First look at the multiphase interstellar medium using synthetic observations from low-frequency Faraday tomography

Faraday tomography of radio polarimetric data below 200 MHz from the LOw Frequency ARray (LOFAR) has been providing new perspectives on the diffuse and magnetized interstellar medium (ISM). One aspect of particular interest is the unexpected discovery of Faraday-rotated synchrotron polarization associated with structures of neutral gas, as traced by atomic hydrogen (HI) and dust. Here, we present the first in-depth numerical study of these LOFAR results. We produced and analyzed comprehensive synthetic observations of low-frequency synchrotron polarization from magneto-hydrodynamical (MHD) simulations of colliding super shells in the multiphase ISM from the literature. Using an analytical approach to derive the ionization state of the multiphase gas, we defined five distinct gas phases over more than four orders of magnitude in gas temperature and density, ranging from hot, and warm fully ionized gas to a cold neutral medium. We focused on establishing the contribution of each gas phase to synthetic observations of both rotation measure and synchrotron polarized intensity below 200 MHz. We also investigated the link between the latter and synthetic observations of optically thin HI gas. We find that it is not only the fully ionized gas, but also the warm partially ionized and neutral phases that strongly contribute to the total rotation measure and polarized intensity. However, the contribution of each phase to the observables strongly depends on the choice of the integration axis and the orientation of the mean magnetic field with respect to the shell collision axis. A strong correlation between the HI synthetic data and synchrotron polarized intensity, reminiscent of the LOFAR results, is obtained with lines of sight perpendicular to the mean magnetic field direction. Our study suggests that multiphase modeling of MHD processes is needed in order to interpret observations of the radio sky at low frequencies. This work is a first step toward understanding the complexity of low-frequency synchrotron emission that will be soon revolutionized thanks to large-scale surveys with LOFAR and the Square Kilometre Array.