Map of M51 showing the spatial distribution of the best-fit model from directly fitting to Stokes Q and U. Sight lines with Q U vs. λ² behaviors best fitted by a uniform external Faraday screen are denoted by the white pixels. Sight lines with Q U vs. λ² behaviors best fitted by an inhomogeneous external Faraday screen are denoted by the black pixels. Pixels with insufficient signal-to-noise detection in polarization have been blanked (gray).