchrotron radiation (right) shows a totally different picture than the unpolarized synchrotron (left). This makes measurements of this polarization and Faraday rotation a valuable method that gives unique information about the magneto-ionized medium.

The Galactic halo is an important ingredient of the Galactic ecosystem, but not much is known about its magnetic field. Magnetic fields in the Galctic halo are expected to exhibit only small amounts of Faraday rotation, which is only observable with low-frequency radio telescopes.

The LOw Frequency ARray (LOFAR) is ideally suited to observe these weak magnetic fields, as part of the LOFAR Magnetism Key Science Project. Also, low-frequency receivers at the Westerbork Synthesis Radio Telescope (WSRT) play a large role in determining Galactic magnetic fields in the Galactic halo. Other parts of the Galaxy, such as the disk of the Milky Way and its connection to the halo, are best probed at higher frequencies, e.g. with the Parkes 64-m single dish telescope with which the maps above are made.

Polarization of interstellar dust

Total intensity map of the Archeops experiment Polarization map of the Archeops experiment Tiny dust grains in the interstellar medium absorb the stellar light. If these dust grains are aligned with the Galactic magnetic field lines, they emit polarized thermal dust radiation. The polarization fraction of this emission depends on the alignment of the dust grains with the field. The process responsible for this alignment is under investigation. The observations of this emission, for instance by the Archeops experiment (see pictures for maps in intensity (I) and in polarization (Q)) allow us to constrain models for the polarization of the interstellar dust. The data from the High Frequency Instrument of Planck will improve these constraints and thus help to determine the alignment process of the dust grains.

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