Irregular Magnetic Fields and the Far-Infrared Polarimetry of Dust Emission from Interstellar Clouds

Abstract

(abridged) The polarized thermal radiation at far infrared and submillimeter wavelengths from dust grains in interstellar clouds with irregular magnetic fields is simulated. We show that the polarization properties of quiescent clouds and of star-forming regions are consistent with Kolmogorov-like turbulent magnetic fields that are comparable in magnitude to the uniform component of the magnetic fields. If the beam size is much smaller than the correlation length $L_{\rm corr}$ of the fields, the calculated percentage polarization $p$ decreases to an asymptotic value when the number of correlation lengths $N_{\rm corr}$ through the cloud exceeds a few tens. For these values of $N_{\rm corr}$, the dispersion in the position angles $σ_α$ is still appreciable--decreasing to only about $20^{\circ}$. However, when the finite size of a telescope beam is taken into account, the asymptotic value of $p$ is reached for fewer correlation lengths (smaller $N_{\rm corr}$) due to averaging over the beam; $σ_α$ becames much smaller and consistent with the observational data. The smoothing of the polarization properties due to the combined effect of the thickness of the cloud and the finite size of the beam can be described by a single variable which we designate as the generalized number of correlation lengths. In addition, we study various factors that may contribute to the decrease in the linear polarization percentage with increasing intensity that is observed at submillimeter and far infrared wavelengths in many, though not in all, dark clouds (the "polarization hole" effect).