Magnetic Field Structure from Synchrotron Polarization

Abstract

Total magnetic fields in spiral galaxies, as observed through their total synchrotron emission, are strongest (up to \simeq 30\mu G) in the spiral arms. The degree of radio polarization is low; the field in the arms must be mostly turbulent or tangled. Polarized synchrotron emission shows that the resolved regular fields are generally strongest in the interarm regions (up to \simeq 15\mu G), sometimes forming 'magnetic arms' parallel to the optical arms. The field structure is spiral in almost every galaxy, even in flocculent and bright irregular types which lack spiral arms. The observed large-scale patterns of Faraday rotation in several massive spiral galaxies reveal coherent regular fields, as predicted by dynamo models. However, in most galaxies observed so far no simple patterns of Faraday rotation could be found. Either many dynamo modes are superimposed and cannot be resolved by present-day telescopes, or most of the apparently regular field is in fact anisotropic random, with frequent reversals, due to shearing and compressing gas flows. In galaxies with massive bars, the polarization pattern follows the gas flow. However, around strong shocks in bars, the compression of the regular field is much lower than that of the gas; the regular field decouples from the cold gas and is strong enough to affect the flow of the diffuse warm gas. -- The average strength of the total magnetic field in the Milky Way is 6\mu G near the sun and increases to 20-40\mu G in the Galactic center region. The Galactic field is mostly parallel to the plane, except in the center region. Rotation measure data from pulsars indicate several field reversals, unlike external galaxies, but some reversals could be due to distortions of the nearby field.Comment: 18 pages, 9 figures. To be published in "Polarisation 2005" (Proc. of the conference held in Paris, 12-15 Sept. 2005), eds. F. Boulanger and M.A. Miville-Deschenes, EAS Publications Series; Two small typos corrected, one reference added and three updated 23/06/200

Constraining the regular Galactic magnetic field with the 5-year WMAP polarization measurements at 22 GHz

“…Finally, note that we do not expect changes in these results if we include a more refined treatment of the turbulent magnetic field in the analysis, since the polarized synchrotron emission comes from the regular pattern of the GMF, which is located in the inter-arms regions (Beck 2007). Indeed, we find that the results obtained with the other two methods for the noise determination (see Sect.…”

The large-scale magnetic field in the fourth Galactic quadrant

“…This is because a very non-uniform weighting of the line-of-sight component of the field may enhance local structure in the field, which can rob the concept of largescale field of its meaning. That is particularly relevant when the small-scale field is comparable to or stronger than the large-scale field, as is thought to be the case in the spiral arms (e.g., Beck 2007;Haverkorn et al 2008). This problem is exacerbated by field reversals, because a very uneven weighting of positive and negative field contributions may even lead to incorrect conclusions about the direction of the mean field.…”

“…In nearby galaxies, the large-scale magnetic field can be mapped fairly well (e.g., Beck 2007). However, because the observations in general still have fairly low linear resolution, structure on scales below a (few) hundred pc is not resolved, and any smaller-scale structure in the magnetic field is averaged out.…”

Joint 3D modelling of the polarized Galactic synchrotron and thermal dust foreground diffuse emission

“…Radio frequency information such as the Leiden 408 MHz and 1.4 GHz surveys (Brouw & Spoelstra 1976;Wolleben et al 2006), the Parkes survey at 2.4 GHz (Duncan et al 1999), and the MGLS survey (Medium Galactic Latitude Survey) at 1.4 GHz (Uyaniker et al 1999) are generally used to provide insight the Galactic diffuse synchrotron emission in polarized intensity. However, Faraday rotation introduces complications into the interpretation of such data since strong depolarization is expected for frequencies lower than 10 GHz (Burn 1966;Sun et al 2008;Jaffe et al 2010;Jansson et al 2009;La Porta et al 2008, 2006. Consequently, the best polarized Galactic diffuse synchrotron tracers are at high frequency such as the Wmap survey at 23 GHz (Page et al 2007).…”