Abstract
The Leiden/Dwingeloo Survey (LDS) offers new possibilities for analyzing Galactic Hi with an outstanding sensitivity. The survey data have been carefully corrected for side-lobe contamination of the antenna and for baseline effects. At present this survey is the most reliable database for analysis of faint, large-scale Hi features. Together with the longstanding dispute whether the Galactic halo is hot or cold, this motivated our investigations which are described in this paper.
We have improved the stray-radiation correction procedure significantly by including reflections from the ground. Hi-gas with an unusually large velocity dispersion (LVD) is revealed when these enhanced LDS Data are massively integrated. Gaussian decomposition of more than 250 integrated profiles for b > 20° yields a complete set of 8500 Hi-components representing the north galactic sky on 10° × 10° fields. LVD components were found in every direction of the sky having a characteristic dispersion of ≥ 60 kms-1 and column densities of ≥ 1.4 · 1019 cm-2. We do not detect the Hi-gas which is associated with the "Lockman-Layer" (σ ∼ 35 kms-1) and conclude therefore that his analysis was biased by instrumental effects. Correction of this bias in the Bell Survey data set makes the "Lockman-Layer" disappear and does show the LVD component reported here.
The LVD gas exhibits minimal sub-rotation and extends several kpc into the Galactic halo. Its scale height is calculated to be ≥ 2 kpc and a non-uniform distribution with respect to the distance from the Galactic center is found. The distribution of the LVD gas is presented. Theoretical spectra are calculated from a simple model of the LVD halo and compared to the real data. The LVD gas seems to be a very sensitive indicator of violent disk phenomena. Two possible Galactic chimneys and a peculiar local velocity field are identified. Implications for the modelling of a Galactic halo with various components are discussed. Strict application of the principle, that the turbulent gas pressure plus magnetic and cosmic ray pressure equals the gravitational pull, leads to a stable halo, extending up to z = 3.3 kpc.
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Westphalen, G., Kalberla, P.M., Hartmann, D. et al. Large Velocity Dispersion Hi in the Galactic Halo. Astrophysics and Space Science 252, 289–300 (1997). https://doi.org/10.1023/A:1000844417268
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DOI: https://doi.org/10.1023/A:1000844417268