The latitude dependence of the rotation measures of NVSS sources

In this Letter I use the variation of the spread in rotation measure (RM) with Galactic latitude to separate the Galactic from the extragalactic contributions to RM. This is possible since the latter does not depend on Galactic latitude. As input data I use RMs from the catalogue by Taylor, Stil & Sunstrum, supplemented with published values for the spread in RM ('sigma(RM)') in specific regions on the sky. I test four models of the free-electron column density (which I will abbreviate to 'DM infinity') of the Milky Way, and the best model builds up DM infinity on a characteristic scale of a few kpc from the Sun. sigma(RM) correlates well with DM infinity. The measured sigma(RM) can be modelled as a Galactic contribution, consisting of a term sigma(RM,MW) that is amplified at smaller Galactic latitudes as 1/sin vertical bar b vertical bar, in a similar way to DM infinity, and an extragalactic contribution, sigma(RM,EG), that is independent of latitude. This model is sensitive to the relative magnitudes of sigma(RM,MW) and sigma(RM,EG), and the best fit is produced by sigma(RM,MW) approximate to 8 rad m(-2) and sigma(RM,EG) approximate to 6 rad m(-2). The four published values for sigma(RM) as a function of latitude suggest an even larger sigma(RM,MW) contribution and a smaller sigma(RM,EG). This result from the NVSS RMs and published sigma(RM) shows that the Galactic contribution dominates structure in RM on scales between about 1 degrees and 10 degrees on the sky. I work out which factors contribute to the variation of sigma(RM) with Galactic latitude, and show that the sigma(RM,EG) I derived is an upper limit. Furthermore, to explain the modelled sigma(RM,MW) requires that structure in < B-parallel to > has a 1 sigma spread less than or similar to 0.4 mu G.