Polarimetry in the Visible and Infrared: Application to CMB Polarimetry

Interstellar polarization from aligned dust grains can be measured both in transmission at visible and near-infrared wavelengths and in emission at far-infrared and sub-mm wavelengths. These observations can help predict the behavior of foreground contamination of CMB polarimetry by dust in the Milky Way. Fractional polarization in emission from aligned dust grains will be at the higher range of currently observed values of 4-10%. Away from the galactic plane, fluctuations in Q and U will be dominated by fluctuations in intensity, and less influenced by fluctuations in fractional polarization and position angle. 1. Some Basics Polarization due to magnetically aligned dust grains in the Milky Way galaxy is subject to both coherent and random processes (see Jones, 1996 for a review of grain alignment). Most observations of interstellar polarization have been at visual wavelengths, with a modest contribution at nearinfrared (NIR) wavelengths. At these wavelengths the polarization is due to selective extinction by aspherical dust grains. In the far-infrared (FIR), these same grains are in emission. This emission is polarized and consequently a foreground contaminant for CMB polarimetry. A few simple facts about interstellar polarization should be noted. • Fractional polarization P is very probably NOT a function of magnetic field strength (Jones, Klebe & Dickey 1991). • Polarization in emission (Pe) behaves differently than polarization in transmission (Pa). • Pa starts low, then increases with increasing optical depth and contains information on the random component of the aligning magnetic field. • Pe starts high, then decreases with increasing optical depth. CMB polarimetry will sample the low optical depth regime. CMB polarization measurements will be concerned with contamination from fluctuations in Stokes vectors Q and U caused by variations in interstellar polarization in emission. Recall that Q and U are intensities, not fractions. Specifically: