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Astron. Astrophys. 332, L61-L64 (1998)

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Letter to the Editor

HI in the galactic halo

P.M.W. Kalberla 1, G. Westphalen 1, U. Mebold 1, Dap Hartmann 2 and W.B. Burton 3

1 Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
2 Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Sterrewacht Leiden, P.O. Box 9513, RA 2300 Leiden, The Netherlands

Received 3 December 1996 / Accepted 6 March 1998


We find solid evidence for diffuse HI gas at substantial z heights in our Galaxy, with a velocity dispersion of [FORMULA] [FORMULA] and a vertical projected column density of [FORMULA] cm-2. This pervasive component of the emission spectrum could be identified in the Leiden/Dwingeloo 21-cm Survey (LDS) after increasing the accuracy further by correcting the observations for reflections from the ground. Investigations of receiver bandpass and stray radiation effects could not explain this component as an artifact of the instrumentation.

Assuming that the distribution of mass and pressure perpendicular to the galactic plane is in hydrostatic equilibrium with the galactic potential, we derive a vertical exponential scale height of [FORMULA] 4.4 kpc for the observed diffuse high-dispersion HI component. The radial distribution is characterized by the corresponding galactocentric scale length [FORMULA] kpc.

Key words: Galaxy: halo – Galaxy: kinematics and dynamics – radio lines: ISM

Send offprint requests to: P. Kalberla


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© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998