Magnetic fields and halos in spiral galaxies

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 15 years, with increasing computing facilities, some authors included them in their simulations of e.g. the interstellar medium and disk-halo interaction (e.g. korpi+99, avillez+05) or in the evolution of spiral galaxies (e.g. pakmor+13). Their result is that magnetic fields play indeed an important role, even if the magnetic and cosmic ray energy density in the interstellar medium is small compared to that of the rotation. The magnetic field energy density is indeed comparable to that of the turbulent gas motion and much higher than that of the thermal gas as has been determined for the nearby galaxies NGC 6946 (beck07) and M33 (taba+08). Hence, magnetic fields are dynamically important in the processes of the interstellar medium. Direct comparison of 3-dimensional MHD simulations of an isolated galaxy with and without a magnetic field show that the magnetic field lead 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 (pakmor+13).
Observationally, the magnetic field in external galaxies can best be studied in the radio continuum emission in the cm wavelength range. The total intensity of the synchrotron emission gives the strength of the total magnetic field. The linearly polarized intensity reveals the strength and the structure of the resolved regular field in the sky plane (i.e. perpendicular to the line of sight). However, the observed polarization vectors suffer Faraday rotation and depolarization (i.e. a decrease of the degree of linear polarization when compared to the intrinsic one) on the way from the radiation’s origin to us. Correction for Faraday rotation is possible with observations at different wavelengths by determining the rotation measure RM (being proportional to $\int n_e{B}_{\parallel }dl$ where $n_e$ is the thermal electron density and $B_{\parallel }$ the magnetic field strength parallel to the line of sight l). The rotation measure itself can be used to correct the observed polarization angle and also to estimate the strength of $B_{\parallel }$, its sign gives the direction of this magnetic field component. The field strength of both components, parallel and perpendicular to the line of sight, together with the information of the intrinsic polarization vectors enables us in principle to get a three-dimensional picture of the magnetic field.

While the polarized intensity gives the orientation of the magnetic field, the magnetic field direction can only be determined by the rotation measure. This implies that a large-scale regular (coherent) magnetic field can only be deduced from the rotation measure pattern, while the polarized intensity may also originate from anisotropic turbulent magnetic fields (e.g. compressed fields with opposite directions) in compressed or shocked regions. As the polarization is only sensitive to the magnetic field orientation, the polarization angle can only be determined with an $n\cdot \pi$ ambiguity. Further, depolarization effects have to be considered. We distinguish between wavelength-independent and wavelength-dependent depolarization. The difference in depolarization at different wavelengths in maps with the same linear resolution should be purely wavelength dependent where three different wavelength-dependent depolarization effects are important to consider: the differential Faraday rotation, Faraday dispersion, and a RM gradient within the beam (burn66; sokoloff+98). Faraday dispersion is due to turbulent (random) magnetic fields within the source and between the source and us, whereas differential Faraday rotation and depolarization by an RM gradient depends on the regular magnetic field within the emitting source. Especially differential Faraday rotation may cause that the source is not transparent in polarization if the internal Faraday rotation reaches values of ${90}^{\circ }$ or more which may be the case for observations of spiral galaxies seen edge-on near the galactic midplane as e.g. in NGC 4631 (mora+13) even in the decimeter wavelength-regime. The coming polarization spectroscopy and RM-synthesis (brentjens+05) will strongly reduce these effects.

The total magnetic field strength in a galaxy can be estimated from the nonthermal radio emission under the assumption of equipartition between the energies of the magnetic field and the relativistic particles (the so-called energy equipartition) as described in beck+05. The mean equipartition value for the total magnetic field strength for a sample of 74 spiral galaxies observed by niklas95 is on average $9\pm 3\mu$G but reaches locally higher values within the spiral arms of up to $20-25\mu$G in M51 (fletcher+2011). The strength of the ordered magnetic fields in spiral galaxies are typically 1–5 $\mu$G, and may reach locally values up to $10-15\mu$G as e.g. in NGC 6946 (beck07) and M51 (fletcher+2011). The field strengths in the halo are comparable to the those in the disk (see Sect. LABEL:anon:sec:structure).