Haud & Kalberla (2007) – Gaussian decomposition of H I surveys. III. Local H I

Aims:We investigate the properties of the 21-cm radio-lines of galactic neutral hydrogen by decomposing, the profiles of “The Leiden/Argentine/Bonn (LAB) Survey of Galactic H I” 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 km s-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 H I 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.

Haud, U.; Kalberla, P. M. W.
2007, Astronomy and Astrophysics, 466, 555-564