Galactic Dust Polarization in Turbulent Multiphase ISM: On the Origin of the $EE/BB$ Asymmetry

TL;DR

This work investigates the origin of the EE/BB asymmetry in Galactic dust polarization by using high-resolution (2048^3) MHD simulations of a turbulent multiphase ISM, coupled with RAT-based dust polarization modeling. By separating the WNM, UNM, and CNM, the study reveals phase-dependent turbulence, anisotropy, and alignment with the magnetic field that shape the E- and B-mode spectra, and it shows that UNM closely reproduces Planck-like EE/BB ratios (~2) while CNM reduces the ratio and causes spectral flattening at small scales. The results implicate UNM as the dominant contributor to polarized foregrounds in high-latitude regions and provide frequency-stable predictions at 150 GHz to aid foreground separation for future CMB B-mode experiments. The analysis highlights the critical role of phase-resolved turbulence and magnetic alignment in interpreting dust polarization and refining component-separation strategies.

Abstract

Polarized thermal emission from Galactic dust is the dominant foreground for CMB polarization measurements at high frequencies, with its statistical properties set by the interplay between turbulence and magnetic fields in the multiphase interstellar medium (ISM). Variations in turbulence regime and density-magnetic-field alignment across the warm (WNM), unstable (UNM), and cold (CNM) neutral media should imprint distinct signatures on the power spectra and $EE/BB$ power ratio, yet the relative phase contributions remain poorly constrained. Using high-resolution 3D magnetohydrodynamic simulations of a turbulent multiphase ISM coupled with synthetic dust polarization maps, we quantify phase-dependent turbulence, anisotropy, and alignment properties. We find that the trans-Alfvénic and transonic WNM and UNM are strongly anisotropic, exhibiting tight alignment of density and velocity structures with the local magnetic field. In contrast, the super-Alfvénic and supersonic CNM displays reduced anisotropy and weak alignment. These dynamical differences are reflected in the statistical scaling of fluctuations: the square root of the second-order velocity structure function exhibits a slope near $1/3$ in the WNM, near $1/2$ in the CNM, and intermediate in the UNM. Our synthetic observations reproduce the polarization power spectra measured by Planck. We find that polarization from UNM dust yields spectral indices most consistent with Planck, whereas WNM and CNM dust produce steeper and shallower spectra, respectively. The WNM yields $EE/BB>2$, the UNM gives $EE/BB\sim2$, and the CNM yields $EE/BB\approx1$. These results indicate that UNM dust could be the dominant contributor to the polarized foreground. We present predictions at 150 GHz to improve foreground separation.

Figures (10)

• Figure 1: Distributions of number density $n$, temperature $T$, and turbulent plasma beta $\beta_{\rm tur}$ in the multiphase ISM simulation. Shown is a two-dimensional slice from the $2048^3$ simulation, displaying the number density (top), temperature (middle), and turbulent plasma beta $\beta_{\rm tur} = \rho v^2 / (B^2 / 8\pi)$ (bottom), where $\rho$ is the gas mass density and $v$ is the local turbulent velocity. For visualization purposes, only half of each slice is shown. The cold neutral medium, characterized by low temperature, is spatially correlated with the densest regions.
• Figure 2: Panels (a), (b), and (c): Distributions of the relative angle $\theta_\rho$, $\theta_v$, and $\theta_B$ between the local magnetic field and the rotated (by 90 degrees) density gradient $\nabla\rho$, velocity gradient $\nabla v$, and magnetic field strength gradient $\nabla B$, respectively. The relative angles are evaluated separately for the warm neutral medium (WNM; $T > 5000$ K), unstable neutral medium (UNM; $200$ K $< T <$ 5000 K), and cold neutral medium (CNM; $T < 200$ K). Here, a relative angle of $0^\circ$ corresponds to parallel alignment, while $90^\circ$ indicates perpendicularity. Panel (d): Distribution of the sonic Mach number $M_{\rm s} = v / c_s$ in each phase. Panel (e): Distribution of the thermal plasma beta, $\beta_{\rm th} = (\rho c_s^2/\gamma) / (B^2 / 8\pi)$. Panel (f): Distribution of the turbulent plasma beta, $\beta_{\rm tur} = \rho v^2 / (B^2 / 8\pi)$. All quantities are phase-separated based on temperature and computed in three dimensions.
• Figure 3: Statistical properties of scale-dependent fluctuations of velocity (1st column), magnetic field (2nd column), and number density (3rd column) in different ISM phases. The fluctuations are computed as the square root of the second-order structure function for each quantity, shown separately for the WNM ($T > 5000$ K), UNM ($200$ K $< T <$ 5000 K), CNM ($T < 200$ K), and the total ISM (including all phases). Dashed and dash-dotted reference lines indicate slopes of 1/3 and 1/2, corresponding to Kolmogorov and Burgers turbulence scaling, respectively. The numerical dissipation scale $r_{\rm dis}$ is around 0.5 pc.
• Figure 4: Statistical properties of scale-dependent velocity and magnetic field fluctuations decomposed into components parallel and perpendicular to the local magnetic field in different ISM phases. Fluctuations are computed as the square root of the second-order structure function. They are shown separately for the WNM, UNM, CNM, and the total ISM (including all phases). Dashed and dash-dotted grey lines represent reference slopes of 1/3 and 1/2, corresponding to Kolmogorov and Burgers turbulence scalings, respectively. The numerical dissipation scale $r_{\rm dis}$ is around 0.5 pc.
• Figure 5: Distributions of the Stokes parameters $I$ (left), $Q$ (middle), and $U$ (right) for synthetic dust polarization. We consider two distinct frequencies: 353 GHz (top), corresponding to Planck observations, and 150 GHz (bottom), a primary target for future CMB experiments.