These clouds are observed in the distant galactic halo of our Milky Way and around other galaxies (e.g., in M31, M87) and are visible throughout the whole electromagnetic spectrum. That class of clouds is defined by two characteristics:

1. They have radial velocities which are not consistent with a simple model of galactic differential rotation. That means, all of them are too fast for the galacto-centric distance where they are located.
2. They mainly consist of neutral hydrogen.

Beside these two features these clouds almost don't share any similarities: some HVCs show molecular content, while some don't. Some HVCs show dust, while some don't. Some HVCs show highly ionized elements, while some don't. Some of the HVCs are very rich in some metals (up to solar), while others are very metal poor. Some HVCs approach the Milky Way, while some recede it. A lot of HVCs are arranged in big complexes, while many are isolated in the sky.

The biggest challenge for observers is the determination of distances to HVCs. Why is the distance this important? Many key parameters of the clouds depend on distance, e.g.,

1. physical radius R ∝ D
2. mass M ∝ D²
3. (mean) matter density ρ ∝ D^-1
4. (mean) pressure p ∝ D^-1

For the majority of HVCs the distances are either unknown or only very loosely constrained. For some there exist only upper or lower distance limits. Thus, the above parameters are very uncertain (up to several 100%).

Interestingly, some HVCs emit Hα photons. These emissions could never be assigned to any stellar counterparts. Nevertheless, there exist some candidates that provide both physical properties and proper boundary conditions which could lead to star formation. In my project I simulate such HVCs considering a variety of physical processes, like heating & cooling, heat conduction, turbulence, and hydrodynamical instabilities. Well-suited for my purposes is the FLASH code, which solves the Euler equations on an adaptively refined grid. This code is parallelized (using the MPI library) and scales fine for many hundreds of processors.

In particular, I follow the interior dynamics of HVCs while they penetrate into the galactic halo. The clouds are initially in both hydrostatic and thermal equilibrium. The latter is maintained by mutual compensation of heating and cooling processes in each region of the cloud. At the cloud's boundary a pressure equality with the ambient hot gas (ISM) is achieved. That implies discontinuities in both density and temperature between the cloud surface and the ISM. Hence, the process of heat conduction across the cloud's surface must not be neglected. Finally, an external gravitational potential due to a Burkert dark-matter halo yields a gravitational acceleration depending on distance. Accordingly, the HVC is subject to an increasing gravitational force while falling toward the galactic disk. On the other hand, the drag of the ISM gas (i.e., exchange of momentum between cloud gas and ISM gas) decelerates the HVC, such that the cloud arrives at a constant terminal velocity.

The above chapter will be continuously extended ...