2021ApJ...921...11M

The physical properties of galactic halo gas have a profound impact on the life cycle of galaxies. As gas travels through a galactic halo, it undergoes dynamical interactions, influencing its impact on star formation and the chemical evolution of the galactic disk. In the Milky Way halo, considerable effort has been made to understand the spatial distribution of neutral gas, which is mostly in the form of large complexes. However, the internal variations of their physical properties remain unclear. In this study, we investigate the thermal and dynamical state of the neutral gas in high-velocity clouds. High-resolution observations (1.'1) of the 21 cm line emission in the EN field of the DHIGLS H I survey are used to analyze the physical properties of the bright concentration CIB located at an edge of a large HVC complex, complex C. We use the Gaussian decomposition code ROHSA to model the multiphase content of CIB and perform a power spectrum analysis to analyze its multiscale structure. The physical properties of some 200 structures extracted using dendrograms are examined. Each phase exhibits different thermal and turbulent properties. We identify two distinct regions, one of which has a prominent protrusion extending from the edge of complex C that exhibits an ongoing phase transition from warm diffuse gas to cold dense gas and filaments. The scale at which the warm gas becomes unstable and undergoes thermal condensation is about 15 pc, corresponding to a cooling time of about 1.5 Myr. Our study characterizes the statistical properties of turbulence in the fluid of an HVC for the first time. We find that a transition from subsonic to transonic turbulence is associated with the thermal condensation, going from large to small scales. A large-scale perspective of complex C suggests that hydrodynamic instabilities are involved in creating the structured concentration CIB and the phase transition therein. However, the details of the dynamical and thermal processes remain unclear and will require further investigation through both observations and numerical simulations.