Line-of-sight extrapolation noise in dust polarization

Abstract

The B-modes of polarization at frequencies ranging from 50-1000 GHz are produced by Galactic dust, lensing of primordial E-modes in the cosmic microwave background (CMB) by intervening large scale structure, and possibly by primordial B-modes in the CMB imprinted by gravitational waves produced during inflation. The conventional method used to separate the dust component of the signal is to assume that the signal at high frequencies (e.g., 350 GHz) is due solely to dust and then extrapolate the signal down to a lower frequency (e.g., 150 GHz) using the measured scaling of the polarized dust signal amplitude with frequency. For typical Galactic thermal dust temperatures of ∼20K, these frequencies are not fully in the Rayleigh-Jeans limit. Therefore, deviations in the dust cloud temperatures from cloud to cloud will lead to different scaling factors for clouds of different temperatures. Hence, when multiple clouds of different temperatures and polarization angles contribute to the integrated line-of-sight polarization signal, the relative contribution of individual clouds to the integrated signal can change between frequencies. This can cause the integrated signal to be decorrelated in both amplitude and direction when extrapolating in frequency.Here we carry out a Monte Carlo analysis on the impact of this line-of-sight extrapolation noise, enabling us to quantify its effect. Using results from the Planck experiment, we find that this effect is small, more than an order of magnitude smaller than the current uncertainties. However, line-ofsight extrapolation noise may be a significant source of uncertainty in future low-noise primordial B-mode experiments. Scaling from Planck results, we find that accounting for this uncertainty becomes potentially important when experiments are sensitive to primordial B-mode signals with amplitude r 0.0015 .

Maps of the Number of H i Clouds along the Line of Sight at High Galactic Latitude

“…). Our determination of the average number of clouds per pixel differs drastically from that assumed in some of the first works mentioned previously (Planck Collaboration 2016a; Poh & Dodelson 2017). These large numbers of "layers" have been reduced in the physically motivated models of Ghosh et al When assuming three dust components, each associated with a discrete phase of the neutral ISM (CNM, WNM, and unstable neutral medium), Adak et al (2020) find that the northern Galactic polar region model lacks a necessary fourth component to account for all the observed dust.…”

“…The estimates differ drastically in the literature. In one of the first works assessing such effects, Poh & Dodelson (2017) studied the region around the north Galactic pole. They used two models: one with nine clouds per kiloparsec and another based on the 3D dust extinction map by Green et al (2015).…”

“…The observed signal could arise from multiple dust components (Tassis & Pavlidou 2015), each with different properties (composition, temperature, magnetic field orientation). A large effort is underway to understand the effects of assumed dust models on the retrieval of cosmological parameters (e.g., Remazeilles et al 2016;Poh & Dodelson 2017;Thorne et al 2017;Hensley & Bull 2018). Innovative, data-driven approaches are being developed to characterize the ISM properties (Clark et al 2015;Clark 2018;Philcox et al 2018;González-Casanova & Lazarian 2019;Zhang et al 2019).…”

Demonstration of Magnetic Field Tomography with Starlight Polarization toward a Diffuse Sightline of the ISM

“…However, the existence of these two strikingly different sources of polarized signal could manifest itself as a variation of the polarization angle as a function of frequency. This effect, which has a wellknown counterpart in the optical 13 (e.g., Serkowksi 1962;Treanor 1963;Coyne 1974;Martin 1974), has been pointed out by Tassis & Pavlidou (2015) and discussed in the context of CMB-foreground subtraction by various authors (e.g., Poh & Dodelson 2017;Hensley & Bull 2018;Planck Collaboration 2018c). For a significant difference between the polarization angle at different frequencies to occur, two conditions must be met: the magnetic fields of the two clouds must have significantly different orientations projected on the plane of the sky and the dust temperatures and/or dust emission spectral indices of the two clouds must not be identical.…”

Evidence for line-of-sight frequency decorrelation of polarized dust emission in Planck data

“…Line-of-sight frequency decorrelation can have a particularly pernicious effect on parametric component-separation methods working at the map level, especially if the SEDs of Stokes Q and U are not modeled with independent parameters (Poh & Dodelson 2017;Ghosh et al 2017;Puglisi et al 2017;Hensley & Bull 2018;Martínez-Solaeche et al 2018; CMB-S4 Collaboration 2020). New techniques employing moment decomposition (Chluba et al 2017) have shown promise for mitigating LOS averaging of dust SEDs in polarization at the expense of additional parameters (Mangilli et al 2019;Remazeilles et al 2020).…”