Variance of dust temperature and spectral index in Planck polarization data using spin-moment expansion

Thermal dust is the major polarized foreground hindering the detection of primordial cosmic microwave background (CMB) B-modes. Its signal exhibits complex behavior in frequency space, arising from the combined variation in our Galaxy of the orientation of magnetic fields and the spectral properties of dust grains aligned with magnetic field lines. In this work, we present a new framework for analyzing the thermal dust signal using polarized microwave data. We introduce residual maps, represented as complex quantities, which capture deviations of the local polarized spectral energy distribution (SED) from the mean complex SED averaged over the sky mask. We present simple predictions that relate the values of the statistical correlation and covariances between the residual maps to the physical properties of the emitting aligned grains. Testing these predictions provides valuable information about the nature of the dust signal. We evaluated our predictions using Planck data over a 97% mask excluding the inner Galactic plane. Despite its simplicity, our model captures a significant part of the statistical properties of the data. For the SRoll2 version of the data, the spectral dependence of the covariances between residual maps is compatible with a dust model that includes only temperature variations rather than spectral index variations. In contrast, for the PR4 Planck official release, it is incompatible with both models. Our methodology can be used to analyze future high-precision polarization data and to build more accurate dust models for use by the CMB community.

💡 Research Summary

The paper addresses one of the most pressing challenges for primordial CMB B‑mode searches: the polarized thermal dust foreground. While the mean dust spectral energy distribution (SED) in high‑latitude sky is well described by a modified black‑body (MBB), spatial variations in dust temperature (T), spectral index (β), and magnetic‑field geometry introduce non‑trivial frequency dependence in both polarized intensity and angle. To capture these effects, the authors develop a novel framework based on complex‑valued residual maps and a spin‑moment expansion of the polarized SED.

The methodology starts by defining a complex polarized intensity Pν = Qν + iUν for each frequency ν. For each sky pixel (or beam) the observed Pν is the line‑of‑sight integral of many local contributions dPν, each assumed to follow an MBB with local T and β (hypothesis HA1). By factoring out a sky‑wide pivot SED εν (evaluated at fixed T₀, β₀) they construct residual maps R_i = P_i/ε_i − ⟨P_i/ε_i⟩_sky, which encode deviations caused by spatial fluctuations of T or β and by magnetic‑field structure.

A Taylor expansion of the emissivity εν(T,β) around the pivot yields a series Pν = P₀ εν