EDP Sciences
Free Access
Volume 487, Number 3, September I 2008
Page(s) 951 - 963
Section Galactic structure, stellar clusters, and populations
DOI https://doi.org/10.1051/0004-6361:20079240
Published online 06 May 2008

A&A 487, 951-963 (2008)
DOI: 10.1051/0004-6361:20079240

Global properties of the H I distribution in the outer Milky Way

Planar and extra-planar gas
P. M. W. Kalberla and L. Dedes

Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
    e-mail: [pkalberla;ldedes]@astro.uni-bonn.de

Received 13 December 2007 / Accepted 31 March 2008

Context. The determination of the global structure of the planar and extra-planar Milky Way $\ion{H}{i}$ disk depends critically on a reliable database but also on reasonable assumptions about the shape of the Milky Way rotation curve.
Aims. We derive the 3D $\ion{H}{i}$ volume density distribution for the Galactic disk out to $R \sim 60$ kpc.
Methods. Our analysis is based on parameters for the warp and rotation curve derived previously. The data are taken from the Leiden/Argentine/Bonn all sky 21 cm line survey.
Results. The Milky Way $\ion{H}{i}$ disk is significantly warped but shows a coherent structure out to $R \sim 35$ kpc. The radial surface density distribution, the densities in the middle of the warped plane, and the $\ion{H}{i}$ scale heights all follow exponential relations. The radial scale length for the surface density distribution of the $\ion{H}{i}$ disk is 3.75 kpc. Gas at the outskirts for $ 40 \la R \la 60$ kpc is described best by a distribution with an exponential radial scale length of 7.5 kpc and a velocity dispersion of 74 km s-1. Such a highly turbulent medium fits also well with the average shape of the high velocity profile wings observed at high latitudes. The turbulent pressure gradient of such extra-planar gas is on average in balance with the gravitational forces. About 10% of the Milky Way $\ion{H}{i}$ gas is in this state. The large scale $\ion{H}{i}$ distribution is lopsided; for $R \ga 15$ kpc there is more gas in the south. The $\ion{H}{i}$  flaring indicates that this asymmetry is caused by a dark matter wake, located at $R \sim 25$ kpc in direction of the Magellanic System.
Conclusions. The $\ion{H}{i}$ disk is made up of two major components. Most prominent is the normal $\ion{H}{i}$ disk which can be traced to $R \sim 35$ kpc. This is surrounded by a patchy distribution of highly turbulent gas reaching large scale heights but also large radial distances. At the position of the Sun the exponential scale height in the z direction is 3.9 kpc. This component resembles the anomalous gas discovered previously in some galaxies.

Key words: Galaxy: disk -- Galaxy: halo -- Galaxy: structure -- Galaxy: kinematics and dynamics -- ISM: structure

© ESO 2008