Network of velocity-coherent filaments formed by supersonic turbulence in a very-high-velocity HI cloud

The warm neutral medium (WNM), which is mainly composed of atomic hydrogen (HI) gas with a temperature of a few thousand K, was thought to be subsonically/ transonically turbulent and diffuse without a distinct internal cloud structure. Here, we report HI 21 cm line observations of a very high- velocity (-330 km s-1 < VLSR < -250 km s-1) HI cloud (VHVC), using the Five-hundredmeterAperture Spherical radio Telescope (FAST) with unprecedented resolution and sensitivity. For the first time, such a VHVC is clearly revealed to be a supersonic WNM system consisting of an network of velocity-coherent HI filaments. The filaments are in the forms of slim curves, hubs, and webs, distributed in different layers within the position-position-velocity (ppv) data cube. The entire cloud has skewed log-normal probability distribution of column density and the filaments themselves show asymmetrical radial density profiles, indicating shock compression by supersonic magnetohydrodynamic (MHD) turbulence, as also confirmed by our MHD simulation (sonic Mach number Ms = 3 and Alfv´en Mach number MA = 1). This work suggests that hierarchical filaments can be established by shocks in a low-density WNM, where gravity is negligible, offering a viable pathway to structure formation in the earliest evolutionary phases of interstellar medium (ISM).