Sky projections of all datasets used in this work. These are pulsar DMs from the ATNF catalog (panel a; Manchester et al. 2005), RMs stemming from the van Eck catalog (panel b; Van Eck et al. 2023) and the EMs calculated from free–free (panel c; Planck Collaboration X 2016) and H-α (panel d; Finkbeiner 2003) emission. The latter two datasets are shown on a logarithmic scale. We show the quality masks employed for the EM data (see the discussion in Sect. 2.3.2) in panels (e) and (f), with 0 indicating a masked pixel. These and all of the following sky maps are presented in Galactic coordinates centered at (l, b) = (0°, 0°), with longitude increasing to the left.