Evidence for Line-of-Sight Frequency Decorrelation of Polarized Dust Emission in $Planck$ Data

Abstract

If a single line of sight (LOS) intercepts multiple dust clouds having different spectral energy distributions (SEDs) and magnetic field orientations, then the frequency scaling of each of the Stokes Q and U parameters of the thermal dust emission may be different, a phenomenon we refer to as LOS frequency decorrelation. We present first evidence for LOS frequency decorrelation in Planck data by using independent measurements of neutral hydrogen (Hi) emission to probe the 3D structure of the magnetized ISM. We use Hibased measurements of the number of clouds per LOS and the magnetic field orientation in each cloud to select two sets of sightlines: (i) a target sample of pixels that are likely to exhibit LOS frequency decorrelation and (ii) a control sample of pixels that lack complex LOS structure. We test the null hypothesis that LOS frequency decorrelation is not detectable in Planck 353 and 217 GHz polarization data at high Galactic latitudes. We find that the data reject the null hypothesis at high significance, showing that the combined effect of polarization angle variation with frequency and depolarization are detected in the target sample. This detection is robust against choice of CMB map and map-making pipeline. The observed change in polarization angle due to LOS frequency decorrelation is detectable above the Planck noise level. The probability that the detected effect is due to noise alone ranges from 5 × 10 −2 to 4 × 10 −7 , depending on the CMB subtraction algorithm and treatment of residual systematics; correcting for residual systematics consistently increases the significance of the effect. Within the target sample, the LOS decorrelation effect is stronger for sightlines with more misaligned magnetic fields, as expected. With our sample, we estimate that an intrinsic variation of ∼ 15% in the ratio of 353 to 217 GHz polarized emission between clouds is sufficient to reproduce the measured effect. Our finding underlines the importance of ongoing studies to map the three-dimensional structure of the magnetized and dusty ISM that could ultimately help component separation methods to account for frequency decorrelation effects in CMB polarization studies.