PFDM Background Induces Systematic EVPA Deflection and Reduced Net Polarization in Kerr-like Photon Rings

The study deploys fully relativistic ray tracing through a PFDM-modified Kerr metric to generate polarized images, varying magnetic field geometry and the PFDM intensity k while tracking EVPA spirals and higher-order image contributions near the photon ring. Image-domain metrics show that increasing k suppresses polarized intensity and produces localized EVPA deflections that accumulate along null geodesics, effects absent in vacuum Kerr calculations. These shifts arise because PFDM alters both photon propagation and parallel transport of the polarization vector in the strong-field regime. The work directly links horizon-scale polarization observables to an additional matter degree of freedom, extending EHT analyses that previously treated only vacuum spacetimes. Comparison with published 2017 M87* constraints indicates that PFDM values above a modest threshold already produce detectable deviations in |m|_net and beta_2. Upcoming higher-sensitivity EHT and ngEHT arrays will tighten bounds by resolving finer azimuthal structure around the ring. Next-generation polarized observations at 230 GHz and 345 GHz will therefore serve as direct probes of dark-matter density profiles on horizon scales.