Properties of extra-planar HI clouds in the outer part of the Milky Way
  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). Data from the 100m telescope
allow for the parameterization of 25 distinct HI 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 NH~10E19 cm^-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 ~3 km/s),
they have volume densities of n > 1.3 cm^-3, masses up to 24 M_{sol}, 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.
