2010A&A...509A..60D

There is mounting evidence for an extra-planar gas layer around the Milky Way disk, similar to the anomalous HI gas detected in a few other galaxies. As much as 10% of the gas may be in this phase. We analyze HI clouds located in the disk-halo interface outside the solar circle to probe the properties of the extra-planar HI gas, which is following Galactic rotation. We use the Leiden/Argentine/Bonn (LAB) 21-cm line survey to search for HI 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 HI halo clouds are studied in detail for their small scale structure using the Westerbork Synthesis Radio Telescope (WSRT) and the NRAO Very Large Array (VLA). Data from the 100m telescope allow for the parameterization of 25 distinct HI halo clouds at Galactocentric radii 10kpc<R<15kpc and heights 1kpc<z<5kpc. The clouds have a median temperature of 620 K, column densities of N_HI∼10¹⁹cm^–2, and most of them are surrounded by an extended envelope of warmer HI gas. Interferometer observations for two selected regions resolve the HI clouds into several arc-minute sized cores. These cores show narrow line widths (FWHM∼3km/s), they have volume densities of n>1.3cm^–3, masses up to 24M_☉, and are on average in pressure equilibrium with the surrounding envelopes. Pressures and densities fall within the expectations from theoretical phase diagrams (P vs. <n_h>). The HI 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.