| This is what the Galactic plasma towards the Perseus cluster looks like, with the line-of-sight B field as arrows, emission in yellow, and the NE2001 electron density in blue. The inset is a top-down view of the WSRT RM-cube of this area.
The strip at +6 rad m2 is the nearby Galactic foreground at 120-250 pc. Then comes emission at a higher Faraday depth in the west, increasing in Faraday depth towards the east due to an intervening ionized cloud at 0.5�0.5 pc.
The bends in the curves of the extragalactic background Faraday depth are caused by this same cloud. The extragalactic Faraday depth is systematically lower because the line-of-sight B field below the Perseus arm points away from us.
More details in Brentjens (2011).
|(c) Michiel Brentjens, ASTRON (2011)