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

TL;DR

This study uses high-resolution simulations to analyze how turbulence and magnetic fields in different phases of the interstellar medium influence polarized dust emission, shedding light on the origin of the $EE/BB$ asymmetry observed in Galactic foregrounds.

Contribution

The paper provides the first detailed phase-dependent analysis of turbulence and magnetic alignment effects on polarized dust emission using 3D MHD simulations and synthetic observations.

Findings

UNM dust has polarization spectra consistent with Planck observations.

WNM and CNM produce steeper and shallower spectra, respectively.

Different ISM phases exhibit distinct $EE/BB$ ratios and anisotropy properties.

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-Alfv\'enic 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-Alfv\'enic 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.