Corrected standard deviation of RMs in bins of electron column density, for sources detected at a < 2 R₅₀₀. The bins for the full sample have equal frequency with 71 sources per bin, while the bins for the sub-samples are defined in logarithmically spaced bins to sample similar densities. The horizontal error bars indicate the bins, and points are plotted at the median N_e. The bottom panel shows the magnetic field estimate assuming Equation (12), with a correlation length that is a factor 100 smaller than the line-of-sight distance, resulting in a mean strength of ∼2 μG.