Statistical Properties of Galactic Starlight Polarization

We present a statistical analysis of Galactic interstellar polarization fromthe largest compilation available of starlight data. The data comprises ~ 9300stars of which we have selected ~ 5500 for our analysis. We find a nearlylinear growth of mean polarization degree with extinction. The amplitude ofthis correlation shows that interstellar grains are not fully aligned with theGalactic magnetic field, which can be interpreted as the effect of a largerandom component of the field. In agreement with earlier studies of morelimited scope, we estimate the ratio of the uniform to the randomplane-of-the-sky components of the magnetic field to be B_u/B_r = 0.8.Moreover, a clear correlation exists between polarization degree andpolarization angle what provides evidence that the magnetic field geometryfollows Galactic structures on large-scales. The angular power spectrum C_l ofthe starlight polarization degree for Galactic plane data (|b| < 10 deg) isconsistent with a power-law, C_l ~ l^{-1.5} (where l ~ 180 deg/\theta is themultipole order), for all angular scales \theta > 10 arcmin. An investigationof sparse and inhomogeneous sampling of the data shows that the starlight dataanalyzed traces an underlying polarized continuum that has the same powerspectrum slope, C_l ~ l^{-1.5}. Our findings suggest that starlight data can besafely used for the modeling of Galactic polarized continuum emission at otherwavelengths.Comment: 31 pages, 11 figures. Minor corrections and some clarifications included. Matches version accepted for publication by the Astrophysical Journa