Dust Polarization From Starlight Data

We present a statistical analysis of the interstellar medium (ISM) polarization from the largest compilation available of starlight data, which comprises ∼ 5500 stars. The measured correlation between the mean polarization degree and extinction indicates that ISM dust grains are not fully aligned with the uniform component of the large-scale Galactic magnetic field. Moreover, we estimate the ratio of the uniform to the random plane-of-thesky components of the magnetic field to be Bu/Br ≈ 0.8. From the analysis of starlight polarization degree and position angle we find that the magnetic field broadly follows Galactic structures on large-scales. On the other hand, the angular power spectrum Cl of the polarization degree for Galactic plane data is found to be consistent with a power-law, Cl ∝ l−1.5 (where l≈ 180◦/θ is the multipole order), for angular scales θ > 10′. We argue that this data set can be used to estimate diffuse polarized emission at microwave frequencies.