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 (‘σRM’) in specific regions on the sky. I test four models of the free-electron column density (which I will abbreviate to ‘DM∞’) of the Milky Way, and the best model builds up DM∞ on a characteristic scale of a few kpc from the Sun. σRM correlates well with DM∞. The measured σRM can be modelled as a Galactic contribution, consisting of a term σRM,MW that is amplified at smaller Galactic latitudes as 1/sin |b|, in a similar way to DM∞, and an extragalactic contribution, σRM,EG, that is independent of latitude. This model is sensitive to the relative magnitudes of σRM,MW and σRM,EG, and the best fit is produced by σRM,MW≈ 8 rad m−2 and σRM,EG≈ 6 rad m−2. The four published values for σRM as a function of latitude suggest an even larger σRM,MW contribution and a smaller σRM,EG. This result from the NVSS RMs and published σRM shows that the Galactic contribution dominates structure in RM on scales between about 1° and 10° on the sky. I work out which factors contribute to the variation of σRM with Galactic latitude, and show that the σRM,EG I derived is an upper limit. Furthermore, to explain the modelled σRM,MW requires that structure in 〈B∥〉 has a 1σ spread ≲0.4 μG.