On the structure of the turbulent interstellar atomic hydrogen. I- Physical characteristics

TL;DR

This study uses high-resolution simulations to analyze the statistical properties of turbulent interstellar atomic hydrogen, revealing structures and spectra consistent with observations and theoretical models, and highlighting the role of turbulence and phase interactions.

Contribution

It provides detailed numerical simulations of interstellar atomic hydrogen across multiple scales, offering new insights into the structure, mass spectrum, and turbulence characteristics of CNM and WNM phases.

Findings

Mass spectrum of CNM structures follows a power-law similar to CO clumps.

Velocity power spectrum aligns with Kolmogorov turbulence predictions.

Small-scale structures are linked to observed tiny features in the ISM.

Abstract

{We study in some details the statistical properties of the turbulent 2-phase interstellar atomic gas.{We present high resolution bidimensional numerical simulations of the interstellar atomic hydrogen which describe it over 3 to 4 orders of magnitude in spatial scales.}{The simulations produce naturally small scale structures having either large or small column density. It is tempting to propose that the former are connected to the tiny small scale structures observed in the ISM. We compute the mass spectrum of CNM structures and find that ${\cal N}(M) dM \propto M ^{-1.7} dM$, which is remarkably similar to the mass spectrum inferred for the CO clumps. We propose a theoretical explanation based on a formalism inspired from the Press & Schecter (1974) approach and used the fact that the turbulence within WNM is subsonic. This theory predicts ${\cal N}(M) \propto M ^{-5/3}$ in 2D and ${\cal N}(M) \propto M ^{-16/9}$ in 3D. We compute the velocity and the density power-spectra and conclude that, although the latter is rather flat, as observed in supersonic isothermal simulations, the former follows the Kolmogorov prediction and is dominated by its solenoidal component. This is due to the bistable nature of the flow which produces large density fluctuations even when the rms Mach number (of WNM) is not large. We also find that, whereas the energy at large scales is mainly in the WNM, at smaller scales, it is dominated by the kinetic energy of the CNM fragments.}