Properties of extra-planar H I clouds in the outer part of the Milky Way

¹ Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
² Max-Planck-Institut für Radiostronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: [ldedes;pkalberla]@astro.uni-bonn.de

Received: 10 June 2009
Accepted: 24 October 2009

Abstract

Context. There is mounting evidence for an extra-planar gas layer around the Milky Way disk, similar to the anomalous H I gas detected in a few other galaxies. As much as 10% of the gas may be in this phase.

Aims. We analyze H I clouds located in the disk-halo interface outside the solar circle to probe the properties of the extra-planar H I gas, which is following Galactic rotation.

Methods. We use the Leiden/Argentine/Bonn (LAB) 21-cm line survey to search for H I  clouds which take part in the rotation of the Galactic plane, but are located above the disk layer. Selected regions are mapped with the Effelsberg 100-m telescope. Two of the H I halo clouds are studied in detail for their small scale structure using the Westerbork Synthesis Radio Telescope (WSRT)[The Westerbork Synthesis Radio Telescope is operated by ASTRON (Netherlands Foundation for Research in Astronomy) with support from the Netherlands Foundation for Scientific Research NWO.] and the NRAO Very Large Array (VLA)[The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.].

Results. Data from the 100 m telescope allow for the parameterization of 25 distinct H I halo clouds at Galactocentric radii 10 kpc <R<15 kpc and heights 1 kpc < z < 5 kpc. The clouds have a median temperature of 620 K, column densities of  ~ 10¹⁹ cm^-2, and most of them are surrounded by an extended envelope of warmer H I gas. Interferometer observations for two selected regions resolve the H I clouds into several arc-minute sized cores. These cores show narrow line widths (FWHM ~ 3 km s^-1), they have volume densities of n > 1.3 cm^-3, masses up to 24 , and are on average in pressure equilibrium with the surrounding envelopes. Pressures and densities fall within the expectations from theoretical phase diagrams (P vs. ). The H I  cores tend to be unstable if one assumes a thermally bistable medium, but are in better agreement with models that predict thermal fragmentation driven by a turbulent flow.