paper

Turbulence measurements from HI absorption spectra

arXiv:0803.2511 · doi:10.1111/j.1745-3933.2008.00473.x

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^α, for v_{nt} larger than \sim 0.7 km s^{-1}. Here l is a derived length scale and α\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.

5 pages, 7 figures. Corrected typos. Accepted for publication in MNRAS Letters. The definitive version will be available at http://www.blackwell-synergy.com

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