Planck dust polarization power spectra are consistent with strongly supersonic turbulence

Abstract

The polarization of the Cosmic Microwave Background (CMB) is rich in information but obscured by foreground emission from the Milky Way's interstellar medium (ISM). To uncover relationships between the underlying turbulent ISM and the foreground power spectra, we simulated a suite of driven, magnetized, turbulent models of the ISM, varying the fluid properties via the sonic Mach number, $\mathcal{M_S}$ and magnetic (Alfvén) Mach number,$\mathcal{M_A}$. We measure the power spectra of density ($ρ$), velocity ($v$), magnetic field ($H$), total projected intensity ($T$), parity-even polarization ($E$), and parity-odd polarization ($B$). We find that the slopes of all six quantities increase with $\mathcal{M_S}$ Most increase with $\mathcal{M_A}$, while the magnetic field spectrum steepens with $M_A$. By comparing spectral slopes of $E$ and $B$ to those measured by Planck, we infer typical values of $\mathcal{M_S}$ and $\mathcal{M_A}$ for the ISM. As the fluid velocity increases, $\mathcal{M_S}> 4$, the ratio of BB power to EE power increases to approach a constant value near the Planck-observed value of $\sim 0.5$, regardless of the magnetic field strength. We also examine correlation-coefficients between projected quantities, and find that $r^{\rm{TE}}\approx 0.3$, in agreement with Planck, for appropriate combinations of $\mathcal{M_S}$ and $\mathcal{M_A}$. Finally, we consider parity-violating correlations $r^{\rm{TB}}$ and $r^{\rm{EB}}.$