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

Planar and extra-planar gas

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

Abstract

Context. The determination of the global structure of the planar and extra-planar Milky Way  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  volume density distribution for the Galactic disk out to  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  disk is significantly warped but shows a coherent structure out to  kpc. The radial surface density distribution, the densities in the middle of the warped plane, and the  scale heights all follow exponential relations. The radial scale length for the surface density distribution of the  disk is 3.75 kpc. Gas at the outskirts for  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  gas is in this state. The large scale  distribution is lopsided; for  kpc there is more gas in the south. The   flaring indicates that this asymmetry is caused by a dark matter wake, located at  kpc in direction of the Magellanic System.

Conclusions. The  disk is made up of two major components. Most prominent is the normal  disk which can be traced to  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.