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

Abstract

Context. The knowledge of the regular (large scale) component of the Galactic magnetic field gives important information about the structure and dynamics of the Milky Way, and also constitutes a basic tool to determine cosmic ray trajectories. It can also provide clear windows where primordial magnetic fields could be detected. Aims. We aim to obtain the regular (large scale) pattern of the magnetic field distribution of the Milky Way that better fits the polarized synchrotron emission as seen by the WMAP satellite in the 5 years data at 22 GHz. Methods. We have done a systematic study of a number of Galactic magnetic field models: axisymmetric (with and without radial dependence on the field strength), bisymmetric (with and without radial dependence), logarithmic spiral arms, concentric circular rings with reversals and bi-toroidal. We have explored the parameter space defining each of these models using a grid-based approach. In total, more than one million models were computed. The model selection was done using a Bayesian approach. For each model, the posterior distributions were obtained and marginalized over the unwanted parameters to obtain the marginal (one-parameter) probability distribution functions. Results. In general, axisymmetric models provide a better description of the halo component, although with regard to their goodnessof-fit, the other models cannot be rejected. In the case of the disk component, the analysis is not very sensitive for obtaining the disk large-scale structure, because of the effective available area (less than 8% of the whole map and less than 40% of the disk). Nevertheless, within a given family of models, the best-fit parameters are compatible with those found in the literature. Conclusions. The family of models that better describes the polarized synchrotron halo emission is the axisymmetric one, with magnetic spiral arms with a pitch angle of ≈24 • , and a strong vertical field of 1 μG at z ≈ 1 kpc. When a radial variation is fitted, models require fast variations.

Planck2015 results

“…Although the precise geometry of these magnetic fields remains uncertain (see, e.g., Ruiz-Granados et al 2010;Jansson & Farrar 2012), the Galactic Faraday depth could be a foreground for the primordial Faraday depth, at least on large scales.…”

Planckintermediate results. XIX. An overview of the polarized thermal emission from Galactic dust

“…In that sense, it is distinct from structure functions often used to describe polarization direction (see Serkowski 1958;Kobulnicky et al 1994;Hildebrand et al 2009) which do not depend on position. Note also that different ways of characterizing the field geometry have been used in the literature, such as the normalized spatial gradient of the polarized intensity (|∇P|/P) discussed in Burkhart et al (2012) and used in Iacobelli et al (2014).…”

“…Some of these structures were detected in polarized light at other wavelengths. For instance, the Fan, Perseus, and Loop I regions seem to have counterparts detected in polarized thermal dust and synchrotron emission, as well as in the WMAP foreground emission (Gold et al 2011;Ruiz-Granados et al 2010;Jansson & Farrar 2012a, and references therein) and in Faraday depth surveys of polarized emission at radio frequencies, such as the Global Magneto-Ionic Medium Survey (GMIMS; Wolleben et al 2010b). In particular, from the Faraday depth data of GMIMS, a significant portion (about 5%) of the sky was found to be dominated by the magnetic field around a nearby H i bubble (at a distance of 100 pc) in the general direction of the Loop I region described above (Wolleben et al 2010a).…”

“…Our knowledge and understanding of the GMF has improved considerably over the past few years, as a result of both progress in the quality (sensitivity and resolution) of radio observations and extensive modelling efforts (e.g., Sun et al 2008;Sun & Reich 2010;Ruiz-Granados et al 2010;Jaffe et al 2010Jaffe et al , 2011Pshirkov et al 2011;Fauvet et al 2012Fauvet et al , 2013Jansson & Farrar 2012a,b). However, the existing radio observations have inherent limitations, as both Faraday rotation measures (RMs) and synchrotron (total and polarized) intensities are quantities integrated over the line of sight (LOS), which depend on the poorly constrained density distributions of thermal and relativistic electrons, respectively.…”

Magnetic Fields in the Milky Way

“…The set of papers which fit magnetic field models to radio polarization data at high frequencies (≥ 22 GHz WMAP data) ( [54,39]) tend to find a higher pitch angle than rotation measure studies of ∼ 24 • −30 • . Planck all-sky maps will provide additional observational constraints, which simulations show suggest the same high pitch angles [86].…”