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

Abstract

<i>Context. <i/>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.<i>Aims. <i/>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.<i>Methods. <i/>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.].<i>Results. <i/>Data from the 100 m telescope allow for the parameterization of 25 distinct H I halo clouds at Galactocentric radii 10 kpc <i><R<<i/>15 kpc and heights 1 kpc < <i>z<i/> < 5 kpc. The clouds have a median temperature of 620 K, column densities of ~ 10<sup>19<sup/> cm<sup>-2<sup/>, 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 (<i>FWHM<i/> ~ 3 km s<sup>-1<sup/>), they have volume densities of <i>n<i/> > 1.3 cm<sup>-3<sup/>, 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 (<i>P<i/> 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.