Gaussian decomposition of HI surveys. III. Local HI

Abstract

Context: To investigate the properties of the 21-cm radio-lines of galactic neutral hydrogen, the profiles of "The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI" are decomposed into Gaussian components. Aims: The width distribution of the obtained components is analysed and compared with similar studies by other authors. Methods: The study is based on an automatic profile decomposition algorithm. As the Gaussians obtained for the complex HI profiles near the galactic plane cannot be directly interpreted in terms of the properties of gas clouds, we mainly study the selected simplest profiles in a limited velocity range. The selection criteria are described and their influence on the results discussed. Results: Considering only the simplest HI profiles, we demonstrate that for Gaussians with relatively small LSR velocities (-9 <= V_C <= 4 km/s) it is possible to distinguish three or four groups of preferred line-widths with mean widths FWHM = 3.9 +- 0.6, 11.8 +- 0.5, 24.1 +- 0.6, and 42 +- 5 km/s. Verschuur previously proposed similar line-width regimes, but with somewhat larger widths. He used a human-assisted decomposition for a nearly 50 times smaller database and we discuss systematic differences in analysis and results. Conclusions: The line-widths of about 3.9 and 24.1 km/s are well understood in the framework of traditional models of the two-phase interstellar medium. The components with the widths around 11.8 km/s indicate that a considerable fraction (up to about 40%) of the HI gas is thermally unstable. The reality and the origin of the broad lines with the widths of about 42 km/s is more obscure. These, however, contain only about 4% of the total observed column densities.