| One of the big open questions about our Milky Way Galaxy is the large-scale form and internal structure of its magnetic field. The Faraday depth of polarized radio emission is a relatively easily measurable quantity that is sensitive to the strength and direction of the magnetic field along a particular line of sight through the Galaxy. In principle, large numbers of Faraday depth measurements can therefore be used to outline the magnetic field of the Milky Way. But from our vantage point inside the Galaxy, it is difficult to see the forest for the trees!
Recently, a team led by Niels Oppermann (MPA, Garching) has built and analyzed a vast catalog of Faraday depth measurements, collected from many polarization studies throughout the literature (and several which are still unpublished). The team used a technique (called the "extended critical filter") from information field theory to reconstruct the underlying signal from the combination of all of these data points, and thereby build a map of the Galactic Faraday depth. Shown above (in units of rad m-2), the reconstructed Galactic Faraday depth sky is beautiful to behold, showing structures on a variety of angular scales! In fact the team used the procedure that generated this map to also determine the angular power spectrum of Faraday depth variations (after having subtracted out the inhomogeneous contribution to the variations, which are strongly dependent on Galactic latitude). The resulting power spectrum was used to derive an approximation of the 3D fluctuation spectrum of the Milky Way's magnetic field.
The paper describing this work is available online at this link, and complete information about the combined data set and resulting reconstructions (including an interactive map!) can be found here. The MPA has also published a press release that includes an extended discussion of the importance of this new map.
|Niels Oppermann et al.