Dust Polarization Fraction

Thermal emission from elongated dust grains aligned with the Galactic magnetic field produces linearly polarized far-infrared and submillimeter emission that serves as both a CMB foreground contaminant and a probe of the Galactic magnetic field topology. The observed dust polarization fraction — the ratio of polarized to total dust emission intensity — varies across the sky and reaches maximum values of 20–25 percent in regions of coherent magnetic field, while typically averaging 5–10 percent across most of the sky. Standard grain alignment models, in which radiative torques align grains with their long axes perpendicular to the local magnetic field direction, predict polarization fractions consistent with observations in most environments. However, systematic discrepancies appear in dense molecular clouds where the observed polarization fraction drops below model predictions, and in certain diffuse regions where the polarization fraction exceeds the maximum expected for a uniform grain alignment efficiency — suggesting either more perfectly aligned grain populations than models produce or multiple alignment mechanisms operating simultaneously.

Successive Collision Theory provides a physical basis for the anomalously high dust polarization fractions in certain diffuse regions through the angular momentum inheritance mechanism applied to grain dynamics. In SCT, the magnetic field of the Milky Way inherits its large-scale organization from the angular momentum field of the collision debris, producing magnetic field lines with coherence lengths much larger than those generated by pure small-scale turbulent dynamo action. The dust grains aligning with these extended coherent field regions achieve higher polarization fractions than grains in a randomly turbulent field because the integration along the line of sight accumulates contributions from grains with nearly identical alignment directions rather than partially canceling contributions from grains with a range of field directions. The SCT prediction is that the highest dust polarization fractions should occur along lines of sight that are most closely perpendicular to the inherited angular momentum axis — the same axis as the CMB quadrupole-octupole alignment — and that a large-scale map of dust polarization fraction should show a coherent pattern aligned with the collision geometry.

The pre-existing grain populations from the colliding pockets include grain species that were processed through radiative environments in the prior epoch — hot stellar atmospheres, supernovae, and AGN radiation fields — that may have produced grains with intrinsically higher electric dipole moments or more efficient radiative torque coupling than grains formed from standard post-collision stellar evolution. These pre-existing grains, distributed along the angular momentum strata of the debris field, would exhibit anomalously high alignment efficiencies in the diffuse ISM where the radiation field is gentler than the environments that shaped them, producing higher polarization fractions than standard grain models calibrated on locally formed grains predict. SCT therefore motivates the development of dust polarization models that include a pre-existing grain component with enhanced alignment properties alongside the standard post-collision grain population.