Galactic dynamos and galactic winds

Abstract

Spiral galaxies host dynamically important magnetic fields which can affect gas flows in the disks and halos. Total magnetic fields in spiral galaxies are strongest (up to 30 µG) in the spiral arms where they are mostly turbulent or tangled. Polarized synchrotron emission shows that the resolved regular fields are generally strongest in the interarm regions (up to 15 µG). Faraday rotation measures of radio polarization vectors in the disks of several spiral galaxies reveal large-scale patterns which are signatures of coherent fields generated by a mean-field dynamo. Magnetic fields are also observed in radio halos around edge-on galaxies at heights of a few kpc above the disk. Cosmic-ray driven galactic winds transport gas and magnetic fields from the disk into the halo. The halo scale height and the electron lifetime allow to estimate the wind speed. The magnetic energy density is larger than the thermal energy density, but smaller than the kinetic energy density of the outflow. There is no observation yet of a halo with a large-scale coherent dynamo pattern. A global wind outflow may prevent the operation of a dynamo in the halo. Halo regions with high degrees of radio polarization at very large distances from the disk are excellent tracers of interaction between galaxies or ram pressure of the intergalactic medium. The observed extent of radio halos is limited by energy losses of the cosmic-ray electrons. Future low-frequency radio telescopes like LO-FAR and the SKA will allow to trace halo outflows and their interaction with the intergalactic medium to much larger distances.

Galactic winds and the symmetry properties of galactic magnetic fields

“…Krause 1989;Beck 2009;Heesen et al 2009b), until quite recently there has been a rather general belief that the magnetic fields throughout the disc and halo of spiral galaxies are of predominantly even (quadrupole-like) symmetry. This conclusion has been supported by a number of numerical investigations of "disc + halo" models (e.g.…”

“…It should be noted that observational information on largescale magnetic fields comes from polarized radio emission and Faraday rotation measures (RM) (Beck 2009). The patterns of B-vectors of polarized emission generally indicate global fields.…”

Ultra-high energy cosmic rays detected by Auger and AGASA

“…We consider seven ordered magnetic field models for our Galaxy (for reviews of Galactic and extragalactic ordered magnetic fields see Beck 2008;Han 2008;Beck 2009). Four of these models, the so-called BS-S, BS-A, AS-S, and AS-A have been used by several authors.…”

“…The line-of-sight integrated GMF of the Galaxy has been measured in various directions via e.g., the rotation measures of pulsars and extragalactic radio sources, and the polarization of starlight or radio synchrotron emission (Heiles 1996;Brown et al 2007;Noutsos et al 2008;Sun et al 2008;Han 2009;Beck 2009). In addition, local magnetic fields can be measured accurately by means of the Zeeman splitting of emission lines.…”

“…Models of the ordered component of the magnetic field are discussed in the next section, and comprehensive reviews can be found in Han (2008) and Beck (2008). Studies of the global magnetic field of other galaxies have greatly facilitated the refinement of models of the Galactic GMF (see Beck 2008Beck , 2009, for a review), even though there is significant variation between galaxies in both the ordered and turbulent GMF, especially among those with high star formation rates.…”