Abstract
Radio continuum and polarization observations reveal best the magnetic field structure and strength in nearby spiral galaxies. They show a similar magnetic field pattern, which is of spiral shape along the disk plane and X-shaped in the halo, sometimes accompanied by strong vertical fields above and below the central region of the disk. The strength of the total halo field is comparable to that of the disk. The small- and large-scale dynamo action is discussed to explain the observations with special emphasis on the rôle of star formation on the α − Ω dynamo and the magnetic field strength and structure in the disk and halo. Recently, with RM-synthesis of the CHANG-ES observations, we obtained the first observational evidence for the existence of regular magnetic fields in the halo. The analysis of the radio scale heights indicate escape-dominated radio halos with convective cosmic ray propagation for many galaxies. These galactic winds may be essential for an effective dynamo action and may transport large-scale magnetic field from the disk into the halo.
Highlights
The effects of magnetic fields on the physical processes in spiral galaxies, their disk-halo interaction and their evolution have been frequently neglected in the past
Direct comparison of three-dimensional MHD simulations of an isolated galaxy withand without a magnetic field show that a magnetic field leads to a lower star formation rate at later times, it reduces the prominence of individual spiral arms and it causes weak outflows from the disk up to several kpc above and below the disk [3]
We detected that the halo scale heights increase linearly with the radio diameters as determined from our maps observed with similar sensitivity and with the radio scale lengths, i.e., exponential fits to the radial total intensity distribution
Summary
The effects of magnetic fields on the physical processes in spiral galaxies, their disk-halo interaction and their evolution have been frequently neglected in the past. Within the last 20 years, with increasing computing facilities, some authors included them in their simulations of, e.g., the interstellar medium and disk-halo interaction (e.g., [1,2]) or in the evolution of spiral galaxies (e.g., [3]) Their result is that magnetic fields play an important role, even if the magnetic and cosmic ray energy density in the interstellar medium is small compared to that of the rotation. Direct comparison of three-dimensional MHD simulations of an isolated galaxy withand without a magnetic field show that a magnetic field leads to a lower star formation rate at later times, it reduces the prominence of individual spiral arms and it causes weak outflows from the disk up to several kpc above and below the disk [3] These results have been supported by cosmological magnetohydrodynamical simulations of galaxies in clusters [6] and will be further researched for isolated Milky Way-like galaxies in dark halos, e.g., with the Auriga Project [7,8]. The field strength of both components, parallel and perpendicular to the LOS, together with the information of the intrinsic polarization vectors enables us in principle to get a three-dimensional picture of the magnetic field
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