PRIMA Vista: far-infrared polarimetry to unveil small-scale magnetohydrodynamics in extragalactic observations

Magnetic fields are a fundamental part of the interstellar medium (ISM) and remain a challenge for building a comprehensive understanding of galactic properties. Their study requires far-infrared polarimetric observations, which provide an unrivaled probe of the dynamics, magnetization, and structure of the coldest and densest interstellar gas and dust at small scales in galaxies, where mass and star formation reside. We use high-resolution magnetohydrodynamical simulations of a face-on Milky Way-like galaxy and show that the alignment of magnetic fields with ISM structures and the turbulence at 100 pc scales decrease with increasing magnetization. We make predictions for extragalactic observations by the proposed PRobefarInfrared Mission for Astrophysics (PRIMA) telescope, comparing them with Stratospheric Observatory For Infrared Astronomy (SOFIA) observations similar to those of the Survey of extragALactic magnetiSm with SOFIA (SALSA). PRIMA will be able to better measure magnetic alignment trends inaccessible by SOFIA. We find that PRIMA will better sample magnetic turbulence, especially in dense environments, and will be able to measure the unresolved intrinsic magnetic field orientations to approximately 6 deg precision. PRIMA will also be capable of resolving observables such as the polarized fraction or the magnetic alignment down to scales comparable to the resolution of our simulations (about 10 pc) for galaxies up to 0.5 Mpc away. The polarization-dispersion relation shows that PRIMA observations will suffer from significantly reduced beam depolarization. Furthermore, PRIMA will recover the correlation between increasing the magnetic alignment parameter and local polarization fraction. Overall, observations of local galaxies with PRIMA will better characterize interstellar magnetism and constrain ISM and galaxy models, advancing our understanding of magnetism in the universe.