Bimodal Behavior and Convergence Requirement in Macroscopic Properties of the Multiphase Interstellar Medium Formed by Atomic Converging Flows

TL;DR

This study uses hydrodynamics simulations to explore how converging flows in the interstellar medium lead to different phases and properties, emphasizing the importance of resolution and initial conditions for accurate modeling.

Contribution

It demonstrates the necessity of high spatial resolution to capture the cooling length and reveals how initial density fluctuations influence post-shock turbulence and phase formation in the ISM.

Findings

Resolution of 0.02 pc is required for convergence of mean properties.

Higher initial density fluctuations (>10%) induce strong turbulence and limit CNM formation.

Lower fluctuations (<=10%) allow efficient CNM formation with less turbulence.

Abstract

We systematically perform hydrodynamics simulations of 20 km s^-1 converging flows of the warm neutral medium (WNM) to calculate the formation of the cold neutral medium (CNM), especially focusing on the mean properties of the multiphase interstellar medium (ISM), such as the average shock front position and the mean density on a 10 pc scale. Our results show that the convergence in those mean properties requires 0.02 pc spatial resolution that resolves the cooling length of the thermally unstable neutral medium (UNM) to follow the dynamical condensation from the WNM to CNM. We also find that two distinct post-shock states appear in the mean properties depending on the amplitude of the upstream WNM density fluctuation (= sqrt(<drho^2>)/rho_0). When the amplitude > 10 %, the interaction between shocks and density inhomogeneity leads to a strong driving of the post-shock turbulence of > 3 km s^-1, which dominates the energy budget in the shock-compressed layer. The turbulence prevents the dynamical condensation by cooling and the following CNM formation, and the CNM mass fraction remains as ~ 45 %. In contrast, when the amplitude <= 10 %, the shock fronts maintain an almost straight geometry and CNM formation efficiently proceeds, resulting in the CNM mass fraction of ~ 70 %. The velocity dispersion is limited to the thermal-instability mediated level of 2 - 3 km s^-1 and the layer is supported by both turbulent and thermal energy equally. We also propose an effective equation of state that models the multiphase ISM formed by the WNM converging flow as a one-phase ISM.