Turbulence measurements from HI absorption spectra

TL;DR

This study analyzes HI absorption spectra to quantify non-thermal line broadening, revealing turbulence characteristics consistent with Kolmogorov scaling and refining the understanding of the warm neutral medium's prevalence.

Contribution

It provides a new method to estimate non-thermal contributions in HI line widths, linking turbulence scaling to physical conditions in the interstellar medium.

Findings

Non-thermal line width scales as l^{0.7} for widths > 0.7 km/s

Turbulence in the Galactic HI follows Kolmogorov-like scaling

Less of the interstellar medium is in the warm phase than previously thought

Abstract

We use the millennium Arecibo 21 cm absorption-line survey measurements to examine the issue of the non-thermal contribution to the observed Galactic HI line widths. If we assume a simple, constant pressure model for the HI in the Galaxy, we find that the non-thermal contribution to the line width, v_{nt} scales as v^2_{nt} \propto l^{\alpha}, for v_{nt} larger than \sim 0.7 km s^{-1}. Here l is a derived length scale and \alpha \sim 0.7 \pm 0.1. This is consistent with what one would expect from a turbulent medium with a Kolmogorov scaling. Such a scaling is also predicted by theoretical models and numerical simulations of turbulence in a magnetized medium. For non-thermal line widths narrower than \sim 0.7 km s^{-1}, this scaling breaks down, and we find that the likely reason is ambiguities arising from Gaussian decomposition of intrinsically narrow, blended lines. We use the above estimate of the non-thermal contribution to the line width to determine corrected HI kinetic temperature. The new limits that we obtain imply that a significantly smaller (\sim 40% as opposed to 60%) fraction of the atomic interstellar medium in our Galaxy is in the warm neutral medium phase.