Turbulence measurements in the neutral ISM from Hi-21 cm emission-absorption spectra

TL;DR

This study investigates the relationship between non-thermal velocity dispersion and spatial scale in the neutral interstellar medium, revealing a break at 0.40 pc and indicating turbulence characteristics influenced by cloud phases and shock dominance.

Contribution

It provides the first detailed analysis of turbulence scaling laws in the neutral ISM across different cloud phases using Hi emission-absorption spectra.

Findings

Power-law correlation between velocity dispersion and scale above 0.40 pc

Steeper than Kolmogorov turbulence law in the cold neutral medium

Distinct turbulence properties in CNM and UNM phases

Abstract

We study the correlation between the non-thermal velocity dispersion ($\sigma_{\rm nth}$) and the length-scale (L) in the neutral interstellar medium (ISM) using a large number of Hi gas components taken from various published Hi surveys and previous Hi studies. We notice that above the length-scale ($L$) of 0.40 pc, there is a power-law relationship between $\sigma_{\rm nth}$ and $L$. However, below 0.40 pc, there is a break in the power-law, where $\sigma_{\rm nth}$ is not significantly correlated with $L$. It has been observed from the Markov chain Monte Carlo (MCMC) method that for the dataset of $L > 0.40$ pc, the most probable values of intensity ($A$) and power-law index ($p$) are 1.14 and 0.55 respectively. Result of $p$ suggests that the power-law is steeper than the standard Kolmogorov law of turbulence. This is due to the dominance of clouds in the cold neutral medium. This is even more clear when we separate the clouds into two categories: one for $L$ is > 0.40 pc and the kinetic temperature ($T_k$ ) is < 250 K, which are in the cold neutral medium (CNM) and for other one where L is > 0.40 pc and T k is between 250 K and 5000 K, which are in the thermally unstable phase (UNM). Most probable values of $A$ and $p$ are 1.14 and 0.67 respectively in the CNM phase and 1.01 and 0.52 respectively in the UNM phase. A greater number of data points is effective for the UNM phase in constructing a more accurate estimate of $A$ and $p$, since most of the clouds in the UNM phase lie below 500 K. However, from the value of $p$ in the CNM phase, it appears that there is a significant difference from the Kolmogorov scaling, which can be attributed to a shock-dominated medium.