Statistical Properties of Galactic Starlight Polarization

Abstract

We present a statistical analysis of Galactic interstellar polarization from the largest compilation available of starlight data. The data comprises ~9300 stars, of which we have selected ~5500 for our analysis. We find a nearly linear growth of mean polarization degree with extinction. The amplitude of this correlation shows that interstellar grains are not fully aligned with the Galactic magnetic field, which can be interpreted as the effect of a large random component of the field. In agreement with earlier studies of more limited scope, we estimate the ratio of the uniform to the random plane-of-the-sky components of the magnetic field to be Bu/Br ≈ 0.8. Moreover, a clear correlation exists between polarization degree and polarization angle that provides evidence that the magnetic field geometry follows Galactic structures on large scales. The angular power spectrum Cℓ of the starlight polarization degree for Galactic plane data (|b| < 10°) is consistent with a power law, Cℓ ℓ-1.5 (where ℓ ≈ 180°/θ is the multipole order), for all angular scales θ 10'. An investigation of sparse and inhomogeneous sampling of the data shows that the starlight data analyzed traces an underlying polarized continuum that has the same power-spectrum slope, Cℓ ℓ-1.5. Our findings suggest that starlight data can be safely used for the modeling of Galactic polarized continuum emission at other wavelengths.