A PRIMA Vista: Far-Infrared Polarimetric Observations to Unveil Small-Scale Magnetohydrodynamics in Galaxies

Magnetic fields are a fundamental component of the interstellar medium (ISM)—the gas and dust that fills the space between stars—and remain a challenge to characterize in our effort to understand the structure and evolution of galaxies over cosmic time. Studying these fields requires far-infrared (FIR) polarimetric observations, which uniquely trace the dynamics, magnetization, and morphology of the coldest, densest parts of the ISM—regions where most of the galaxy’s mass is located and where star formation actively occurs. We employ high-resolution cosmological magnetohydrodynamical (MHD) simulations of a Milky Way-like galaxy viewed face-on to investigate two key magnetic observables: (1) the alignment between magnetic fields and ISM density structures, and (2) the degree of magnetic turbulence—chaotic motions of the magnetic field—on ∼100 parsec (pc) scales. By systematically varying the initial magnetization in our models, we find that both the turbulence fraction and the alignment between magnetic fields and density structures decrease as magnetization increases. Stronger, more ordered magnetic fields are less turbulent and less aligned with the structure of the gas. We use these simulations to make predictions for the proposed PRobe far Infrared Mission for Astrophysics (PRIMA). Our results suggest that PRIMA’s observations of surface density and magnetic field orientation will be sensitive enough to resolve these magnetic features down to the smallest physical scales present in our simulations (∼10 pc), for galaxies ≤ 0.5 Mpc. We predict that PRIMA will also be able to detect variations in magnetic turbulence across different galactic environments and measure unresolved intrinsic magnetic field orientations to a precision of about 6◦. We conclude that PRIMA will be capable of distinguishing a wide range of galactic magnetizations, allowing us to more accurately characterize magnetic properties in galaxies, place stronger constraints on ISM and galaxy formation models, and ultimately deepen our understanding of how magnetic fields shape the Universe.