Turbulence measurements from HI absorption spectra
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, vnt scales as v2nt lα, for vnt larger than 0.7 km s-1. Here l is a derived length scale and α 0.7 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 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 ( 40% as opposed to 60%) fraction of the atomic interstellar medium in our Galaxy is in the warm neutral medium phase.