Gaussian decomposition of $\ion{H}{i}$ surveys

Abstract

Aims. We investigate the properties of the 21-cm radio-lines of galactic neutral hydrogen by decomposing, the profiles of Leiden/Argentine/Bonn (LAB) Survey of Galactic H into Gaussian components. 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 H I 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 is discussed. Results. Considering only the simplest H I profiles, we demonstrate that for Gaussians with relatively small LSR velocities (-9 ≤ Vc < 4 kms -1 ) it is possible to distinguish three or four groups of preferred line-widths. The mean widths of these groups are FWHM = 3.9 ± 0.6, 11.8 ± 0.5, 24.1 ± 0.6, and 42 ± 5 km s -1 . 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 -1 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 -1 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 -1 is more obscure. These, however, contain only about 4% of the total observed column densities.