Turbulence measurements from H I 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 H I linewidths. If we assume a simple, constant pressure model for the H i in the Galaxy, we find that the non-thermal contribution to the linewidth, v_nt scales as v²_nt^OC/α, for v_nt 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 linewidths 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 linewidth to determine corrected H I kinetic temperature. The new limits that we obtain imply that a significantly smaller (^~40 per cent as opposed to 60 per cent) fraction of the atomic interstellar medium in our Galaxy is in the warm neutral medium phase.