Origin of realistic magnetized cold neutral media in multiphase interstellar media : A simulation perspective

The cold neutral media (CNM) is perhaps the most important astrophysical observable in the last decade. Observations like HI4PI, GALFA and FAST have advanced our understanding of CNM, including its spatial distribution, alignment to B-field and its connection to underlying molecular phase. However, why fundamentally HI emission map is highly filamentary along B-field is still a mystery. Different proposals such as the density perturbations, MHD turbulence and reconnection are proposed recently. However, simulations suggest that CNM are significantly shorter in length and less aligned to B-field. In this talk, we demonstrate our recent high-resolution simulations on how CNM forms and its physical properties both spatially and dynamically. In particular, we showed that (1) CNM filaments are subjected to the bounding forces from the warmer, unstable phases, which restricts its aspect ratio to half of the GS95 estimate. (2) Despite the CNM does not follow the GS95 cascade, statistically CNM still follows the Kolmogorov cascade. (3) The high density CNM features that are perpendicular to the magnetic field lines are the progenitors of the star formation sites. This work will update the lack of understanding of UNM since the proposal of two-phase model by Mckee & Ostriker (1977).