Sky projections of the data set used in this work, the associated observational errors and the source density. Panel a shows the Faraday data, panel b the observed error bars. Data points falling into the same pixel have been averaged. These plot has been made with a four-times decreased resolution (corresponding to an pixel area of 0.21 deg² or 7.56 × 10² arcmin²) compared to that of the inference in order to enable better visualization. Panel c shows the source density of the data set used in this work. The pixels in this map have an area of about 3.36 deg² or 1.2 × 10⁴ arcmin², again for visualization purposes. These, and all subsequent maps, are presented in Galactic coordinates centered at .